Lift at Altitude
As a balloon ascends, it is generally true that temperature, atmospheric pressure, and gross lift all decrease. Gross lift
decreases as pressure decreases, but increases as temperature decreases. Thus, as a gas balloon rises in the atmosphere,
the decreasing pressure and temperature oppose each other. The decreasing temperature increases lift while the decreasing
pressure decreases lift. Atmospheric pressure changes are more significant than temperature changes. Thus, net lift
decreases as altitude increases in a standard atmosphere.
When calculating the effect of changing pressure and temperature, it is necessary to multiply the sea level lift by the ratio
of pressures and temperatures. For a nonstandard ambient pressure, multiply the lift at the ISA level either by 29.92 "Hg
or 1,013.25 millibars (mb).
Temperatures at altitude must also be calculated and compensated for. The factor for temperature is a ratio of absolute
temperatures expressed in either degrees Kelvin or Rankine. To get temperature in degrees Rankine, simply add 459 to the
normal Fahrenheit temperature. For a new temperature, multiply the lift calculated at the ISA by the factor: (59 °F+ 459)/
(new temperature + 459). When using temperature in degrees Centigrade (°C), add 273 to convert to absolute temperature
(i.e., Kelvin). This is: (15 °C + 273)/(new temperature + 273). Various lift factors at differing altitudes, comparing helium
and hydrogen, are illustrated in Appendix F.
Pressure Ceiling
The pressure ceiling is the altitude at which the lifting gas inside the envelope would expand to just completely fill the
envelope, assuming the balloon rose to that altitude. Rising above the pressure ceiling causes lifting gas to be expelled
from the appendix and establishes a new, higher pressure ceiling. Exceeding the current pressure ceiling causes loss of
lifting gas, reduces gross lift, and typically causes the balloon to eventually begin to descend. Ballast must be expended
to maintain the new higher altitude. However, maneuvers that result in altitude changes below the pressure ceiling, do not
result in loss of lifting gas or gross lift. Very little ballast is required to ascend while below the pressure ceiling. For these
reasons, the gas pilot should always be aware of what the approximate current pressure ceiling is and should consider the
consequences of penetrating that ceiling.
The following approximations generally apply to a balloon below 18,000 mean seal level (MSL).
1. For a 1,000 cubic meter balloon at its pressure ceiling, an ambient pressure decrease of 1 "Hg causes a decrease in
gross lift of about 80 pounds.
2. For a 1,000 cubic meter balloon at its pressure ceiling, an ambient and gas temperature decrease of 3.3 ºF causes a
lift increase of about 16 pounds.
3. For a 1,000 cubic meter balloon at its pressure ceiling, a discharge of about 64 pounds of ballast results in
approximately a 1,000 foot increase in altitude.
Additional Factors That Affect Lift
1. A balloon flying below its pressure ceiling (i.e., a flaccid balloon) responds differently from one flying at its pressure
ceiling.
2. When the lifting gas inside the balloon is warmer (i.e., super heating) than the ambient air, additional lift is generated.
The reverse happens when the lifting gas is colder than the ambient air.
3. Nonstandard atmospheric conditions, such as inversions, affect a balloon’s stability.
4. The atmospheric humidity has a small effect on lift with more humidity resulting in slightly less lift.
5. The purity of the lifting gas directly affects lift. Most commercially produced gas is assumed to be greater than 99
percent pure, but purity can be reduced as a result of improper filling technique.
For further discussion of gas balloon calculations, the book A Short Course on the Theory and Operation of the Free
Balloon, by C. H. Roth, Goodyear Tire and Rubber Company, is recommended reading. This manual provides a good
overview of the physics and operation of gas ballooning as of 1917. It is long out of print, but photocopies are readily
available.
Weather Considerations for Gas Ballooning
When studying weather for gas ballooning, one must look for trends both further into the future and higher above the
ground. The best weather for any flight is determined by the flight’s objectives. A flight to set a duration record (maximum
time aloft) benefits from light winds and clear skies while a distance competition requires high winds aloft with lighter
winds in the landing zone. A competitor in a long competition is likely to encounter several different weather patterns
during flight simply due to the length of the flight. Examples of these include precipitation, snow, icing, thunderstorms,
lightning, high winds, mountain winds, unstable air, or convective currents.
This discussion again focuses on the most common type of competitive flight, a Gordon Bennett type, with the objective
to maximize great circle distance covered. Since winning distances can be well over a thousand miles at altitudes of up to
18,000 feet MSL with times aloft possibly exceeding seventy hours, a much larger area of the weather map must be studied
than for a typical hot air flight.
