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(NASA CR OR TUX OR AD N U M B E R ) " W A Y S I N CROSSWINDS . / Since 1959 t h e Civil Aeronautics Board has attributed 30 accidents, o r incidents, t o aircraft tire hydroplaning. Most of these incidents also occurred under runway crosswind conditions.' Thirty such occurrences i n the thousands of landings and take-offs made i n t h a t same period seem infinitesimally small; however, there were probably numerous additional times when a i r c r a f t were par- t i a l l y out of control and i n potentially dangerous situations.
Since t h e p i l o t s were able t o regain a i r c r a f t control, and no damage was done, these latter cases were not reported.
Many a r t i c l e s have been w r i t t e n on the effect of crosswinds on a i r c r a f t take-off and landing performance f o r dry runways. I n t h i s article, w e w i l l discuss t h e combined effects of crosswind and slippery runways caused by accu- mulations of water, snow, ice, and slush on a i r c r a f t ground performance.
Runway slipperiness.- Aircraft designers depend upon pneumatic tires t o These func- perform three basic functions during a i r c r a f t ground operation.
are: tions (1) To support the weight of the aircraft while at rest or under high- speed rolling (landing o r take-off) (2) To develop high symmetrical or asymmetrical retardation forces (during wheel braking) f o r stopping o r f o r directional control purposes during landings, aborted take-offs, and taxiing I i i [J t l L ' C A B Bureau of Safety Pam-phlet BOSP 7-4-2, October 1963.
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- 2 - (3) To develop high cornering forces (nose wheel steering) and side forces (on main wheels1 t o overcome skid-producing external forces acting on the a i r c r a f t due t o crosswinds o r changes i n a i r c r a f t direction ( f o r example, high-speed turns onto taxiways) The operational problems connected with the first finction are t i r e f a i l u r e s from foreign-ob ect damage o r blowouts from protracted locked wheel resulting The advent of automatic skid control f o r a i r c r a f t skids during wheel braking.
wheal, braMllg ryrstme has greatly alleviated the l a t t e r problem. The ability Of pneumatic tire8 t o perform the 2nd an8 3rd function8 depends upon the slip- Some typical runway slipperiness values a r e shown i n periness of the runway.
For exanrple, t h i s figure indicates t h a t snow-covered pavements are, figure 1.
Ice- at the least, twice as slippery as dry bituminous o r concrete pavements.
covered pavements can be 4 t o 16 times as slippery as dry pavements, depending upon t h e temperature of the ice, with ice near the melting point (32' F) being In contrast t o dry, snow-covered, and ice-covered pavements the most slippery.
which appear t o have l i t t l e speed effect, water-covered and slush-covered pavements tend t o become more slippery (lower f r i c t i o n coefficients) as the A t high speeds on these deeply flooded pave- a i r c r a f t ground speed increases.
ments, f r i c t i o n coefficients can drop t o values as low as those found f o r icy pavements covered with a water film. Investigations at t h e NASA Langley Research Center have shown t h a t t h i s condition results from the phenmenon of t i r e hydroplaning. For t h i s puddled o r flooded runway condition, hydrodynamic f l u i d pressures develop between the t i r e footprint and pavement. These pres- sures grow l a r g e r as ground speed increases, and at a c r i t i c a l speed called t h e t o t a l hydroplaning speed, the hydrodynamic lift resulting fromthese pres- sures equals the weight riding on the t i r e . Any increase i n ground speed above I I .
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- 3 - I t h i s c r i t i c a l speed lifts the t i r e off the pavement, leaving it supported by the f l u i d alone. (See f i g . 2.) Any automobile driver knows about the dif- f i c u l t i e s he experiences i n stopping or wintaining directional control of h i s vehicle on wet icy pavements. A glance a t t h e comparative f r i c t i o n coeffi- cients f o r hydroplaning tires i n figure 1 shows t h a t similar d i f f i c u l t i e s will exist when a i r c r a f t or automobile t i r e s hydroplane on water o r slush-covered pavement s .
Tires hydroplane only when certain c r i t i c a l f l u i d depths are exceeded on These c r i t i c a l depths can range f r o m approximately 0.1 t o 0.4 inch, runways.
depending upon t h e character of tire-pavement surfaces. Smooth-tread t i r e s operating on the smoother pavement surfaces require the l e a s t f l u i d depth, whereas rib-tread t i r e s operating on open-textured and transverse-grooved pave- ment surfaces require the greatest f l u i d depths. When t h i s c r i t i c a l f l u i d
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depth i s exceeded f o r any combination of tire and pavement surface, the c r i t i c a l ground speed (hydroplaning speed) required for t o t a l hydroplaning t o occur w a s This found t o be almost entirely dependent upon t i r e i n f l a t i o n pressure.
r e s u l t led t o the derivation of the following simple relation f o r estimating t i r e hydroplaning speed: vp = 9 f 5 where Vp is t h e hydroplaning speed i n knots and p i s the tire i n f l a t i o n pressure i n lb/in.2.
