CJ 0 

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US. Department 

ofTrtJnsportation 

Federal Aviation 

Administration 

FAA-P-8740-39 

AF0-800-0582 

Balloon Safety Tips 

false lift 

.shear & 

rotors 

..... :. . . .. . ····· .. 

/) at:eldent· prevention program. 
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FOREWORD 

The purpose of this series of publications is to provide the general 

aviation flying public with information that may improve aircraft operational 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 

Balloons respond to various air currents to a greater degree 

than other aircraft. Except for momentary delays wherein inertia of the balloon mass resists the energy of a new air current, 

the flight path of a balloon in equilibrium exactly mirrors the 

direction and velocity of the air current in which it is operating. 

This simple fact of balloon operation holds true for horizontal 

(the kind we like to fly in), vertical, and rotary air currents (when 

Mother Nature flies). This article is about flying with Mother 

Nature and includes a few tips on what to do when Mother 

Nature's flying becomes -too exciting. 

FALSE LIFT 

False lift Is an aerodynamic phenomena which occurs during the initial acceleration of the balloon. A balloon standing 

in the wind acts as an obstacle to normal air flow causing the 

wind speed to increase on the surface of the balloon. Changing the wind speed causes a pressure conversion to occur 

(static pressure decreases and velocity pressure increases) 

which results in false lift. During launch false lift couples with 

the balloon's normal lifting forces to cause the balloon to take 

off. The problem with having false lift is that it quits when the 

balloon reaches wind speed. Ready or not, false lift dissipates 

rapidly after takeoff causing the pilot to apply heat constantly 

in order to gain real lift before acceleration is complete. In a 

practical sense, ·false lift does not pose any operating problems until the wind speed exceeds approximately 10 mph. 

False lift is unavoidable in a fast wind making takeoffs a 

challenge for the balloon pilot. Simultaneously, the pilot must 

watch burner operation, fabric, control the crew, assess the 

balloon's physical readiness for flight and judge the lift. The 

confusion of burner noise, paSsenger and crew demands, and 

the. physical jolting caused by the gondola dragging across 

the gro!Jnd can quickly overload an unprepared pilOt. The pilot 

should carefully prepare, inspect and rig the balloon prior to 

inflation to minimize the tasks required during inflation and 

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, 

that the envelope lift is inadequate, and continue heating until 

balloon acceleration is complete and a positive climb rate is 

established. During takeoff and acceleration It is not possible 

to differentiate between real lift and false lift. The best policy 

is to heat past equilibrluril temperature and then vent as 

necessary to maintain a comfortable rate of climb. False lift is 

easier to overcome when burner output is high, therefore fuel 

pressure Is the best information available to judge the 

balloon's ability to overcome false lift . Small burners on big 

envelopes are obviously a poor choice of equipment for'fast 

wind operation. Obviously the takeoff field is important in fast 

wind operations. Obstructions downwind require additional 

clearance depending on wind speed due to the reduced initial 

climb rate of the balloon after losing the false lift. 

BURNER MUST OPERATE CONTINUOUSLY 

FLIGHT PATH 

. ,...,....-- ~ . ---. 

e BALLOON IS DISTORTED 

AND AT LOW VOLUME . 

e FALSE LIFT IS MAXIMUM 

• BALLOON HAS HOT 

BUBBLE IN CROWN 

e BALLOON IS INHALING 

COLD AIR 

e FALSE FIFT IS 

DIMIMISHING 

. LAUNCHING WITH FALSE LIFT 

e BALLOON IS FULLY 

INFLATED 

e EQUILIBRIUM HAS 

OCCURED 

• FALSE LIFT IS ZERO 
WIND SHEAR 

Wind shear is a phenomena that resembles. false lift in many 

ways. The principle difference is that the·balloon is airborne 

and may be accelerating or decelerating. In a practical sense, 

there is no difference between acceleration oi' deceleration 

since the balloon is symmetrical and responds uniformly to air 

floW in any direction. The real difference from a pilot's viewpoint is the location of the occurrence. False lift occurs from a 

known position selected by the pilot. Wind shear locations are 

usually unknown to the pilot which can increase the hazard. 1 

recall a windy landing which was unfortunately expedited by a 

strong decelerating wind shear. At that time my balloon was 

not fitted With a skirt. The wind had increased unexpectedly to 

a force of about 25 to 30 mph. On a previous approach 1 accidently struck a bank and tree causing the deflation valve to 

open over3 panels. Frankly,! was scared, nervous arld excited 

and probably not thinking too clearly.! was flying down a river 

valley and the landing sites were typically small and surrounded 

by trees. I decided to descend to tree top eiev~tion so that 1 

would be ready when a suitable landing site appeared. 1 wasn't 

experienced enough to realize that strong shears are typically 

present just above tree lines. The shear destroyed the 

balloon's lift and I descended through the trees and into a 

small clearing. We hit hard but there was no damage to occupants or balloon. I learned 3 big lessons that day. First, that 

velcro tops are not rated for the life of the envelope. Second, 

wind shears can deflate the ·balloon sufficiently to lose control. Third, that skirts are a necessity because they allow you 

to heat during a shear. 

Let's examine what is happening dynamically and thermodynamically to the envelope in a wind shear. The initial 

reaction of the envelope to a wind shear Is to distort and gain 

false lift. False lift occurs instantaneously and is momentarily 

beneficial. Distortion occurs a little slower and is proportional 

to the gross weight of the balloon, envelope volume, and the 

strength of the shear. There Is no evidence to suggest that one 

envelope design is more resistant to wind shear forces· than 

another. The effect of distortion is to reduce the envelope 

volume causing exhalation through the throat which Is follow-

-ed--by cold a_i{ inhalatlon.-The-distortion--or breathing of-the 

envelope may, and typically does, cycle several times. Each 

distortion causes the average envelope temperature to 

decrease because of the inhalation of cold air. The heat 

transfer rate of the balloon envelope is also gre~tly increased. 