Meteorological Differences From Hot Air Ballooning
In contrast to hot air flights, landing conditions are most likely to be different from those at launch and several weather
patterns may be encountered during the flight. Freezing levels and the moisture content of the air should be checked to
predict the possibility of icing. Any icing that occurs has multiple negative impacts on the flight. It adds weight to the
balloon and can interfere with the functioning of the valve by preventing it from either opening properly or from sealing
tightly after activation. The weight of the ice should initiate a natural descent to a lower altitude and warmer temperatures.
If this does not happen naturally, the pilot may initiate a descent by valving. Be aware that the melting ice at lower altitudes
lightens the system and additional valving is necessary to prevent a second ascend back above the freezing level. Falling
ice shards have also been blamed for equipment damage on occasional flights. A second strategy against icing is to seek
drier air above the saturated air but the higher altitude is unlikely to provide the warmer temperatures needed to melt the
accumulated ice.
Encountering thunderstorm activity is much more common in gas versus hot air flights. This is especially dangerous when
flying an explosive gas such as hydrogen. Isolated thunderstorms can develop due to solar heating of moist air in the
afternoon over the great plains of the United States with very little warning. Prediction and avoidance are the best tools
against thunderstorms. Real time contact between the pilots and a ground-based meteorologist with access to forecasting
tools to predict the formation of thunderstorms can help to avoid these storms. During the different stages of a thunderstorm,
rapid accelerations due to inflows or outflows, as well as rapid ascents or descents, are very likely. The presence of any of
these effects, in conjunction with any thunder or lightning are indications that an immediate landing is prudent.
Meteorological Flight Planning
Flight planning for a gas balloon flight starts several days prior to the planned flight. The gas balloon pilot examines
numerous meteorological tools, looking at frontal movement to get the big picture, wind speed and direction forecasts
at several altitudes, and future times to predict a flight path. Then, they will study forecast precipitation probabilities
and freezing levels over the flight period. Modern trajectory predictors, such as the HYSPLIT program, maintained by
National Oceanic and Atmospheric Administration (NOAA) can be a great help. However, the HYSPLIT model gives no
forecast for precipitation along the route, so the possibility of rain, snow, or icing must be assessed using other weather
models. Often, ideal weather is found just after a frontal passage, after any moisture has cleared but while the air mass is
still moving with the departing front.
For serious competitions, a professional meteorologist is an invaluable team member and is consulted before and during
the flight. A good starting point for weather investigations is NOAA’s aviation weather website.
Flying in Inversions
Proper utilization of atmospheric temperature inversions during gas balloon flights can result in increased flight stability
and ballast conservation. Figure 11-7 shows a simplified atmospheric temperature lapse chart when no inversion is present.
Figure 11-8 is similar but with an inversion present. In both charts, altitude increases up the vertical scale and temperature
increases going to the right on the horizontal scale. Figure 11-7 shows temperature decreasing consistently with altitude,
while Figure 11-8 has an inversion zone (inside red circle) from 2,000 to 4,000 feet in which the temperature increases
with increasing altitude. A gas balloon flying in this inversion has the advantage of increased stability as compared to the
altitudes above or below the inversion which exhibit normal lapse rates.
14,000
12,000
10,000
8,000
6,000
4,000
2,000
Temperature (°F)
Altitude (feet)
–20.00 0.00 20.00 40.00 60.00 80.00
Standard Lapse Rate
Figure 11-7. Standard temperature lapse chart
14,000
12,000
10,000
8,000
6,000
4,000
2,000
Temperature (°F)
Altitude (feet)
–20.00 0.00 20.00 40.00 60.00 80.00
Inversion Aloft
Figure 11-8. Atmosphere exhibiting an inversion.
To explain this, it is important to understand the term “stability” with respect to a gas balloon. Stability can be imagined
as an invisible hand that gently pulls the balloon down whenever it starts to rise or alternately pushes the balloon back up
as it starts to fall. A balloon flying in stable weather tends to fly level with very little intervention from the pilot. However,
stability is a weak condition and can be overcome by many factors, such as gain or loss of solar heating, orographic winds,
and ballasting or valving.
To understand why an inversion creates stable flying conditions, think of a gas balloon flying at 3,000 feet in the middle
of the inversion. If, for some reason, the balloon starts to ascend, two things happen. First, the balloon enters warmer
ambient air. Second, the lifting gas inside the balloon expands and cools adiabatically as it reaches the slightly lower
pressure atmosphere of the higher altitude. Both of these effects cause the balloon to lose lift and to descend. It is helpful
to remember that, as a hot air balloon either cools or enters hotter air, the temperature differential between outside and
inside air decreases and lift is lost. The same principle applies here, as applied to gas balloons. Subsequently, if the gas
balloon descends from its starting point, it encounters cooler outside ambient air and its interior gas compresses and warms
adiabatically, therefore gaining and ascending back to its original altitude. Remember, in an inversion, either motion
(ascending or descending) tends to cause an opposing force to passively initiate a return to the original altitude.
