Document
FAA-P-8740-39
AF0-800-0582
Balloon Safety Tips
CJ 0
US. Department ofTrtJnsportation
false lift
Federal Aviation Administration
.shear &
rotors
..... :. . . .. . ····· ..
/) at:eldent· prevention program.
FOREWORD
The purpose of this series of publications is to provide the general
aviation flying public with information that may improve aircraft opera-
tional safety. Because of the increasing activity in balloon operations and
the increase in accidents involving balloons, safety tips for balloon pilots
will be included in the Accident Prevention Program pamphlet series.
This article, contributed by the Albuquerque Aerostat Ascention
Association, was written by Mr. Robert L. Ruppenthal, former Chief Design
Engineer for Research and Development for Thunder Hot-Air Balloons arid
author of several papers on safety in balloon operations.
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BALLOON SAFETY TIPS
False lift is unavoidable in a fast wind making takeoffs a Balloons respond to various air currents to a greater degree challenge for the balloon pilot. Simultaneously, the pilot must than other aircraft. Except for momentary delays wherein iner- watch burner operation, fabric, control the crew, assess the tia of the balloon mass resists the energy of a new air current, balloon's physical readiness for flight and judge the lift. The the flight path of a balloon in equilibrium exactly mirrors the direction and velocity of the air current in which it is operating. confusion of burner noise, paSsenger and crew demands, and This simple fact of balloon operation holds true for horizontal the. physical jolting caused by the gondola dragging across (the kind we like to fly in), vertical, and rotary air currents (when the gro!Jnd can quickly overload an unprepared pilOt. The pilot should carefully prepare, inspect and rig the balloon prior to Mother Nature flies). This article is about flying with Mother inflation to minimize the tasks required during inflation and Nature and includes a few tips on what to do when Mother Nature's flying becomes -too exciting. prior to takeoff. This procedure is necessary to insure that the balloon is safe in the event takeoff occurs unexpectedly. The pilot must always assume false lift is present during takeoff, FALSE LIFT that the envelope lift is inadequate, and continue heating until False lift Is an aerodynamic phenomena which occurs dur- balloon acceleration is complete and a positive climb rate is ing the initial acceleration of the balloon. A balloon standing established. During takeoff and acceleration It is not possible in the wind acts as an obstacle to normal air flow causing the to differentiate between real lift and false lift. The best policy wind speed to increase on the surface of the balloon. Chang- is to heat past equilibrluril temperature and then vent as ing the wind speed causes a pressure conversion to occur necessary to maintain a comfortable rate of climb. False lift is (static pressure decreases and velocity pressure increases) easier to overcome when burner output is high, therefore fuel which results in false lift. During launch false lift couples with pressure Is the best information available to judge the the balloon's normal lifting forces to cause the balloon to take balloon's ability to overcome false lift . Small burners on big off. The problem with having false lift is that it quits when the envelopes are obviously a poor choice of equipment for'fast balloon reaches wind speed. Ready or not, false lift dissipates wind operation. Obviously the takeoff field is important in fast rapidly after takeoff causing the pilot to apply heat constantly wind operations. Obstructions downwind require additional in order to gain real lift before acceleration is complete. In a clearance depending on wind speed due to the reduced initial practical sense, ·false lift does not pose any operating prob- climb rate of the balloon after losing the false lift.
lems until the wind speed exceeds approximately 10 mph.
BURNER MUST OPERATE CONTINUOUSLY FLIGHT PATH . ,...,....-- ~ . ---.
e BALLOON IS DISTORTED • BALLOON HAS HOT e BALLOON IS FULLY AND AT LOW VOLUME .
BUBBLE IN CROWN INFLATED e FALSE LIFT IS MAXIMUM e BALLOON IS INHALING e EQUILIBRIUM HAS COLD AIR OCCURED e FALSE FIFT IS • FALSE LIFT IS ZERO DIMIMISHING . LAUNCHING WITH FALSE LIFT WIND SHEAR . act like you doubled the load insofar as the operation and feel- ing of the balloon is concerned. The net result is a significant Wind shear is a phenomena that resembles. false lift in many decrease in lift.
ways. The principle difference is that the·balloon is airborne I mentioned that the reaction of the envelope to shea~:] and may be accelerating or decelerating. In a practical sense, also dependent upon, the internal pressure, the volume of\ .
there is no difference between acceleration oi' deceleration envelope and its gross weight. The internal pressure of' ... _ since the balloon is symmetrical and responds uniformly to air balloon envelope of a given volume is directly proportional to floW in any direction. The real difference from a pilot's view- its lift. The limiting pressure is a function of e·nvelope height.
point is the location of the occurrence. False lift occurs from a Given an identical load, a small envelope will have a higher in- known position selected by the pilot. Wind shear locations are ternal pressure than a larger envelope. A large envelope, loaded usually unknown to the pilot which can increase the hazard. 1 to its rated gross weight, will have a higher internal pressure recall a windy landing which was unfortunately expedited by a than a small envelope loaded to its rated grosS weight strong decelerating wind shear. At that time my balloon was because of its greater height. What does all .thi.s mean to a not fitted With a skirt. The wind had increased unexpectedly to pilot? Simply that a lightly loaded balloon will distort more a force of about 25 to 30 mph. On a previous approach 1 ac- than a heavily loaded balloon when it encounters a shear or cidently struck a bank and tree causing the deflation valve to gust because the internal pressure offers the only resistance open over3 panels. Frankly,! was scared, nervous arld excited to. those wind forces.