The wind shear wipes away the dead air layer which is laying 

on the surface of the envelope. This dead air layer, or bound?ry 

lay~r. represents significant insulation to the envelope. When 

the boundary layer Is wiped away by a gust the envelope heat 

loss nearly doubles. To view this in perspective the ballool) will 

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. act like you doubled the load insofar as the operation and feeling of the balloon is concerned. The net result is a significant 

decrease in lift. 

I mentioned that the reaction of the envelope to shea~:] 

also dependent upon, the internal pressure, the volume of\ . 

envelope and its gross weight. The internal pressure of' ... _ 

balloon envelope of a given volume is directly proportional to 

its lift. The limiting pressure is a function of e·nvelope height. 

Given an identical load, a small envelope will have a higher internal pressure than a larger envelope. A large envelope, loaded 

to its rated gross weight, will have a higher internal pressure 

than a small envelope loaded to its rated grosS weight 

because of its greater height. What does all .thi.s mean to a 

pilot? Simply that a lightly loaded balloon will distort more 

than a heavily loaded balloon when it encounters a shear or 

gust because the internal pressure offers the only resistance 

to. those wind forces. 

There is a serious hazard to flight associated with wind 

shear which could be easily overlooked. A heavy shear gust 

can displace the vertical axis of the balloon. This displacement from vertical can be extreme, perhaps as much as 30 to 

40 degrees: in severe turbulence. If the burner is operating at 

the moment the envelope displacf;!S, severe structural damage 

to the throat can occur. Structural damage is not loss of fabric; 

fabric Is not significant In the throat area because there Is no 

Internal pressure. Structural damage is loss of load tapes or 

ropes. The gravity of this problem is not easily visualized. The 

scenario involves the Joss of lift and thus pressure in the 

envelope and the loss of two or mo(e load tapes or ropes. 

When the load elements are burned free the fabric of that portion of the envelope is no longer supported. If the throat 

pressure Is low or negative the fabric repositions Itself Into the 

balloon throat thus blocking the burner from the envelope. As 

the balloon begins to descend the velocity pressure created bY. 

the downward flight of the balloon presses on this unsl'/) 

po~ed fabric causing it to close the throat further. At sol 

pomt, probably over 1500 feet/min., the force on fabr1 ... 

becomes so great that the fabric tears. This enormous internal 

flap of fabric effectively splits the balloon from the inside". This 

_ i~ -~-~n_<?wn __ fa_i!u~ _which _has _contributed to at least _c:me fatal 

balloon accident. AdViCe? DOn't use Ccir1S018 nloLinted -bi..irner 

controls In turbulent air or high winds because they have too 

much afterburn. In turbulence and shears be ready to shut off 

the burner Immediately. When at altitude It is sei'dom urgent to 

maintain eqUilibrium thus there is no particular reason to 

operat_e the burner when heavy envelope distortion is present. 

Be aware that high burner pressures can cause the flame to 

reach to distorted fabric on the side of envelope and result in 

major damage. 
~ ) 

FLAME PATTERN 2: 

SHEAR 

SHEAR 

e HAZARD OF HIGH ALTITUDE SHEER e 

e WIND SHEAR HAZARD AT LANDING e 

3 

INTERIOR-STILL AIR 

"R/' = 1.60 

ENVELOPE FABRIC 

"Rz" = 0 

EXTERIOR-STILL AIR 

"Rt = 1.60 or0.25@ 7'12 MPH 

R1 + Rz + R3 (STILL AIR) = 3.20 BTU/Hr, Ft', of 

R, + Rz + R3 (7'!12 MPH WIND} = 1.85 ·• 

WIND 

0 

7112 

HEAT LOSS, BTU/HR 

4,250,000 

7,351,000 

e TERMINAL EFFECT OF WIND SHEER e 

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ROTOR 

Rotor Winds are associated with mountainous terrain and 

strong winds aloft. Mountain flights should anticipate rotor 

winds, and in particular, when wind aloft forecasts exceed 15 

· knots at mountain peak altitude. Prediction of rotor winds is 

not a well developed science. Typically a rotor wind will have 

forces which exceed the performance of the balloon. 

Downward airflow will therefore carry the balloon to the 

ground even with the envelope at red line temperature. Further, 

it is difficult to escape from a rotor. The only apparent escape 

is climbing out through the top. The vertical segment o

rotor enhance·s balloon performance. A good rate of clim

the upside of the rotor will force the balloon out the top by 

tia. OnCe the rotor area is defined, the area can b.e avoide

clearing it at high altitude. An approximate safe altitude is 

4 thousand feet above highest terrain. Surprisingly, 

winds can be completely free of turbulence; however, th

not always true. Fortunately balloon flight over mountain

not the typical flight. Those who choose to fly in mount

should realize that there are serious hazards which 

supersede the sc6nery;.rotors are just one of those haza

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Jf/ ESCAPE 

.. -----· • __ . __ _. ~- ---SHEAR ZONE /'-::- --:=-------~ .>- r :--. -~· - TURBULENTZONE HEATTO ~ . • / (l'L CLIMB ~\~~~~~~') . / / \ •• §OUILIARIUM TEMP ' _...--

'

/ / { • ROTORZONE ) ") 

/ ~' '\MOUNTAINRE~ ./ 

MECHANICAL // • J".' \. .....__ ·-. _., }/ LIFT # -'/ ""-....._ ~ 

BALLOON FLIGHT IN ROTOR WIND 

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