With proper planning, the gas balloon pilot can take advantage of this scenario. Weather theory teaches that inversions
often set up at night either right at the surface (sometimes referred to as “surface inversion”) or at some altitude above
the ground (generally referred to as an “inversion aloft”) as shown in Figure 11-8. Visible signs of an inversion may be
pollution trapped below the inversion causing reduced visibility and dirty looking air. Invisible signs of an inversion may
be significantly more stable flying conditions in the inversion area. If a slow, steady initial ascent is initiated (best done by
launching with a flaccid balloon), the balloon may find an inversion with no help from the pilot by leveling off as it enters
the inversion zone. More likely the pilot has to hunt for the inversion; inversion levels can be determined from the use of
the Skew-T charts as previously discussed in Chapter 4, Weather Theory & Reports.
As an example, during flight at night very near the ground, a pilot may feel they are continually ballasting to fly level. If a
pilot suspects an inversion aloft may exist above, they can initiate a slow ascent. If the envelope is flaccid and the ambient
air at this altitude is not inverted, then the balloon tends to continue rising until it either encounters an inversion or becomes
full as it reaches its pressure ceiling. If the balloon levels out while it is still flaccid, this may indicate that it has entered an
inversion zone. If the balloon continues to fly level passively, it is flying in an inversion.
The Practice of Gas Ballooning
Gas Balloon Regulations
For a pilot with a hot air rating, certification to fly gas balloons requires the removal of the “with airborne heater” limitation
on their certificate. For a private pilot gas rating, aeronautical experience requirements in Title 14 of the Code of Federal
Regulations (14 CFR) part 61, section 61.109(h)(1) require “…at least two flights of at least 2 hours each that consists of:
i. At least one training flight with an authorized instructor in a gas balloon in preparation for the practical test
within the preceding 2 calendar months from the month of the test;
ii. At least one flight performing the duties of pilot in command in a gas balloon with an authorized instructor; and
iii. At least one flight involving a controlled ascent to 3,000 feet above the launch site. ”
The regulation for removal of the airborne heater restriction from an existing hot air balloon pilot’s certificate (14 CFR part
61, section 61.115 Balloon ratings: Limitations) states:
(2) The limitation may be removed when the person obtains the required aeronautical experience in a gas balloon and
receives a logbook endorsement from an authorized instructor who attests to the person’s accomplishment of the
required aeronautical experience and ability to satisfactorily operate a gas balloon.
Note: Only a logbook entry is required to complete the process (no check ride), since the gas authorization is a removal of
a restriction from an existing rating rather than an issuance of a new rating.
The regulations for the aeronautical experience for a commercial rating are virtually identical according to 14 CFR part 61,
section 61.129(h)(4)(i) except that the student must act as pilot in command on both flights and the controlled ascent must
be to “…5,000 feet above the launch site.”
Additional areas of specific interest to gas balloon pilots are the regulation for currency for night flight according to 14
CFR part 61, section 61.57(b)(1):
“…no person may act as pilot in command of an aircraft carrying passengers during the period beginning 1 hour after
sunset and ending 1 hour before sunrise unless within the preceding 90 days that person has made at least three takeoffs
and three landings to a full stop during the period beginning 1 hour after sunset and ending 1 hour before sunrise.”
Note: This is similar to the currency for day flight and only applies to flights with passengers. It does not prohibit night
flight when only pilots are on board.
Also of interest are the sections on aircraft lights (14 CFR part 91, section 91.209) for night flight; the use of supplemental
oxygen (14 CFR part 91, section 91.211), and the use of transponders (14 CFR part 91, section 91.215).
Flight Planning
During the planning stage of the flight, the objectives of the flight should be established. Possibilities include: training,
pleasure, competition, record setting, new equipment checkout, or possibly scientific investigation. Then, the expected
flight parameters should be developed. These include the number of pilots and passengers, balloon parameters (type,
size, and lifting gas to be used), launch time and location, expected duration, required weather, maximum altitude, and
predicted landing zone. If a fixed date has been selected for the flight, an initial meteorological assessment should be made
approximately three days out and subsequent weather developments are used to make a “go/no-go” decision. If the flight
date is flexible over an extended time window, continual monitoring of the weather is required until proper conditions
develop.
