Operation of Hot Air Balloons with Airborne Heaters

Advisory Circular 91-71: A Commentary

by Tom Hamilton

In June of this year the Federal Aviation Administration issued Advisory Circular 91- 71, Operation of Hot Air Balloons with Airborne Heaters. The purpose of this 14 page document is to “provide guidance for the safe and practical operation of hot air balloons with airborne heaters in compliance with appropriate portions of Title 14 of the Code of Federal Regulations (14CFR) part 91, General Operating and Flight Rules.”

The FAA has issued this AC because, “several recent court decisions regarding low flights of hot air balloons indicate a possible lack of understanding by the ballooning public of how various sections of the CFR are applied to hot air balloon operations.”

The FAA continues to explain that the AC has been prepared to help “alleviate misunderstanding or misconceptions and to assist hot air balloonists in operating their aircraft within the requirements of part 91.”

Fair enough.

The AC is roughly divided into six areas: Definitions; Launch site, takeoff, and climb; Cruise flight; Minimun altitudes; Approach and landing; and Wind.

In the concluding paragraph it is noted that, “the procedures and techniques discussed in this AC are advisory in nature. They are general guidance and should not be construed as required operating practices. This AC also contains numerous excerpts from, and references to, compliance with part 91. The regulations themselves are not advisory, and compliance is required. Applicable operating limitations and procedures contained in manufacturer’s FAA-approved flight manuals, markings, and placards, or as otherwise prescribed by the certificating authority of the country of registry, take precedence over the information contained in this AC.”

It is clear from the outset that this document will undoubtly be used by the Administrator when pressing certificate action before an Administrative Law Judge or on appeal to the National Transportation Safety Board. The AC covers areas of operation that are frequently cited in certificate action cases. The guidelines proposed in the AC are clearly designed to establish parameters of operation from which the Administrator can argue the government’s case.

While it might be argued that the above paragraph is a bold and unsubstaniated statement, the tone of the AC clearly indicates that someone has a distaste for how balloons fly. While it is generally recognized that the FARs were written with powered flight in mind this AC is intended to reinforce those rules on a different type of aircraft.

It is doubtful that great thought about balloon flight was put into this AC. Below is Balloon Life’s anaylsis and comment of the various sections defined above.


On the surface, the definitions given by the FAA in the sidebar nearby appear acceptable. Or are they?

Aeronaut, the American Heritage Dictionary defines the word as: A pilot or navigator of a balloon or lighter-than-air craft. The FAA says it’s anybody who goes up in a balloon or airship. If that is the case why bother to define the word? Later in the section on approach and landings the AC refers to the aeronaut’s duties. Is that the pilot or the passenger aeronaut?

Basket, in the 1996 Balloon Life Buyer’s Guide seven manufacturers specify that their baskets are made from rattan, three include the use of wicker, one specifies wicker, and three do not specify. Wicker is not the main component of balloon baskets. Or is that gondola? The FAA defines gondola as containing addition materials. Sloppy, if any, research.

Cruise Flight, a new term in ballooning. Checking in four different training manuals (Flight Training File, BFA Flight Instructor Manual, Taming The Gentle Giant, and How To Fly A Balloon) I could not find this term. Level flight yes, crusie flight no. Fixed wing terminology forced on ballooning? Cruise is defined in the dictionary as: To travel at a speed providing maximum operating efficiency for a sustained period.

Weight Off, training aids that contained a reference to this procedure refered to the term as weigh off (a term derived from gas ballooning which means to find neutral buoyancy).

Velocity, a term used with Pibal and Windshear. In the former velocity refers to speed and in the latter the FAA indicates that it could mean direction. Better to have not used velocity and stuck with words like speed and direction.

Launch site, takeoff, and climb

In selection of launch site the FAA hits the nail on the head. “The appropriateness of any launch site involves much more than its physical size and absence of obstructions, even though these are important considerations. Of equal or greater importance is the direction the balloon will track following the launch. Any site selection made should include consideration of local winds as determined by pibal, smoke, trees, or other physical indications; forecast and reported winds aloft; and local phenomena peculiar to the specific site (determined by personal knowledge or by consulting with local balloonists). Also, location of obstructions along the projected flightpath and available suitable landing sites down range should be considered.”

The launch site selection continues by advising that any adverse changes in conditions might cause the pilot to consider moving their launch or even cancelling the flight. The Administrator in a number of certificate action cases has argued that launch site was inappropriate because there were no “suitable” landing sites down wind.

Pilots are cautioned about obstacle clearance during lift off and climb out from a launch site. The rule of thumb for computing the minimum distance between the launch point and obstacles downwind is given. Mulitply the windspeed expressed in miles per hour by 100 to calculate the suggested minimum horizontal distance in feet from obstacles that the balloon should be positioned for launch. This is the distance that the balloon is expected to travel during the first minute of flight.

The AC suggests establishing an initial rate of climb of by adding the height of the obstacle plus a minimum desired clearance to arrive at an initial rate of climb. Thus, a 100-foot obstacle and a 200 feet of desired clearance over the obstacle totals 300 feet. Lay the balloon out at least the computed distance from the obstacle. Upon takeoff, establish an initial rate of climb of 300 feet per minute.