Table I lists hydroplaning speeds and other chmscteristics including touchdown speeds of typical a i r c r a f t types currently being operated i n this country.
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- 4 - TABLE I.- B Y D R O P L A N I N G SPEEDS F O R SOME TYPICAL AIRCRAFT Main wheel MaXirmun T i re Touchdown t &e- of f hydroplaning pres sure Aircraft gross speed, speed, (main), weight, knots knots p s i l b Light-Twin Recip.
4,830
Twin Recip. Transport 32, ooo
Four Eng. Recip. 122,200 Twin Turbo Prop 35,100
113, ooo
Four Q. Turbo Prop Exec. Twin Jet 17,800 Four Eng. Jet 246,000 Service Jet Fighter 30,500 Note t h a t all of these a i r c r a f t a r e susceptible t o hydroplaning under the right conditions, since t h e hydroplaning speed is l e s s than t h e touchdown speed.
Crosswind.- Crosswinds a c t over the e n t i r e side area of a i r c r a f t and
- produce side forces which tend t o push a i r c r a f t off the downwind side of run-
ways. These forces are proportional t o t h e square of the crosswind velocity; thus, a 10-knot crosswind would quadruple the side force developed by a 5-knot the center of pressure of t h i s crosswind crosswind on an aircraft. Generally, a c t s aft of t h e center of rotation (main landing gear), so t h a t a yawing moment which tends t o make the aircraft weather cock, o r weather vane, into t h e wind is usually produced. Ekceptions t o t h i s behavior may be encountered when t h e I n this lateral ere= fa?xard cf the zeritzr of rotatiom exceeds that behind it.
case, t h e a i r c r a f t ( f o r example, t h e F-102) w i l l yaw downwind.
Combined effects of crosswind and slippery runway.- One of t h e worst con- t r o l situations occurs when there is a crosswind i n conjunction with water o r slush-covered runways, and t h e conditions t h a t are encountered produce t o t a l t i r e hydroplaning. FAA-NASAtests with a four-engine j e t transport i n slush - 5 - demonstrated a l o s s i n directional control and an approximately doubling o r I t r i p l i n g of t h e dry runway stopping distance (without use of reverse t h r u s t ) when hydroplaning occurred.
During t h e i n i t i a l low-speed portion of the take-off r o l l (a) Take-off: The a i r c r a f t heading can (see f i g . 3(a)), the tire-ground traction is good.
be maintained by nose-wheel steering, differential braking, d i f f e r e n t i a l for- and t h e resistance t o sideways motion can be produced w a r d thrust, and rudder; Thus, the a i r c r a f t maintains runway heading by tire-ground, reaction forces.
on runway center line.
As t h e a i r c r a f t approaches t i r e hydroplaning speed, tire-ground f r i c t i o n forces approach zero, and a point w i l l be reached where the side force from If runway heading the crosswind overcomes the counteracting tire-ground force.
i s maintained, t h e aircraft will skid toward t h e downwind side of t h e runway.
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This downwind skid can occur even on a completely dry runway, if t h e crosswind component i s large enough. To prevent d r i f t (see f i g . 3(b)), the p i l o t yaws t h e a i r c r a f t i n t o t h e wind so t h a t the side component of engine thrust opposes the crosswind component, because tire-ground forces a r e not available f o r t h i s puspose.
Aside from t h e basic a i r c r a f t control problem, t h e following points should be considered: Slush o r standing water on the runway increases take-off distance as a result of t h e added drag developed by t i r e s displacing t h e f l u i d cover from t h e wheel paths.
The F A A has recognized t h i s problem by i n s t i t u t i n g the
1- inch rule which prevents turbine-powered transport aircraft f r o m taking off
o r landing on runways covered with slush or standing water exceeding - 1 inch i n
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For slush o r water depths less than - inch, an additional take-off
depth.
as w e l l as an additional accelerate-stop distance, must be allowed distance, for.
A t temperatures near freezing, slush can accumulate around moving com- ponents of t h e aircraft during the take-off r o l l and freeze after the a i r c r a f t becomes airborne. This slush can hinder o r even prevent subsequent operation of landing gear, flaps, etc. It i s recommended t h a t such devices be cycled before final retraction t o minimize the effects o f frozen slush o r snow i n the storage wells.