After this, an equipment list can be developed and a system weight and ballast calculation should be performed. Permission
to use the desired launch site must be confirmed and the availability of adequate gas supply and launch and chase crew
must be assured. A concise schedule should be sent to all crew members, defining when decisions are made and how these
decisions are communicated to the crew.
Finally, all inflation and flight equipment must be assembled and checked out for proper functioning. The chase vehicle
selected should be able cover the expected distance and bring all participants back home.
Layout & Inflation
When the day comes, all equipment is transported to the launch site. A site “walk-around” is performed to remove debris
and trash and to check for obstructions to inflation or takeoff. Layout and assembly differ by balloon type, but should
proceed according to the balloon’s flight manual. For quick fill balloons, layout should be downwind, similar to hot air
balloons. A crew briefing should be performed and a crew chief should be designated. Safety must be emphasized; new
crew members must be given specific instructions and be assigned to a more experienced crew member for guidance. Gas
balloon launches still tend to draw a crowd and some form of crowd control may be required. [Figure 11-9]
Figure 11-9. America’ s Challenge launch at Fiesta Park, Albuquerque, New Mexico.
A launch restraint should be secured and additional inflation ballast (weighing several hundred pounds more than the pilots
and supplies that eventually are on board) should be added to the gondola.
The start of gas flow is a critical point in the inflation. A slow initial flow allows a last minute check of cable routings
and crew positions. Surface weather conditions determine flow rate after the initial checkout. For quick fill systems, a
partially filled envelope is much more subject to twisting in wind gusts. A flaccid envelope tends to present a concave
(spoon-shaped) surface to the wind, producing a higher drag factor and placing more stress on the entire system. Once the
envelope’s shape has filled out to a convex (beach ball) shape, it is more able to stand by shedding wind gusts around it.
Quick fill systems are much more subject to this effect than are netted balloons.
As described in the section on balloon systems, a netted balloon envelope is laid out flat on the ground and its shape during
inflation resembles a sphere rising out of the ground. It always presents a convex (shedding) shape to the wind. A separate
step is required after inflation to install the basket and attach it to the load lines coming down from the net.
A good rule of thumb is to take no more time than is necessary to complete the fill. This is especially true in windy
conditions.
Launch
With the use of a checklist, confirm that all required equipment items and pilots are on board. A launch master is usually
assigned to direct removal of excess inflation ballast until the system is neutrally buoyant. Desired ascent rate determines
how much additional ballast is removed to attain the proper amount of positive buoyancy. The launch master should be an
experienced gas balloonist and direct crew to allow the balloon to rise several feet off the ground several times to test the
buoyancy before instructing the crew to bring it back to the ground one last time before final release. If the ascent rate is
too slow, additional ballast are removed until the proper rate is achieved. After a final check for airspace clearance above,
the “Hands off!” command is given and the balloon is allowed to fly free. [Figure 11-10]
Figure 11-10. A gas balloon, shortly after the weigh off procedure, ascends into the sky.
In-flight Procedures
All other things being equal, a gas balloon’s natural tendency is to find its equilibrium altitude and to fly level at that
altitude. By contrast, a hot air balloon’s natural tendency to descend must be counteracted with periodic infusions of heat.
In a gas balloon, pilot action is only required to initiate or arrest an ascent or descent or to counter atmospheric or other
disturbances. A gas balloon may fly for an hour or longer with no intervention by the pilot. Pilot initiated altitude changes
with gas balloons tend to occur at a slower rate over a longer time period as compared to hot air ballooning.
Ascents are initiated by jettisoning ballast (usually sand or water) while descents result from releasing lifting gas from a
valve at the top of the envelope.
Significant mid-flight altitude changes are often undertaken as part of a long-term strategic plan rather than for short term
tactical reasons. The consequences of any maneuver should be considered carefully before being undertaken. It is often
stated that ballast is the fuel of a gas balloon and well before all ballast has been expended, the aircraft must be safely back
on the ground. Use of large amounts of ballast to execute a major ascent invariably shortens the potential duration of a
flight.
For example, in a distance competition, an ascent from the surface to 12,000 feet MSL may be executed to enter more
favorable winds. This may take 1 hour to accomplish but that altitude may then be maintained for the next 8 hours if
weather conditions are stable.
Two additional concepts that must be understood to pilot gas balloons are solar heating and lifting gas purity.
Solar Heating
Solar heating (also called super-heating) occurs when the heat of the sun is trapped inside the balloon’s envelope and causes
the temperature of the lifting gas to exceed the outside air temperature. As the heated lifting gas expands, one of two things
will happen. If the envelope is flaccid, the less dense gas occupies a larger fraction of the envelope’s volume and displaces
more air and the system’s gross lift temporarily increases. This causes the balloon to rise towards its pressure ceiling and