Document
AND
NASA TECHNICAL
NOTE
NASA TN D-7775
G1/02 4 9733
DAMAGE TO
LIGHTNING
-
AVIATION AIRCRAFT
A GENERAL
DESCRIPTION AND ANALYSIS
by Paul T. Hacker
Lewis Research Center
Cleveland, Ohio 44135 uT
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION * WASHINGTON, D. C. * SEPTEMBER 1974 Government Accession No. 3. Recipient's Catalog No.
1. Report No. 2.
NASA TN D-7775 I 5. Report Date and Subtitle 4. Title - SEPTEMBER 1974 AVIATION AIRCRAFT DAMAGE TO A GENERAL LIGHTNING Code Performing Organization 6.
AND ANALYSIS DESCRIPTION 8. Performing Organization Report No.
7. Author(s) E-7967 T. Hacker Paul No.
10. Work Unit 9. Performing Organization Name and Address 501-38 Lewis Research Center 11. Contract or Grant No.
Space Administration National Aeronautics and Cleveland, Ohio 44135 13. Type of Report and Period Covered 12. Sponsoring Agency Name and Address Technical Note National Aeronautics and Space Administration 14. Sponsoring Agency Code Washington, D.C. 20546 Notes 15. Supplementary 16. Abstract by a single lightning discharge damage sustained by a Beechcraft King Air Model B90 aircraft The is presented and analyzed. The incident occurred during landing approach at Jackson, Michigan, damage at the lightning attachment on Feb. 19, 1971. In addition to the usual melted-metal damage over a large area on the lower right side of the points, there was severe implosion-type and crushing-type damage on the upper and lower surfaces on the left aircraft and impact- indicate that the implosion-type damage was probably wingtip near the trailing edge. Analyses damage was caused by lightning-generated shock waves, that the impact- and crushing-type and that the lightning discharge was a multiple strike with at least caused by magnetic forces, The evidence indicates that the lightning 11 strokes separated in time by about 4. 5 milliseconds.
and severity.
discharge was rather different from the average in character Key Words (Suggested by Author(s)) 18. Distribution Statement 17.
Aircraft hazards; Operational hazards; Unclassified - unlimited Weather; Lightning; Thunderstorms Category this page) 21. No. of Pages 22. Price* 19. Security Classif. (of this report) 20. Security Classif. (of Unclassified 53 $3.75 Unclassified * For sale by the National Technical Information Service, Springfield, Virginia 22151
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LIGHTNING DAMAGE TO A GENERAL AVIATION AIRCRAFT
AND ANALYSIS
DESCRIPTION
T. Hacker
Paul
by
Center
Lewis Research
SUMMARY A Beechcraft King Air Model B90 aircraft was struck by lightning at an altitude of 2743 meters (9000 ft) during landing at the Jackson, Michigan, airport on February 19, 1971. Witnesses on the ground and in the aircraft reported that there was only one lightning discharge at the time of the incident and that it was between ground and cloud.
prior to or following No other thunderstorms were reported in the area within 2 hours the incident.
The damage sustained by the aircraft was widespread, rather severe, and unusual in several respects. The lightning attachment points on the aircraft were (1) the out- board trailing edge of the left wingtip, (2) the right engine propeller tip, (3) the ventral fin on the aft end of the fuselage, and (4) the navigation light on the top of the vertical stabilizer. In addition to the usual melted metal and cracked nonmetallic materials at the attachment points, there was (1) severe implosion-type damage to the aircraft skin on the lower right wing from the fuselage to a short distance outboard of the engine nacelle, including the nacelle and both sections of flaps; (2) impact- and crushing-type damage over an area of about 900 square centimeters (1 sq ft) on the top and bottom sur- faces of the left wingtip at the lightning attachment point; (3) pitting by electrical arcing of all support and control rod bearings on both sections of flaps on the left side of the aircraft; and (4) interruption of electrical power due to tripping of the circuit breaker on the generator on the right engine.
Photographs showing the damage in detail are presented. Analyses are made that show (1) that the implosion-type damage was probably due to shock waves generated by the high-current portions of the lightning discharge, (2) that the impact-type damage was probably due to magnetic forces created by the lightning currents flowing along dif- ferent paths in the aircraft structure, and (3) that the lightning discharge was a multiple- stroke type with at least 11 high-current strokes (spikes) with an average time between strokes of about 4.5 milliseconds.
INTRODUCTION dis- Marathon Oil Company was owned by Air Model B90 aircraft Beechcraft King A 19, 1971, from Findlay, on Friday, February passenger mission patched on a routine The air- Ohio.
and back to Findlay, Michigan, to Jackson, Illinois, Ohio, to Robinson, plan. The instrument flight 1600 e. s. t. on an for Jackson at Robinson craft departed of Jackson and in the vicinity (13 000 ft) enroute, to 3962 meters aircraft was cleared (7000 ft).
meters to 2134 meters descend from 3962 was cleared to (Reynolds Field) at a speed of flaps extended and (9000 ft) with through 2743 meters While descending strike.
a lightning experienced knots), the aircraft m/sec (160 side of the in the lower right a ball of fire a large flash and witnessed The pilot were indicated that they and working systems of the instruments cockpit. An inspection was off line. Follow- on the right engine the electrical generator all normal except that crew continued normally and the worked the generator the circuit breaker, ing reset of Landing time the flaps were retracted.
descent and landing. After landing, on a normal e.s.t.
was 1713 under limited lighting con- the crew inspected the aircraft After landing at Jackson, right nacelle had the upper cowl on the were made: The following observations ditions.
about 1. 25 centimeters by safety fasteners, a hole unlocked and was held become and a of the ventral fin, the lower aft corner had been burned in in.) in diameter (1/2 burned in length had been centimeters (4 in.)
approximately 10 discharge area lightning the aircraft was observed, so wingtip. No other damage trailing edge of the left on the on this leg and its systems operated normally ferried to home base. The aircraft was but a safe landing was the flaps failed to extend, of the flight; however, during landing, for takeoff. ) (Extended flaps are not required accomplished.
it had apparently of the hangar revealed that the aircraft in the light Inspection of to slightly right side, from the fuselage an external explosion on the lower suffered from paint was in, rivets were missing, nacelle. Panels were caved outboard of the right showed three and the outboard flap flap was bowed backwards, discolored, the inboard wrinkles.
damage Murphy, of the damage prompted M.
The unusual characteristics and extensiveness Of- NASA Headquarter's Safety Oil company to contact the Chief Pilot for the Marathon re- They, in turn, damage to aircraft.
to be interested in lightning fice, whom he knew NASA Lewis Research and Data Institute, Aerospace Safety Research quested that the This aircraft and document the incident.
Ohio, inspect the damaged Center, Cleveland, lightning analyze both the damage and the the damage and attempts to report describes the damage.
strike that caused are Ohio, and M. Murphy, their Chief Pilot, The Marathon Oil Company, Findlay, the airplane available for for notifying NASA of the incident, making to be commended photographs and inspection, and providing information on the flight and the aircraft.
Without this excellent cooperation, this report could not have been written and valuable information would have been lost.
OBSERVATIONS AND COMMENTS OF WITNESSES The pilot, J. R. Day, and the copilot, J. W. Maxie, reported after the incident that radar indicated no thunderstorm cells, that no heavy turbulence was encountered, and that only the one lightning flash was seen. The air traffic controller on duty, T. Stevens, stated that the aircraft was on approach to Reynolds Field, Jackson, Michigan, from the northeast and was preparing to land on runway 23, which is 1615 meters (5300 ft) long.
He then observed one large, very bright, ground-to-air lightning flash and notified the pilot of this observation. The pilot responded that he thought they were hit by it. The controller believes that the thunderclap lasted for more than 30 seconds and that it was one continuous roll. The off-duty controller, R. Grove, who lives 3. 2 kilometers (2 miles) east of Reynolds Field was at home at the time of the incident. The long, roll- ing clap of thunder shook his house, jarring some plaques on the wall.
These observations indicate that the lightning discharge or at least the resulting thunder was probably more severe than the average lightning discharge.