Water or slush spray thrown up by nose wheels can be ingested into engine intakes, especially on some wing-root o r fuselage-mounted engines, and can \ The i k o v a t i o n of "chine" type nose-wheel cause loss of thrust o r flame-outs.
which depress nose-wheel spray patterns away from engine intakes, has tires,
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alleviated t h i s problem i n a few cases, and t h e i r use i s expected t o increase.
( b ) Landing: "he landing is usually more c r i t i c a l than t h e take-off f o r In the approach, the p i l o t can be flying solely by t h e following reason: instruments and h i s attention i s concentrated entirely within h i s cockpit.
Suddenly he breaks out a t "minims" and must make an instantaneous transition t o v i s u a l flight; and he must immediately thereafter touch down on a runway of unknown slipperiness. This is one of t h e worst situations possible, but it can frequently be encountered with slush- o r water-covered mimays when v i s i b i l i t y ~ is poor. This is i n contrast t o the take-off where t h e a i r c r a f t starts from rest w i t h good tire-ground traction available t o t h e pilot.
Consider the landing shown i n figure 3(c), where t h e a i r c r a f t touches down on runway heading and center line. This touchdown i s accomplished i n a crosswind by e i t h e r a "wing down" or "crab" correction. Since t h e touchdown I * !
I - 7 - speed ib greater than the t o t a l hydroplaning speed (see table I), the t i r e - With no p i l o t corrections, t h e a i r c r a f t w i l l probably ground traction i s n i l .
I n weathercock i n t o the wind and drift toward t h e downwind side of the runway.
t h i s situation, application of reverse thrust increases the d r i f t downwind, since t h e side component of reverse thrust acts i n t h e same direction as the If t h i s condition is allowed t o
wind force. (See vector diagram, fig. 3( c) . )
continue and t h e crosswind component is large enough, the a i r c r a f t w i l l d r i f t o f f the side of t h e runway with perhaps thousands of f e e t of usable runway (1) he can remaining. Only three alternatives a r e available t o the pilot: ( 2 ) he can apply enough continue as before and run off t h e side of t h e runway, the a i r c r a f t i n the center of t h e runway, or (3) he forward t h r u s t t o maintain and apply reverse thrust. The first is obviously can yaw t h e a i r c r a f t downwind The second increases the stopping distance appreciably, and t h e unacceptable .
third would seem t o be contrary t o any maneuver a p i l o t has attempted before.
Fortunately, i n most cases, the a i r c r a f t can be slowed below t h e hydroplaning speed before it is pushed off the side of t h e runway.. Then the tires can begin t o take part of t h e load, maintain a i r c r a f t control, and help t o slow .
that aircraft braking tests t h e a i r c r a f t . It should be remembered, however, have demonstrated t h a t stopping distances (without reverse t h r u s t ) can be increased 60 percent on wet runways without hydroplaning occurring.
The purpose of t h i s a r t i c l e has been t o point out some of the problems and principles involved during take-off and landing operations i n crosswinds on slippery runways and not t o advise experienced p i l o t s on how t o control t h e i r own f a m i l i a r a i r c r a f t . The strongest and most important f a c t the p i l o t If the destination has a known crosswind should remember is t o plan ahead.
condition, and t h e conditions t o be encountered can possibly cause t i r e I ' 1
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I n a marginal h>dropldng, he should think twice before landing there.
I situation without p r i o r planning there may be insufficient t i m e f o r the p i l o t If t h e t o analyze what is happening and take the proper corrective action.
landing must be accomplished i n these conditions, the p i l o t should make sure .
t h a t t h e approach is not high o r fast and t h a t t h e actions of t h e a i r c r a f t a f t e r touchdown are anticipated w i t h subsequent p i l o t reactions planned.
Further information on t i r e m p l a n i n g 2 J 3 can be obtained on request t o t h e Langley Station, Hampton, Va.
NASA I;angley Research Center, ?Eazards of T i r e Hydroplaning t o Aircraft Operation. Langley f i l m serial No. L775 (15-minute, 16 mm color film with sound narrative).
TN D-2056. "Phenomena of Pneumatic !Mre Hydroplaning," by Walter B. Home and Robert C. Dreher.
I Figure Tiles f o r Cobb-Horne Article Figure 1.- Some t y p i c a l runway s l i p p e r i n e s s v a l u e s .
Figure 2 . - Detachment o f aircraft t i r e f o o t p r i n t from runway surface due to tire hydroplaning.
Figure 3 . - Crosswind t a k e - o f f and landings on flooded runways.
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