and probably not thinking too clearly.! was flying down a river There is a serious hazard to flight associated with wind valley and the landing sites were typically small and surrounded shear which could be easily overlooked. A heavy shear gust by trees. I decided to descend to tree top eiev~tion so that 1 can displace the vertical axis of the balloon. This displace- would be ready when a suitable landing site appeared. 1 wasn't ment from vertical can be extreme, perhaps as much as 30 to experienced enough to realize that strong shears are typically 40 degrees: in severe turbulence. If the burner is operating at present just above tree lines. The shear destroyed the the moment the envelope displacf;!S, severe structural damage balloon's lift and I descended through the trees and into a to the throat can occur. Structural damage is not loss of fabric; small clearing. We hit hard but there was no damage to oc- fabric Is not significant In the throat area because there Is no cupants or balloon. I learned 3 big lessons that day. First, that Internal pressure. Structural damage is loss of load tapes or velcro tops are not rated for the life of the envelope. Second, ropes. The gravity of this problem is not easily visualized. The wind shears can deflate the ·balloon sufficiently to lose con- scenario involves the Joss of lift and thus pressure in the trol. Third, that skirts are a necessity because they allow you envelope and the loss of two or mo(e load tapes or ropes.
to heat during a shear.
When the load elements are burned free the fabric of that por- Let's examine what is happening dynamically and ther- tion of the envelope is no longer supported. If the throat modynamically to the envelope in a wind shear. The initial pressure Is low or negative the fabric repositions Itself Into the reaction of the envelope to a wind shear Is to distort and gain balloon throat thus blocking the burner from the envelope. As false lift. False lift occurs instantaneously and is momentarily the balloon begins to descend the velocity pressure created bY.
beneficial. Distortion occurs a little slower and is proportional the downward flight of the balloon presses on this unsl'/) to the gross weight of the balloon, envelope volume, and the po~ed fabric causing it to close the throat further. At sol strength of the shear. There Is no evidence to suggest that one pomt, probably over 1500 feet/min., the force on fabr1 ...
envelope design is more resistant to wind shear forces· than becomes so great that the fabric tears. This enormous internal another. The effect of distortion is to reduce the envelope flap of fabric effectively splits the balloon from the inside". This volume causing exhalation through the throat which Is follow- _ i~ -~-~n_<?wn __ fa_i!u~ _which _has _contributed to at least _c:me fatal -ed--by cold a_i{ inhalatlon.-The-distortion--or breathing of-the balloon accident. AdViCe? DOn't use Ccir1S018 nloLinted -bi..irner envelope may, and typically does, cycle several times. Each controls In turbulent air or high winds because they have too distortion causes the average envelope temperature to much afterburn. In turbulence and shears be ready to shut off decrease because of the inhalation of cold air. The heat the burner Immediately. When at altitude It is sei'dom urgent to transfer rate of the balloon envelope is also gre~tly increased.
maintain eqUilibrium thus there is no particular reason to The wind shear wipes away the dead air layer which is laying operat_e the burner when heavy envelope distortion is present.
on the surface of the envelope. This dead air layer, or bound?ry Be aware that high burner pressures can cause the flame to lay~r. represents significant insulation to the envelope. When reach to distorted fabric on the side of envelope and result in the boundary layer Is wiped away by a gust the envelope heat major damage.
loss nearly doubles. To view this in perspective the ballool) will FLAME PATTERN 2: ~
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SHEAR SHEAR
e HAZARD OF HIGH ALTITUDE SHEER e
INTERIOR-STILL AIR "R/' = 1.60 ENVELOPE FABRIC "Rz" = 0 EXTERIOR-STILL AIR "Rt = 1.60 or0.25@ 7'12 MPH R + Rz + R (STILL AIR) = 3.20 BTU/Hr, Ft', of 1 3 R, + Rz + R (7'!12 MPH WIND} = 1.85 ·• WIND HEAT LOSS, BTU/HR
e WIND SHEAR HAZARD AT LANDING e
0 4,250,000 7112 7,351,000
e TERMINAL EFFECT OF WIND SHEER e
is climbing out through the top. The vertical segment of the ROTOR rotor enhance·s balloon performance. A good rate of climb on Rotor Winds are associated with mountainous terrain and the upside of the rotor will force the balloon out the top by iner- strong winds aloft. Mountain flights should anticipate rotor tia. OnCe the rotor area is defined, the area can b.e avoided by winds, and in particular, when wind aloft forecasts exceed 15 clearing it at high altitude. An approximate safe altitude is 3 to · knots at mountain peak altitude. Prediction of rotor winds is 4 thousand feet above highest terrain. Surprisingly, rotor not a well developed science. Typically a rotor wind will have winds can be completely free of turbulence; however, this is forces which exceed the performance of the balloon. not always true. Fortunately balloon flight over mountains is Downward airflow will therefore carry the balloon to the not the typical flight. Those who choose to fly in mountains ground even with the envelope at red line temperature. Further, should realize that there are serious hazards which can it is difficult to escape from a rotor. The only apparent escape supersede the sc6nery;.rotors are just one of those hazards.
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Jf/ ESCAPE
.. -----· • __ . __ _. ~- ---SHEAR ZONE
/'-::- --:=-------~ .>- r :--.
-~· - TURBULENTZONE HEATTO ~
. • / (l'L CLIMB ~\~~~~~~')
. / / \ •• §OUILIARIUM TEMP ' _...-- / / { • ROTORZONE ) ")
/ ~' '\MOUNTAINRE~ ./
MECHANICAL // • J".' \. .....__ ·- . _., }/
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LIFT # -'/
""-....._ ~ BALLOON FLIGHT IN ROTOR WIND