METEOROLOGICAL CONDITIONS The weather reports for Jackson, Michigan, for February 19, 1971, near the time of the incident are as follows: Time, Cloud Cloud Visibility Conditions Temperature Dewpoint Wind Wind speed Barometric Remarks e.s.t. height coverage direction pressure m milesC F C oF m/sec knots m ft rad deg kN/m in. Hg a 152 500 Broken 1609 1 Light rain 1 6 5 9 and fog 5.6 42 208 37 2.62 150 7.7 15 100.7 29.74 -- 305 1000 Overcast b1710 152 500 Overcast 1207 3/4 Thunderstorm, --- -- ----- 2.62 150 7.2 14 ----- ----- Thunderstorm overhead moderate rain started at 1710 e.s.t.
showers, and fog b1728 152 500 Overcast 1207 3/4 Light rain showers ----- ----- 2.01 115 7.2 14 100.7 29.74 Thunderstorm ended at and fog e.s.t.
a 183 600 Broken 2414 11 Light rain and fog 7.2 45 4.4 40 2.62 150 7.2 15 100.6 29.72 1 7 5 4 305 1000 Overcast aRegular report.
bspecial report.
one thun- The low cloud ceiling, fog, and rain persisted throughout the entire day. Only Sunset derstorm was observed at Jackson, Michigan, from 1500 e.s.t. to 2000 e.s.t.
The weather across the on February 19, 1971, at Jackson, Michigan, was 1815 e.s.t.
was very similar to that in Jackson around the southern portion of lower Michigan situation at 2000 e. s. t. (about 3 hr after the time of the incident. The synoptic weather showed a region of low barometric pressure in central Illinois. A cold front incident) the low, and a warm front extended eastward from the extended southwestward from low across Indiana. The direction of travel of the low was north-northeast toward Lake Michigan.
DESCRIPTION OF AIRCRAFT The aircraft involved in the lightning strike is shown in figure 1. According to ref- Air is a pressurized 6- to 10-seat business aircraft powered by two erence 1, the King characteristics: Pratt & Whitney PT6A-20 turboprop engines with the following Wing span, m (ft) . . . . . . . . . . . . . . . . . . . . . . . . . . 15.32 (50.25) Overall length, m (ft) . . . . . . . . . . . . . . . . . . . . . . . . 10. 82 (35. 5) Height over tail, m (ft) . . . . . . . . . . . . . . . . . . . . . . . 4. 47 (14. 66) Empty weight equipped, kg (Ibm) . .................. . 2578 (5685) Maximum takeoff weight, kg (Ibm) . .................. 4377 (9650) Maximum cruising speed at 4875 m (16 000 ft), km/hr (mph) . ....... 407 (253) Stalling speed with wheels and flaps down, km/hr (mph) . ......... 137 (85) The wings are cantilevered, two-spar, aluminum alloy structures. The fuselage is an aluminum alloy semimonocoque structure. The standard avionics comprise FAA Cate- gory II complete all-weather navigation and communication systems and weather radar.
DESCRIPTION OF DAMAGE AND HAZARDOUS EFFECTS The damage and hazardous effects produced by the lightning discharge were rather severe and widespread and resulted from the various phenomena associated with a light- ning discharge. There was melting of metals and cracking of nonmetals at lightning stroke attachment points on the aircraft.
Metal skins were distorted due to the "mag- netic pinch effect" as the lightning current flowed through them. Metal skins were also distorted by the overpressure in shock waves generated by the lightning discharge. In- terior movable mating surfaces were pitted by electrical arcing as the lighting current flowed through the structure. The circuit breaker in the electrical generator system in the electrical system. The damage and was tripped by transient voltages induced hazardous effects are described in detail in the following sections.
Lightning Attachment Points is a flow of current between two regions of electrical charge of opposite Lightning sign. These charged regions may be in the atmosphere or in the atmosphere and the ground. When lightning strikes an aircraft, the aircraft becomes a part of the electrical path. So there are at least two attachment points, an entrance and an exit. Both en- trance and exit points may shift positions, however, during a discharge due to motion of the aircraft with respect to the relatively stationary lightning path. Thus, several attachment points are created.
Left wingtip. - One attachment point of the lightning strike was at the trailing edge left wingtip. The damage at this point is shown in figure 2. Besides the metal of the which is typical lightning discharge damage, the trail- being melted (figs. 2(b) and (g)), ing edge has been pushed forward (fig. 2(b)). This forward motion of the trailing edge created two pronounced ridges in both the upper and lower surfaces of the wing which were approximately parallel to the trailing edge. The rearmost ridge coincided with a normal bend line in the wing and was so sharp that the metal was torn (figs. 2(b) and (g)). Forward of the ridges, both surfaces of the wing skin were pushed inward. This crumpling damage is probably due to electromotive forces created by the interaction of magnetic fields generated by the lightning electrical currents flowing in the upper and lower surfaces of the wing. This phenomenon is commonly called the "magnetic pinch effect. " An analysis and an estimation of the magnetic forces involved in the magnetic section.
pinch effect are presented in a later Propeller. - One blade of the right propeller was another lightning attachment point.
Damage to the aluminum propeller was minor, as can be seen in figure 3. The amount of damage is typical for lightning strikes to propellers.
The massiveness of the propel- ler provided a good heat sink and thus not much melting of material occurred.
Ventral fin. - Another area of lightning attachment was the bottom edge of the ven- tral fin. These damage points are shown in figure 4. The lightning attachment on the fin is typical of what is generally referred to as a "swept stroke. " The stroke first attached itself at a forward position on the fin.
Then, because of the motion of the air- craft relative to the nearly stationary lightning discharge path, the attachment point moved rearward along the fin, creating damage that ranged from small pitmarks to relatively large holes.
Vertical stabilizer. - Another possible lightning attachment point was the navigation light on the top of the vertical stabilizer. There was no evidence of burning or melting, but the plastic mounting for the light was cracked, as can be seen in figure 5.
Implosion- Type Damage of the aircraft suffered damage that ap- An extensive area on the right bottom side damaged area included the fuselage peared to be caused by an external explosion. The outboard of the engine, both sides near the wing root, the wing from the fuselage to just and the bottom of the aft end of the engine nacelle, and both sections of the wing flap.
skin over the inward buckling of skin was sufficient in some sections to pull the The rivet heads.
The damage to the inboard section of the wing, the inboard side of the engine nacelle, to 13.
the fuselage is shown in figures 6 and and the in- Figure 6 is a general view of the damage to the lower surface of wing to two skin board side of the engine nacelle. The major damage on the wing occurred Closeup panels: the hatch cover for the battery compartment and a wing root fairing.
8. The views of the implosion-type damage to these panels are shown in figures 7 and heads.
inward force on the fairing panel was sufficient to pull the skin over the rivet had a stiffener as is The fairing panel had no internal stiffeners, but the hatch cover panels evidenced by the rows of weld spots visible in figure 7. All the damaged skin are given in table I.
were aluminum alloy. Typical thicknesses of damaged skin panels in an area containing a part of the In addition to the implosion-type damage, the paint and a section of the fuselage was discolored as hatch cover, the wing root fairing, to high temperature. This area of discoloration is shown in though it had been exposed 9. Most of the discoloration was slight, except on the fuselage, where it figures 8 and 9) is not was moderate. The longitudinal black streak on the bottom of fuselage (fig.
but exhaust deposits from the cabin heater.
paint discoloration A drain fitting for a cabin coffee bar was located in the area of moderate paint dis- holes for this fitting are shown in figures 8 and 9. The fitting coloration. The mounting is shown in figure 10. The fitting flange was mounted on the internal surface of the skin and completely separated WIL Liv rivets. The fitting was found inside the fuselage skin from it.
to the lower inboard side of the right engine nacelle is shown in figures 11 Damage to 13. The damaged area was below the wing chord line and extended from about 35 cen- of the nacelle (fig. 12).
timeters (13. 8 in. ) in front of the wing leading edge to the aft end The severity of the damage increased markedly in the aft direction. The forward panels showed some wrinkling and sharp creases at an internal structural member. The aft skin panels were severely buckled. The landing gear doors showed no evidence of dam- age. However, the bottom of the nacelle aft of the landing gear doors was damaged, as is shown in figure 13.
The damage to the outboard bottom side of the right engine nacelle is shown in fig- ures 14 and 15. The area of damage is almost identical to that on the inboard side of the nacelle. The magnitude of the damage also increased in the aft direction but overall was not as severe as that on the inboard side. Damage to the lower wing surface on the outboard side of the engine was minor, as shown in figure 14. Figure 15 presents another view (see fig. 13) of the damage to the aft bottom of the nacelle.
Each wing of the aircraft is fitted with about 3. 6 meters (11. 8 ft) of wing flap which is divided into two sections of about 1.8 meters (5.9 ft) each. The inboard section ex- tends from the fuselage to just beyond the engine nacelle. The flaps are made of alumi- num alloy and have an airfoil shape, with the lower surface nearly straight in both the chordwise and spanwise directions.
The flaps were extended and lowered at the time of the lightning strike, and both sections on the right side of the aircraft sustained severe damage, as shown in figures 16 to 22.
As might be expected from the damage shown in previous photographs of the wing and nacelle, the inboard flap received the greatest damage. In general, the inboard flap was bowed upward, as can easily be seen in figures 16 and 17. The centerline of the bow appears to be oriented in approximately the chordwise direction and located about 13 centimeters (5. 1 in. ) from the inboard side of the engine nacelle fairing that is at- tached to the flap (figs. 16 and 18). In addition to the bowing, both upper and lower sur- faces sustained extensive damage in the form of buckling and dents. The severest buck- ling occurred at the leading edge of the flap near the centerline of the bow. Both the lower surface (point A, fig. 18) and the upper surface (point A, fig. 19) were sharply creased inward. The creasing was sufficient to tear the metal on the leading edge. A depression in the metal at the inboard end of the inboard flap at the leading edge (point B, fig. 18) was sufficient to create a sharp crease in the metal on the leading edge.
There was a similar depression in the lower-surface leading edge of the inboard flap at the outboard end (point C, fig. 20). The trailing edge of the inboard flap near the midsection was sharply bent downward, as can be seen in figures 16 and 17. The skins of the flaps, especially on the upper surface at the aft end of the flap attachment fittings, were dis- torted (figs. 19 and 21). The metal on each side of the engine nacelle fairing adjacent to the lower flap surface was dented inward (figs. 16 and 20).
Damage to the outboard flap was minor compared with that to the inboard flap.
There was no evidence of general bowing or widespread buckling.
The lower surface near the leading edge was dented inward in three areas.
These are shown at points D, E, and F in figure 22. Point D is also shown in figure 20. There was also some distor- tion of the upper surface flap skin at the aft end of the flap attachment fitting on the in- board side (fig. 21).
Other Damage and Hazardous Effects The lightning strike produced the following additional damage and hazardous effects: generator on the right - The circuit breaker on the electrical Electrical system.
breaker.
the circuit by resetting Power was restored the strike.
tripped during engine physical damage.
showed no evidence of the generator system Inspection of found un- right engine were latches on the - The engine cover cover latches.
Engine the lightning strike.
after landing following by safety latches latched but secured slotted to the wing by two the flaps is mounted Each section of bearings. - Flap slot) at- four bearings (two bearings per (fig. 19) attached to the wing in which tracks is moved by a is moved. Each section of flap to the flap can roll when the flap tached to a fixture on the the pivot point through a ball bearing at rod which is connected control electrical pitting due to the two left flaps showed eight bearings on flap (fig. 19). All that con- race. This is the surface of the outer bearing on the outside surface arcing the pair of bear- pitting occurred to in the track. The severest tacted the slot surface attach- to the lightning which was closest of the outboard flap, at the outboard end ings decreased in an inboard direction The degree of pitting point on the left wingtip.
ment outer of bearings. Damage to the on the inboard pair until it was just barely detectable the mounting bearings were is shown in figure 23. Although surface of the outboard pair race race with respect to the outer by turning the inner not disassembled, it was obvious were also damaged. This difficult and their mating surfaces that the rolling elements rod bear- of bearings. Both control marked for the outboard pair turning was especially the bolt holes showed the mounting bolts, and The side faces, ings were also damaged.
to the in figure 24. Damage for the outboard flap is shown melted areas. The bearing difficulty in turning the mating surfaces was also indicated by rolling elements and bearing.
RESULTANT DAMAGE OF LIGHTNING STRIKE AND DISCUSSION AND ANALYSIS Damage Implosion-Type engine nacelle, and damage to the lower right wing surface, The implosion-type generated in the shock wave resulted from overpressures flaps (figs. 6 to 22) probably of energy (of the order of releases a large amount discharge. Lightning by the lightning into a relatively small vol- path) at a very high rate (1 to 10 psec) 105 J/m of discharge (of the in the channel is heated to extremely high temperatures ume of air. The air As the creates a very high pressure in the channel.
order of 20 000 to 30 000 K), which which moves radially out- expand, a shock wave is generated gases in the heated channel the channel. As the shock wave moves outward, ward from the center of the discharge and the shock wave even- overpressure immediately behind the shock front decreases wave of thunder. Analytically predicted values of shock tually degenerates into the sound as a function of distance as given in reference 2 for typical lightning wave overpressures discharges are shown in figure 25 as curves A and B. The calculations for the two curves are based upon different assumptions, as indicated in the figure and give widely different results at large distances. Curve B at distances of about 5 meters (16. 4 ft) gives overpressures equivalent to 0. 454 kilogram (1 lbm) of TNT, curve C (ref. 3).
When a traveling shock wave is intercepted by a solid surface, the load imposed on the surface is greater than that due to the overpressure in the incident shock wave itself.
The kinetic energy of motion of the incident shock wave is transformed into a pressure rise, which increases the overpressure in the reflected shock wave. For shock waves traveling in a direction normal to a surface that does not absorb any energy from the wave, the ratio of the overpressures, reflected to incident, varies from 2 for very low overpressures in the incident shock wave to many times for incident shock waves with large overpressures.
When the direction of travel of the shock wave is not normal to the surface, the imposed load is less.
The predicted overpressures given in figure 25 would be sufficient to have caused the observed damage if the lightning discharge path was close enough.
Therefore, it may be of interest to theorize on the probable lightning discharge path and lightning characteristics.
Lightning Discharge Path and Characteristics An indication of the probable lightning discharge path with respect to the airplane and some of the characteristics of the discharge may be obtained from the observed damage and lightning attachment points. First, however, some general characteristics of lightning should be reviewed. The lightning phenomenon is well described in refer- ences 2 and 4. Lightning discharges generally consist of a series of high-current, short-duration strokes (spikes) separated by relatively low-level, long-duration con- tinuing currents. The duration of the high-amperage current is of the order of micro- seconds, while the duration of the continuing currents is of the order of milliseconds.
Shock waves are created by the high-current strokes. The melting of metal at the at- tachment points is caused primarily by the continuing currents.
As pointed out earlier, two lightning attachment points on the aircraft were the trailing edge of the left wingtip and a tip of the propeller on the right engine. If it is assumed that these points were the initial attachment points, a probable discharge path was from a charged region in a cloud above and ahead of the aircraft to the left wingtip, through the aircraft to the right propeller tip, and then to the ground to the rear of the aircraft.
If the propeller tip at the time of initial attachment was near or below the plane of the wing, then the ground portion of the discharge would be under the wing and relatively close to the wing surface.
Inasmuch as there was implosion-type damage on the lower wing surface and the nacelle surfaces on both sides of the nacelle, (figs. 6 one on each side of the two high-current strokes, must have been at least to 22), there (viewed from in direction in a counterclockwise nacelle. Since the propeller rotates tip was on the the effected propeller had to occur when of aircraft), the first stroke front of the nacelle.
outboard side in- than that on the substantially less of the nacelle was sustained outboard Damage farther factors: the lightning was to either or both of two side. This could be due board was less than in level in the first stroke or the current from the aircraft surfaces, away lightning dis- is the distance since in the usual stroke. The probable reason the second propeller tip the largest (ref. 2). As the effected the first stroke is generally charge the factors would tend to cause nacelle to inboard, several from outboard of the moved aircraft. Such a dis- the lower surface of the path to move toward lightning discharge was moving toward the lightning.
result, for example, if the aircraft charge path would lower wing sur- discharge path would also move toward the The air containing the ionized edge and also it approached the wing leading of the upward flow of air as face because the propeller. The slipstream downstream of of the contraction of the propeller because root and the adjacent damage at the wing and severe implosion-type paint discoloration the path for a stroke was very near indicate that the discharge fuselage (figs. 8 to 10) tip was near the plane occurring when the effected propeller aircraft surface. A stroke almost with the lightning discharge path wing surface on the inboard side of the lower from the propeller tip with the lightning attachment point moving horizontal is consistent fin (fig. 4).
to the ventral aircraft cannot be easily on the lower right surfaces of the Some damage sustained strokes. These damages are the shock waves created by only two associated with the the wing flaps near their lead- sustained by the lower surfaces of localized indentations these indentations on 22. The locations of in figures 18 to 20 and ing edges as indicated shown in fig- and propeller tip circle are with respect to the wing, nacelle, the flaps and D is A. The damage at points B, C, The most severe damage was at point ure 26.
that and at point F very light. It is proposed Damage at point E was slight moderate.
was different strokes while the lightning indentations were caused by five these localized re- from the orientation of the stroke with to the propeller tip. They resulted attached wave, and of the lightning-generated shock spect to the aircraft, the cylindrical nature and of the lightning strike, the flaps were extended the wing-flap geometry. At the time the wing. The along the junction of the flap leading edge and lowered, creating a corner were all in the flap and located along this corner line.
localized indentations we assumed that the occurrence of these localized indentations, In order to explain propeller tip to the ground was ori- the portion of the lightning discharge path from the as illustrated in ented in a general longitudinal or chordwise direction under the wing, that the lightning discharge path was parallel 27(a). There is a high probability figure on the neither to the underneath surfaces of the flap or wing, except maybe at a point forward portion of the wing where the surface is highly curved, nor to the wing-flap corner line. Also shown in figure 27 are positions of the cylindrical shock wave at ar- bitrary points in time as it moved radially away from the lightning discharge path, in- wing and flap surfaces, and was reflected. As was mentioned previously, tercepted the the greatest load imposed on a surface by a shock wave with a given overpressure occurs direction of travel of the shock wave is normal to the surface. That is, the when the load occurs at points on the surface where the direction of travel of maximum imposed to the lightning discharge path and the surface.
the shock wave is mutually perpendicular wing-flap corner line can be considered as two non- The lightning discharge path and the that is mutually perpen- parallel lines. There is, therefore, only one line between them figure 27 by line O-P. In the spanwise direction dicular to both. This is represented in the surface wing-flap corner line from point O (fig. 27(b)), the imposed load on along the the direction of travel of the shock wave is not normal to the decreases not only because further away from the lightning discharge path surface but also because the surface is to a lower overpressure (fig. 25). In the chordwise direction, the and thus subjected imposed load for points near point O may be less than the load imposed at point O be- cause of the nonnormal direction of travel of the shock wave. At greater distances from point O the imposed load may become greater as the distance between lightning discharge path and the wing and flap surfaces becomes less. This distribution of the imposed load in the chordwise direction is in agreement with the damage sustained, especially on the lower inboard sections of the wing and the flap. On the wing the damage was greater on the forward portion than on the aft portion (figs. 6 to 12). The flap was also damaged near the trailing edge (fig. 17). The preceding argument can be extended to show that localized damage can also occur at a corner point formed by three intersecting surfaces.
It may appear odd that all the localized indentations occurred in the flap and not in the wing material, but there is a possible explanation. The wing skin near the corner line is reinforced by doubling the material, whereas the flap is single thickness. The flap skin thickness is also only 0. 56 millimeter (0. 022 in. ), which is thinner than for representative wing skin panels for which measurements are available (table I).
The preceding discussion indicates that localized damage can occur along the wing- flap corner line. Based upon the location of the damaged areas and a reasonable as- sdmption of the orientation of the lightning discharge path with respect to the aircraft, the possible location of the propeller tip to which the lightning was attached when the high-amperage strokes occurred can be determined. From these locations and the known propeller rotational speed, the time interval between strokes can be estimated.
Consider first point A (fig. 26), which sustained the greatest damage, and assume the lightning discharge path from the propeller tip was oriented in a chordwise direction.
A line normal to the wing-flap corner line at point A (line A-A' in fig. 26) and the light- ning discharge path which it intersects defines a plane which intersects the propeller circle at point 4. Point 4 is a possible position of the effected propeller tip at the time of the stroke that caused the damage at point A.
Dl, and D (fig. 26) on the outboard side of the nacelle.
Next consider points C, The overall damage here was not as great as it was at point A. There appear to be three distinct damage points, but it can be argued that all three were the result of a flap is swept forward with single stroke. The wing-flap corner line for the outboard respect to the inboard wing-flap corner line. Thus, the wing and the two flaps create a corner where they meet. Based on the argument presented previously, this geometry could lead to four distinct localized damage areas. One on each of the three corner lines (those formed by the junctions of the wing and the outboard flap, the wing and the inboard flap, and the inboard and outboard flaps), and the corner formed by these three surfaces. The damage at point D is, therefore, explainable. The damage on the inboard wing-flap corner line could be that shown at point C. It could be, however, that the damage that could occur on the inboard wing-flap corner line did not occur because of the shielding effect of the nacelle. If this is true, points C and D could be associated with the damage that might normally occur in the corner. Points C and D are very near the corner but are on different flaps. The ends of each flap contain end plates which stiffen the flaps near the corner so that the resulting damage occurred in the weaker material on either side of the corner. There was no localized damage area along the inboard-outboard flap corner line. There would be no damage along this corner line, if the corner line and the lightning discharge path were in the same plane. If they were not in the same plane, there may have been no visible localized damage because the point of maximum imposed load was either foreward or aft of the flap. Now if it is as- sumed that the lightning discharge path was oriented under the wing in an almost chord- wise direction and almost in the same plane as the inboard-outboard flap corner line, another position of the effected propeller tip when a stroke occurred, point 3, can be established.
The angle subtended by points 3 and 4 on the propeller tip circle is 0. 87 radian (490). At the nominal propeller rotational speed of 1800 rpm (ref. 1), the time between strokes 3 and 4 is about 4. 5 milliseconds. This time interval is much less than the most frequently observed interval of about 40 milliseconds and only slightly larger than the minimum time of 3 milliseconds for multiple-stroke discharges that do not have continu- ing currents between each stroke (ref. 2).
It is frequently observed in multiple-stroke lightning discharges that the time inter- val between successive strokes is fairly constant throughout the discharge (ref. 4).
The time interval or distance between points 3 and 4 on the propeller tip circle can then be used to locate the position of the effected tip for the three other strokes which are as- sumed to have occurred. The other positions are shown as points 1, 2, and 5 in fig- ure 26.
Points 2 and 5 are very close to the localized-damage points E and B, respec- tively. This leaves point 1 to be associated with the damage at point F. Since the propeller rotates in a counterclockwise direction, the successive strokes in the dis- charge go from point 1 to point 5. After point 5, the lightning discharge path would be interrupted by the wing, and the attachment point for this portion of the discharge path would have to change.
The lightning attachment point did move to the lower edge of the ventral fin and be- came what is generally referred to as a swept stroke, as shown in figure 4. The swept stroke left a series of damaged areas that ranged from scorched paint with small pits in the metal to large holes through the sheet metal. The positions of the damaged areas along the ventral fin are shown in figure 28. The relative magnitude of the damage at each position is qualitatively indicated by the length of the line marking the position.
Altogether there were 12 damaged areas, wifh six of these more pronounced than the others. The spacing of these pronounced damaged areas along the fin was fairly uniform, 35. 6 to 43.2 centimeters (14 to 17 in. ), except the one at'the aft end which was 20.3 cen- timeters (8. 0 in. ) from the preceding area. The average spacing, not including the 20. 3 centimeters (8. 0 in. ), was 39. 7 centimeters (15. 6 in. ). At the time of the light- ning strike, the aircraft speed was 82.3 m/sec (160 knots). The time required for the aircraft to move 39. 7 centimeters (15. 6 in. ) was 4. 8 milliseconds. This is very close to the 4. 5 milliseconds calculated earlier for the time between the high-amperage strokes of the discharge. This suggests that the change in attachment point for a swept stroke is triggered by the high-amperage strokes. After attachment, the continuing cur- rent flow caused the melting-type damage. The amount of current flow required to melt aircraft structural materials is discussed in reference 5. Meanwhile, because of the motion of the aircraft, the lightning discharge path near the aircraft was stretched and bent with respect to the original path which was relatively fixed in space. When the next stroke occurred, the large magnetic field created around the path ruptured the discharge path at the bend, causing a new attachment at a point on the aircraft near the original discharge path. The minor damage occurring between the major damage positions (fig. 28) could have been caused either by momentary contact between the discharge path and the structure as the attachment point was carried forward with the aircraft or by momentary contact as the attachment point changed position.
The last attachment point for the ground portion of the lightning strike was probably the aft end of the ventral fin. How long it remained attached here cannot be estimated but was probably fairly long since the damage here was the greatest. During most of the time interval between the attachment of the ground portion of the lightning discharge to the propeller tip until it moved to the aft end of the ventral fin, the cloud portion of the discharge was probably attached to the left wingtip. Near the end of the discharge, the cloud-portion attachment point moved to the plastic light housing on the tip of the vertical stabilizer. The probable lightning path with respect to the aircraft as a function of time is shown in figure 29.
the discharge was at foregoing analysis, the number of strokes in Based upon the was greater than 42 milliseconds. Statistics on lightning least 11 and the total duration strokes per dis- presented in reference 2 give (1) the average number of discharges occurring in only about 5 percent of charge as about four, with 11 strokes per discharge with 11 strokes as about discharges; and (2) the average total duration of discharges discharge was far from 350 milliseconds. According to these statistics, this lightning average.
Damage due to Magnetically Induced Forces crumpling-type damage sustained at the lightning attachment point on the trail- The on the edge of the left wingtip (fig. 2) appears to have been caused by an impact ing and forward. The ap- trailing edge since the trailing edge has been pushed downward of a chordwise cross section of the wingtip trailing edge before and proximate geometry is shown in figure 30. In the analysis that follows, it will be after the lightning strike from magnetically induced forces created by shown that the damage probably resulted as it flowed through the aircraft wing skin.
the lightning current known that a current-carrying conductor is surrounded by a magnetic It is well a second current-carrying field. If the magnetic field for one conductor encompasses produces a force on the second con- conductor, the magnetic field of the first conductor the second conductor produces a force on the first con- ductor and the magnetic field of simple case of two parallel current-carrying conductors, the forces are ductor. For the as to create an attraction of the conductors when the currents are in the same such direction and a repulsion when they are in opposite directions. In the case of a single current-carrying conductor which is not straight, the magnetic field generated in one section can produce a force on another section of the conductor. For the general case, the force on conductor 2 by a current in conductor I is given by the vector differential equation (ref. 6) dF = i dl x 2 ll1 2 2 (1) where F force on conductor 2 i current in conductor 2 dl element of length along conductor 2 in direction of current flow p permeability of media surrounding conductors in conductor 1 2 due to current flow of field at conductor i1 magnetic intensity The magnetic field intensity H is given by the vector differential equation (ref. 6) d1 x r dH 1i (2) r where
ii current in conductor 1
d element of length along conductor 1 in direction of current flow rvector distance from a point on conductor 1 to a point on conductor 2 r magnitude of r (The units for the parameters of eqs. (1) and (2) depend upon the system of units used.)
In order to illustrate the direction and order of magnitude of the magnetic forces on the wingtip surfaces, equations (1) and (2) were applied to a system of conductors whose geometry represents the wingtip trailing-edge chordwise cross section shown in fig- ure 30. In addition, the lightning discharge path was included as part of the circuit.
The lightning path was assumed to be in the plane of the wing cross section and attached to the trailing edge (point A, fig. 30) at an angle of 0. 7854 radian (450) with respect to the trailing-edge section. The lightning current I was assumed to flow into the wing at a point A (fig. 30) and to divide equally at point O into the conductor segments OBE and OCD, representing the upper and lower surfaces of the wing. Equation (2) can be inte- grated to give the magnetic field intensity H produced by the current flow in one seg- ment of the circuit at various points along other segments of the circuit. The resulting equation is 1 cos S 1 cos bI S 1 - 1 2 - H1 (3) Ssin (21 cos )S )S2 + 2 12 - (21 cos )S + 1 S where I current in conductor 1, A b conversion factor to convert amperes to abamperes, b = 0. 1 0 angle between conductor 1, the field-producing conductor, and conductor 2, along which the field is to be calculated such as points C points of conductor 1, vertex of angle P to end S ' S distances from 1 2 respectively, in fig. 30, cm and D, from vertex of angle 0, cm 1 distance along conductor 2 measured due to current in conductor 1, Oe magnetic field intensity at conductor 2 H one- lightning current I or to (3) is either equal to the The current I used in equation the conductor system involved.
upon the segment of the lightning current, depending half system is the along one segment of the conductor The total magnetic field intensity all segments of the all the other segments. Since sum of the contributions from vector field along these segments is perpen- system are in one plane, the magnetic conductor current for intensity per ampere of lightning the plane. The magnetic field dicular to positions along of the conductor system is presented in table II for various each segment the conductor.
integrated form of equa- field intensities in table II and the By using the magnetic of lightning current of conductor per ampere squared tion (1), the forces per unit length II in newtons per meter per calculated. These forces are also presented in table were proportional to the appears because force is directly squared. Current squared ampere strength is current I and because the field magnetic field strength H and product of to the current.
also directly proportional configuration, the mag- pointed out, for the assumed conductor system As has been the plane of the conductor to the direction of current flow and netic field is perpendicular vector to be perpendicular to both The vector equation (1) requires the force system.
the various the magnetic field. Thus, the forces on the direction of the current and are in the plane of the conductor system.
segments of the conductor system on a schematic repre- calculated forces presented in table II are shown plotted The section in figure 31, in order to show sentation of the wing trailing-edge chordwise cross and to compare the forces with the relation between the forces on the various segments of the calculated forces correlate very the damage sustained (fig. 30). The directions well with the damage.
The forces on the trailing-edge segment (A-O) are downward, The forces on A-O, es- agrees with the final configuration in the damaged area.
which are mainly due to the lightning, which was assumed for the calculation pecially near A, selected as coming from to come from above and behind the trailing edge. If it had been above and forward, the downward forces on the upper surfaces and especially on the the trail- trailing edge would have been greater. If the lightning had come from below, and ing edge would have been forced upward. The calculated forces on segments O-B O-C near point O are such as to force the two segments together, which actually oc- curred. At points B and C, the calculated forces are such as to cause the points to move away from the interior and also to cause separation of segments O-B and O-C from seg- ments B-E and C-D, respectively. Points B and C were actually pushed outward, and the metal was torn at point B. The calculated forces on most of the upper surface (seg- ment B-E) and lower surface (segment C-D) are such as to cause the surfaces to move inwardly toward each other, which was the actual case.
The forces presented in table II and figure 31 are the calculated forces per ampere squared for a 1-meter-long conductor. (The maximum current measured in a stroke portion of a lightning discharge was slightly greater than 200 kA, with 2 percent of all strokes greater than 100 kA (ref. 2). ) For the trailing-edge segment (A-O), whose length is 1.25 centimeters (0. 48 in. ), the average force over its length would be about - 5 10 N/(A2)(m).
Thus, the average force for a 100-kiloampere stroke on segment A-O would be 1250 newtons (281 lbf). On the upper and lower surfaces, B-E and C-D, re- - 5 spectively, at about 2-centimeter (0. 79-in. ) position the force would be about 0. 5x10 N/(A2)(m), which gives for a 100-kiloampere stroke a force of 500 N/cm (286 lbf/in.).
The foregoing calculated forces are based on the assumption that all the lightning current flows in a line circuit defined by a chordwise section of the wing trailing edge.
In the case of a stroke to the aircraft wingtip, the lightning current flowing in the upper and lower wing skins will spread out as it flows away from the attachment point. This diffusion of the current decreases the intensity of the magnetic field and thus the forces.
The actual current diffusion pattern in the wing skin is not known and if it were known, it might be difficult to calculate the magnetic field and the forces. An estimate of the forces on the upper and lower wing surfaces can be made, however, if it is assumed that the current flow paths on the two surfaces, or conductors, are parallel ribbons of infinite length, as shown in figure 32(a). If it is further assumed that the current dis- tribution across the width is uniform, the x- and y-components of the magnetic field intensity, H and Hy, respectively, produced by x one of the conductors derived from a vector potential equation given in reference 7 are given by Hx _ 2 tan-1 + 2 tan-1 a - x (4) 87ra y y and Hy = lo(a + x) + y - log (a - x) + y2] (5) where I total current in conductor a half-width of conductoi (fig. 32(a)) y distance between conductors 1 and 2, m centerline of conductor 2, meter x distance from For a 2 cm (0.79 in.)
1A I in.)
y 1 cm (0.39 to current on conductor 2 due field intensity of the magnetic The x- and y-components of the force The x- and y-components in figure 32(b).
conductor 1 are presented flow in of conductor 1 magnetic field 2 due to the along conductor per ampere squared per meter field inten- from the magnetic The forces were derived in figure 32(c).
are presented by sities of figure 32(b) (6) F = ii H 1 2 2 0 - con- . Since the force on a value of 1.257x10 6 N/A permeability p has a where the and the direction of of the magnetic field to both the direction ductor is perpendicular a force in the negative magnetic field produces the x-component of the the current flow, as equal and were chosen currents in both conductors Inasmuch as the y-direction.
but opposite in direc- on conductor 1 are equal, same direction, the forces flowing in the other.
will move toward each is, the conductors 32(b). That to those given in figure tion, by multi- unit current are obtained the conductors for other than The forces per meter of For individual currents.
the product of the two in figure 32(c) by plying the forces given -8. 6 N/(A2)(m).
from figure 32(c) is about y-component of force example, the-average to a 50 kiloamperes (equivalent in each conductor is If it is assumed that the current on con- the conductors), the force stroke that is divided equally between 100-kA lightning negative y-direction is ductor 2 in the x 25x10 = 215x102 N/m 8.6x10-6 previously force per unit length calculated N/cm (123 lbf/in. ). The corresponding or 215 cur- was 500 N/cm. Thus, spreading the circuit for the same total current for the line the forces on (1. 58-in.) wide conductors reduces from a line circuit to 4-centimeter rent high. The forces per unit length of 57 percent, but the forces are still the conductors by rib- or force per unit area, since the conductors can be converted to pressure the ribbon 5375x102 N/m 4-centimeter (1. 58-in.) ribbon is have width. The pressure on the bons to the design wing loading for high pressure when compared (78 psi). This is extremely 1 as about 1. 7x103 N/m (0.25 psi).
which is given in reference the King Air airplane, currents flowing in the the magnetic force generated by lightning Thus, it appears that damage sustained.
aircraft wingtip was sufficient to cause the Flap Bearing Pitting The pitting of the left wing-flap bearings (figs. 23 and 24) by electrical arcing is typical of damage that can occur on mating surfaces which are not adequately electri- the flaps have to be free to move cally bonded. Bonding in this case is difficult because with respect to the wing. The bonding provided on the aircraft consisted of flexible cables, one for each of the four flap support and attachment points (two per flap). These and 25 centimeters (10 in. ) long cables were about 0. 5 centimeter (3/16 in. ) in diameter and were connected to the main wing structure near the flap support structure and to the sheet metal structure to which the flap bearings were attached, as shown in figures 33 and 20. The flap actuators (fig. 19) also provided, through the bearing surfaces, a con- nection between the flaps and the main wing structure.
Arcing at mating surfaces is easy to understand when the direct lightning-current flow path is through the mating surfaces. In this case, the lightning did not attach itself to the flap. The primary lightning path was through the main section of the left wing.
The complex geometry of the wing structure, the flap structure, and the associated at- tachments presents a very complex electrical circuit. A simplified representation of this electrical circuit is presented in figure 34. Because of the transient nature of lightning, especially the high rate of change of current in the high-current portion of the discharge, the metallic structure presents an impedance to the current flow. This im- pedance is primarily composed of resistance and inductance. The bearing mating sur- face in the flap support structures and flap actuator acts like a capacitor because of the presence of oil or oxide films. The effective resistance of the structural member of the circuit is much greater than the direct-current resistance because of the "skin effect" produced by the rapidly changing current. When the lightning current begins to flow through the wing structure, the primary lightning flow path, the impedance of the wing structure establishes a voltage drop between points A and B. This voltage drop can be fairly large, even though the impedance is small, because the lightning current is very large.
As the voltage drop develops between points A and B, current begins to flow be- tween the wing and the flap through the various attachments. Most of the current flow between the wing and the flap is through the bonding cables since the capacitance of the capacitor formed by the bearing mating surfaces is small. The current flow through the bonding cables because of impedance creates a voltage difference between the wing and the flap. Although the bonding cables are rather short and fairly large in diameter, their impedance to the high-frequency lightning current can be sufficient to create a large voltage difference between the wing and the flap.
If this voltage difference exceeds the breakdown potential of the dielectric material (oil or oxide) of the capacitors formed by the bearing surfaces, electrical arcing will begin. With the onset of arcing, the cur- rent flow through the bearings will increase sharply and probably exceed the current flow through the bonding cables since the bearing support structure is more massive (larger cross-sectional area) than the bonding cable. Once arcing is started it will con- tinue until the voltage difference between the wing and the flap reaches a critical mini- mum value, which may be considerably less than the voltage necessary to start the arc- ing. Thus, arcing across inadequately bonded mating surfaces which is initiated by the short-duration, high-current portion of the lightning discharge may continue throughout the continuing-current (long duration, relatively low current) portion of the lightning discharge. Most of the arcing damage is caused by the continuing current.
Although bonding cables were provided around each of the flap bearing assemblies, they were obviously inadequate for the lightning strike encountered. Electrical arc pitting damage of mating surfaces on movable components may not immediately create a serious problem but with time may lead to a serious problem unless the damage is de- tected and repaired. Detection of arcing on internal or hidden components such as these bearings may be a time-consuming task.
Arcing inside integral fuel tanks is a hazard to be avoided. Thus, not only is good electrical bonding required in the design and fab- rication of an aircraft, but periodic inspection and maintenance of bonds should also be made.
Loss of Electrical Power Tripping of circuit breakers in aircraft electrical generator circuits by lightning is not an uncommon occurrence. In an airlines lightning reporting project (ref. 8), trip- ping of the alternating-current generator circuit breaker was reported for 2 of 46 inci- dents. Tripping of circuit breakers can result from either direct passage of the light- ning current into a circuit (direct strike to a conductor) or by voltages induced in circuits by the lightning current flowing in the aircraft structure. The mechanism by which lightning trips a circuit breaker is not fully understood. Circuit breakers usually have a built-in time lag to prevent actuation by transients, and lightning is a rather short transient phenomenon.
Tripping by direct strikes to conductors is more easily compre- hended than tripping by induced effects since the currents and voltages in the lightning are orders of magnitude greater than those induced indirectly.
Measurements and dis- cussions on induced voltages and currents in aircraft electrical systems can be found in references 9 to 11. The magnitude of induced voltages and currents depends upon many factors: the electrical characteristics of the lightning strike itself, the length and loca- tion of the lightning path through the aircraft, the length and location of the electrical circuit with respect to the lightning path, and the physical and electrical characteristics of the circuit. In simulated lightning tests (ref. 9), induced open-circuit voltages of 96 volts and short-circuit currents of 23 amperes have been measured.
direct strike to any elec- Since inspection of the King Air aircraft did not reveal any probably due to induced voltages.
trical circuit, the tripping of the circuit breaker was is the circuit for the navigation The most likely circuit in which voltages were induced full length of the wing and nearly par- light on the left wingtip. This circuit extends the flow path when the lightning was attached to the trailing edge of the allels the lightning propeller on the right engine. Another possible circuit is the left wingtip and to the the lightning also struck the navigation light on the top of the vertical stabilizer since mounting for the light.
Damage to Navigation Light Mounting light on the top of the vertical The crack in the plastic mounting for the navigation in- stabilizer (fig. 5) is typical of damage produced when lightning strikes an electrical the plastic sulating or dielectric material. When the lightning attaches to the exterior of housing, the insulating plastic is in the current path and an electric field is generated across the material. When the electric field exceeds the dielectric strength or break- down potential of the dielectric material, the material is punctured or cracked. The 10 volts per centi- dielectric strength of most insulating materials ranges from 10 to are easily obtained when the meter. I otential gradients of this order of magnitude path. Dielectric material can be dielectric material is a part of a lightning discharge protected by use of electrical conductive paints or metallic strips bonded on the exterior surfaces and connected electrically to the metal aircraft.
CONCLUDING REMARKS The aircraft in this incident suffered most of the hazards associated with lightning except the ignition of fuel and a direct strike to the electrical system. The photographic documentation and analysis presented may serve to inform designers and operators of aircraft of the many types of lightning hazards and their magnitudes. Operators and maintenance personnel should also be aware of the possibility that some lightning dam- age, such as the pitted flap bearing that occurred in this incident, may not be imme- diately obvious.
Lewis Research Center, National Aeronautics and Space Administration, Cleveland, Ohio, May 22, 1974, 501-38.
REFERENCES ed.: Jane's All the World's Aircraft. McGraw-Hill Book Co., 1. Taylor, John W., Inc., 1969.
Book Co., Inc., 1969.
2. Uman, Martin A.: Lightning. McGraw-Hill Storage Vessels.
3. Olson, Richard E. : Safety and Design Considerations for Gas M-64-137, Martin Co., 1964.
4. Chalmers, John Alan: Atmospheric Electricity. Second ed., Pergamon Press, 1967.
Heating of Titanium Aircraft Skins.
5. Kofoid, Melvin J.: Lightning Discharge Lab. (AD-677352), 1968.
D1-82-0752, Boeing Scientific Research Second ed., D. Van Nostrand 6. Page, Leigh: Introduction to Theoretical Physics.
Co., Inc., 1935.
Kenneth J.; and Lawrenson, P. J.: Analysis and Computation of Electric 7. Binns, and Magnetic Field Problems. MacMillan Co., 1963.
8. Plumer, J. A.: Data from the Airlines Lightning Strike Reporting Project.
Electric Company, 1971.
GPR-72-008, General Measurements and Analysis of 9. Lloyd, K. J.; Plumer, J. A.; and Walko, L. C.: Lightning-Induced Voltages in Aircraft Electrical Circuits. NASA CR-1744, 1971.
in Aircraft 10. Plumer, J. A.: Analysis and Calculation of Lightning-Induced Voltages Electrical Circuits. NASA CR-2349, 1974.
11. Walko, L. C.: A Test Technique for Measuring Lightning-Induced Voltages on Air- craft Electrical Circuits. NASA CR-2348, 1974.
TABLE I. - TYPICAL SKIN THICKNESSES Component Thickness Figure Panel number mm in.
Upper wingtip 0.71 0.028 2(b) Lower wingtip .80 ,031 2(c), (d) 2 Wing-to-fuselage fairing .64 .025 2 Panel around battery hatch .94 .037 8 4, 5 Skin on aft end of nacelle .64 .025 15 6 .038 2(g) --- Wingtip fairing .97 Wing flap .56 .022 ------ --- TABLE BI. - MAGNETIC FIELD INTENSITIES AND FORCES ON SEGMENTS OF CONDUCTOR SYSTEM REPRESENTING CHORDWISE CROSS SECTION OF WINGTIP TRAILING EDGE (FIG. 30) c c d e Location,a Field, Force, Location, Field, Force, Location, Field, Force, Location, Field, Force, Location, Field, Force, d, H/I, F/I21, d, H/I, F/I 1, d, H/I, F/I21, d, H/I, F/I21, d, H/I, F/I21, 2 2 2 2 2 cm (A/m)/Ab N/(A )(m) cm (A/m)/Ab N/(A )(m) cm (A/m)/Ab N/(A )(m) cm (A/m)/Ab N/(A )(m) cm (A/m)/Ab N/(A )(m) Part A (segment A-O) Part B (segment O-B) Part C (segment O-C) Part D (segment C-D) Part E (segment B-E) 0 + . + - 0 + - + - 0 --m - 0 +0 + ' 0 -0 -m - 6 - 6 6 6 - 6 .01 254.72 320. 00l0 .02 448.01 281.42x10 .02 -504.93 -317.16x10 .10 1.42 0.89x10 .10 3.18 2. 00x10 .03 135.32 170.00 .04 238.80 150.00 .04 -218.90 -137.50 .20 -. 96 -. 60 .20 7.08 4.45 .05 66.06 82.99 .06 175.12 110.00 .06 -157.61 -99.00 .40 -5.49 -3.45 .40 9.15 5.75 .10 20.70 26.00 .08 127.36 80.00 .08 -118.49 -74.43 .60 -7.08 -4.45 .60 10.03 6.30 .20 11.94 15.00 .10 93.90 58.98 .10 -93.13 -58.50 .80 -7.76 -4.88 .80 10.20 6.41 .30 9.63 12.10 .20 45.77 28.75 .15 -57.31 -36.00 1.00 -7.83 -4.92 1.00 10.13 6.36 .40 8.04 10.10 .30 33.03 20.75 .20 -41.39 -26.00 2.00 -7.58 -4.76 2.00 8.96 5.63 .50 6.79 8.53 .40 25.07 15.75 .25 -33.43 -21.00 4.00 -6.81 -4.28 4.00 7.72 4.85 .60 5.81 7.30 .50 20.44 12.84 .30 -27.06 -17.00 6.00 -6.11 -3.84 6.00 6.80 4.27 .70 4.86 6.10 .60 16.72 10.50 .35 -22.29 -14.00 8.00 -5.53 -3.48 8.00 6.08 3.82 .80 4.38 5.50 .70 14.01 8.80 .40 -19.90 -12.50 10.00 -5.05 -3.17 10.00 5.51 3.46 .90 3.82 4.80 .80 10.98 6.90 .45 -14.33 -9.00 12.00 -4.64 -2.92 12.00 5.03 3.16 1.00 3.73 4.69 .90 4.66 2.92 .50 -10.35 -6.50 14.00 -4.30 -2.70 14.00 4.63 2.91 1.10 3.58 4.50 .92 0 0 .55 -3.98 -2.50 16.00 -4.07 -2.56 16.00 4.30 2.76 1.20 3.47 4.37 .94 -6.29 -. 39 .60 +13.53 +8.50 18.00 -3.72 -2.34 18.00 4.00 2.52 1.25 3.38 4.24 .96 -15.92 -10.00 .62 +35.80 +22.99 20.00 -3.57 -2.25 20.00 3.75 2.36 .98 -35.49 -22.29 .64 +- + 22.00 -3.30 -2.07 22.00 3.53 2.22 1.00 -m -m 24.00 -3.10 -1.95 24.00 3.33 2.09 aDistance from point A.
bOersteds (cgs electromagnetic system) = 79.6 A/m (SI system).
SCDistance from point O.
dDistance from point C.
eDistance from point B.
Figure 1. - Beechcraft King Air Model B90 aircraft.
Attachment point
(a) General view.
Normal bend lines Normal trailing- edge line (b) Top surface of wingtip.
Figure 2. - Damage at lightning attachment point on trailing edge of left wingtip.
IGAL pATY
S arp ridge Normal bend nes C-71-886 (d) Lower surface of wingtip looking aft.
Figure 2. - Continued.
I DtCmm~rry OF THE
ORIGINAL PAGE IS POOR C-71-626 (e) Trailing edge from below.
C-71-625 (f) Trailing edge from above.
Figure 2. - Continued.
Tip fairing Lower surface Upper surface (g) Wingtip disassembled - view of outside surfaces.
(h) Wingtip disassembled - view of inside surfaces.
Figure 2. - Concluded.
REPRODUCIBILITY OF TH QRIGINAL PAGE IS POOR -Damagedar C-71-637 Figure 3. - Tip of blade on right propeller, showing lightning erosion damage.
Point C figs.4(d) and (0- !See figs, (a) General view indicating location of major damage.
(a) General view indicating location of major damage.
Point A (see C-71-6888 (b)i Left side of forward section of fin.
Figure 4. - Lightning attachment points on ventral fin.
(c) Right side of forward section of fin.
C-71-89 (d) Aft section of fin.
Figure 4. - Continued.
(e) Aft end of fin.
Figure 4. - Concluded.
C-71-639 (a) General view of light in place.
C-71-890 (b) Closeup of light removed from aircraft.
Figure 5. - Crack in plastic mount for navigation light located on top front of vertical stabilizer.
Figure 6. - General view of damage to lower surface of wing and inboard side of engine nacelle.
C-71-633 and wing-to-fuselage fairing.
Figure 7. - Damage to battery compartment cover R2RODUCIBILITY OF THE POOR PAGE IS ORIGINAL MouFigure 8. - Area ofnting and exit h oles for coffee bar drain drain fitting (see figs. 9 and 107 C-71-899 Figure 8. - Area of paint discoloration on wing and fuselage.
coffee bar (see tig, o01 ,-71-1108 Figure 9. - Area of moderate paint discoloration around mounting and exit holes for coffee bar drain fitting on bottom of fuselage.
C-71-1121 location on bottom of fuselage. ) Figure 10. - Coffee bar drain fitting. (See fig. 9 for C-71-631 Figure 11. - Damage to lower inboard side of right engine nacelle and wing.
Figure 12.
Damage to lower inboard side of right engine nacelle.
C -71 -895 Figure 13. - Inboard view of damage at aft bottom of right engine nacelle.
(See also figs.
15 and 16. ) C-71-634 engine nacelle and lower wing surface.
to outboard bottom side of right Figure 14. Damage C-71-892 Figure 15. - Outboard view of damage at aft bottom of right engine nacelle. (See also figs. 13 and 16. ) C-71-635 Figure 16. - Forward view of damage to lower surface of right inboard flap and enqine nacelle fairing.
C-71-627 Figure 17. - Damage to top surface of right inboard flap.
OF THE
r#toDUCIBLIY
ORIGINAL PAGE IS POOR 39 Figure 18. - Rearward view of damage to lower surface of right inboard flap and aft end of engine nacelle. Flap control rod disconnected with flap hanging from flap track. (See fia. 19.)
bearing track - i- B ngs LPoint A Figure 19. - Damage to top surface and leading edge of right inboard flap Flap control rod dis- connected with flap bearing normally in forward track supporting flap in rear track.
Figure 20. - Damage to lower surfaces of right inboard and outboard flaps. Flap control rod dis- connected with flaps hanging from flap tracks.
Figure 21. - Damage to upper surfaces of right inboard and outboard wing flaps. Flap control rods disconnected with flaps hanging from flap tracks.
Points D and D 1 2 F see fig. 26) - Point E (see fig. 26) (see fig. 26)7\ Point C-73-2707 Figure 22. - Damage to lower-surface leading edge of right outboard flap.
Damage ,- Damage - Aft bearing Forward bearing Side face of bearing forward C-73-2667 Figure 23. - Damage to outboard pair of bearings on left outboard flap.
Damage -Damage - Mounting bolt Side 1 Side C-73-2668 Figure 24. - Damage to flap control rod bearing on left outboard flap.
10- . r-Curve C (0. 454 kg (1 Ibm) of TNT (ref. 3)) . CurveA E --- (discharge w*th release of iO: J/m (ref. 2)) B (discharge with current growth E- Curve rate of 0. 3 kAIpsec (ref. 2)) I , II .01 .01 .1 1 10 100 1000 atm Overpressure, Figure 25. - Predicted shock wave overpressure generated by typical lightning discharge.
PointE
IP
rop
(see fig. 22) 118. 1 cm spinner r (46. 5 in. ) ;Flap hinge location, H I Nacelle?
H7 D D Hi Point F / , H- se fig. 2- 5 (see fig. 22-' /2 , w Outboard Point B flap / -Point C I It- surfae 18) (see fig.
(see fig.-20) of wing 0 s Points D (see fig. 20)1 I 4 Inboard flap fig. 18) Point A (see ,A' Figure 26. - Locations of sharp indentations (points A to F) on leading edge of flaps and estimated positions (1 to 5) of propeller tip at times strokes occurred.
OF T"I1 REPRODUCIBILITY IS POOR PAGE ORIGINAL Shock waves Incident Reflected Time, Lower surface of wing- t ,-O (point of maximum imposed load) r3 , uuu'Y ju " " FLower surface of flap From propeller 2 Toground \ \-Cylindrical Lightning discharge path-s shock wave (a) Chordwise direction.
load) of maximum imposed -0 (point /-Wing -Wing-flap corner line Flap t-1 t=2 t=3 (b) Spanwise direction - perpendicular to lightning path at point P of part (a).
Figure 27. - Orientation of lightning path and shock wave with respect to lower side of aircraft wing -flap combination.
20. 3cm (8 in.
- Major damage Minor damage 40.6cm 35.6cm 43.2cm 39.4cm h-(16 in., --- t--( in. )-..- (17 in. )--- (15. 5 in.-) ---- Aft end
i S I i I I I I I i Ii
0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 Distance from forward end of fin, cm
I I i I I I I I I i I
l i I
0 10 20 30 40 50 60 70 80 90 100 110 120 130 Distance from forward end of fin, in.
Figure 28. - Location of damage points along ventral fin due to swept stroke.
(Length of line marking damage points indicates magnitude of damage.
) From ground Time = 0 Initial attachment points Time = 4.5 msec Time =9.0 msec Time = 13.5 msec Figure 29. - Probable lightning path with respect to aircraft. (Indicated times are for high-current-stroke portions of discharge. ) Time = 18.0 msec Time = 22.5 msec Time = 27.0 msec Time = 31.5 msec Figure 29. - Continued.
Time = 36.0 msec 'no Time = 40.5 msec 0 0 Time >45.0 msec Final attachment points Figure 29. - Concluded.
1- . 25 cm 6 \\ 300 A / Lightning path 2. 40 E Reference line _ (a) Before lightning strike. ~1. 60 A C (b) After lightning strike.
Figure 30. - Approximate geometry of chordwise cross section of wingtip trailing edge.
B- To -~at 0 A -0 \ C-D Wing cross section S -12x10 Force on- To at B o AO o OC 0 OC , U// o BE C D -, -4/ 10 cm 2 pp Trailing A edge 0 6 D 2 4 _ 1cm SLower surface - ,O_,,O Tom at C -5 12x10 N/(A2)(m) 16 ' To atA o -5 20xlO N/(A2(m) (Force is in newtons per due to interaction of magnetic fields and electrical currents.
of forces on trailing-edge surfaces of aircraft wing Figure 31. - Direction and magnitude ampere squared of lightning current per meter of surface length of circuit.)
Conductor 2 SConductor 2 / / (a) Schematic of conductors and magnetic flux lines.
- 6 7. 5x10 x-component x-component 8 -5.0 - 2.5 4- y-component 0 yy-omponent t -6 -2 5 Qc 0 ~ -5.0 y-component -2- -7.5 / -4 -10.0 -a a 2 -a 2 a 1 -12.5 -6 1 -4 -2 0 2 -4 -2 0 2 Width of current-carrying conductor, cm (b) Magnetic field intensity on (c) Force on conductor 1 per conductor 2 per ampere of ampere of current in con- current in conductor 1. ductor 2 for magnetic field intensity shown in part (b).
Figure 32. - Magnetic fields and forces created by two infinitely long, 4- centimeter-wide, current-carrying conductors separated by 1 centi- meter (assuming uniform current density in each conductor).
Bonding wires C-71-1113 on left wing flaps.
Figure 33. - Electrical bonding wires _wwivw_ Resistance K Inductance -1 F- Capacitance Wing structure - primary lightning current flow path B A Bonding Flap support Flap Flap support Bonding structure actuator cable cable structure Bearings Bearings Bearings structure Flap attachments.
of wing-flap structure and interface Figure 34. - Electrical circuit representation NASA-Langley, 1974 E- 7967