The AC gives the following example: “The windspeed is 4 mph and there is a 75- foot line of trees downwind at the end of the field. Using the above formula, multiply the windspeed by 100 (4 X 100 = 400). In order to pass 100 feet over the trees after takeoff, add 100 feet of the tree height of 75 feet, resulting in a total of 175 feet. Lay the basket at least 400 feet upwind of the treeline. Upon takeoff, establish a minimum initial rate of climb of 175 fpm. The balloon should pass 100 feet over the trees.”

The AC goes on to suggest an allowance for errors in distance estimation and changes in surface wind or other unforeseen occurences. It is suggested that a landmark be selected halfway to the obstruction. “If it should appear that the balloon will not achieve a minimum of half the clearance altitude by that halfway point, immediately terminate the flight by activating the deflation port before reaching a critical altitude (usually about 20 feet above the surface).”

While this might work well for low obstacles, those under 100 feet, additional considerations in wind speed and direction must be considered for taller obstacles. For a more detailed discussion of launch consideration read Box Canyon (avoiding radio/TV towers) in the February, 1991 issue of Balloon Life.

At the conclusion of this section the advice continues, “It is also important to maintain a positive rate of climb during the departure until the balloon is at or above the appropriate minimum safe altitude prescribed in § 91.119 in VFR conditions. (Emphasis mine.)

Cruise flight

“Once established in cruise flight, care must be exercised to maintain visual flight rules (VFR) visibility and cloud clearance requirements (§ 91.155).” Some of the examples given are illustrated nearby.

Balloons are often ascending or desending during flight in order to use the winds for navigation. As discussed earlier, the selection of the term “cruise flight” by the FAA is an interesting one.

Minimum altitudes

This area has generated debate between the balloon community and the FAA for many years. It is the regulation most often used against a balloonist in certificate action. There have been numerous attempts to petition the FAA to lower the minimum altitudes for balloons.

Whether the current § 91.119 is just a powered flight rule being forced on balloons or really a good idea for all aircraft, it is the law. The Administrator makes that point with the following discussion.

“In accordance with § 91.119, balloons must maintain a safe minimum altitude above the surface and safe minimum distances from obstacles. Section 91.119(b) requires that all aircraft, except when necessary for takoff and landing, operate over congested areas of cities, towns, or settlements or over open air assemblies of persons at an altitude no lower than 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.

“Section 91.119(c) requires that all aircraft, except when necessary for takeoff and landing, operate over other than congested areas at an altitude of 500 feet above the surface except over open water or sparesely populated areas.

“In the case of open water and sparsely populated areas, § 91.119(c) further requires that aircraft operate not closer than 500 feet from persons, vessels, vehicles or structures.”

Approach and landing

“When a landing site is being considered, consideration should be given to the site’s suitablility. For example, city streets and highways, small fields occupied by large gatherings of people not associated with the ballooning activity, and fields containing obstructions (such as power lines) may be inapropriate landing sites in certain instances. When considering the prevailing surface wind, aeronauts should make certain that adequate ingress/egress is available with respect to the above obstructions. Before descending below the cruise altitude, aeronauts should keep in mind the need to fly a reasonable descent path to the proposed landing site. The approach to the site may be accomplished in several ways. In a no wind situation, simply settling vertically may suffice; however, when winds are involved, an approach path involving track and descent will be required.” (Empahsis mine.)

The AC suggests two methods to accomplish an approach path: the stairstep approach and the three degree constant rate descent path.

In the January through April, 1991 issues of Balloon Life Bill Bird wrote an excellent four part series on landings. Part two in the series said there are “bascially three types of approaches… These three types are: The Step Down Approach, The Rapid Descent and Round Out Approach, and the Constant Rate Approach.

On the subject of the constant rate approach Mr. Bird wrote, “(the so called ‘glide slope’ approach) is one that people who fly airplanes always seem to want to attempt. In the hundreds of hours that I have given balloon flight instruction, invariably a person who has an airplane license, no matter the number of hours, will always attempt to approach and land the balloon using this style approach. To successfully execute this style approach to a landing at a preselected site requires a lot of skill, a set of perfect climatic conditions (constant velocity, constant direction winds) from altitude to the landing site. I do not recommend this approach.” (Emphasis his.)

Of what use is the constant rate approach? The AC states, “A shallow glidepath of approximately three degrees has been established as a general standard by the aviation industry for descents over congested areas. For hot air balloons, this general standard may need to be modified, but it is a good basic guideline. Achievement of a three degree glidepath at a forward groundspeed of five knots requires a descent rate of 26 to 27 fpm.”

From cruise altitude of 1,000 feet it will take 38 minutes to reach the ground covering a distance of just over three nautical miles. Okay, let’s see a show of hands of how many balloonists have experienced conditions that would sustain this approach.

While the AC suggests that the three degrees might have to be modified it does provide an interesting argument, one presented by an FAA Inspector at a safety seminar several years ago. The presenter demonstrated how the three degree, or steeper, angle might apply to balloon flight.

Powered aircraft on approach basically operate in a two dimensional plane, straight flight and a descent rate. During that descent rate the aircraft might be on, above, or below glide slope for a variety of reasons. Because of a change in conditions during the approach it might be necessary for the aircraft to excecute a go-around.

Okay, how does that relate to balloons? If the balloon is on approach, even at a low rate of descent, to a suitable landing area everything is fine as far as the § 91.119 is concerned. However, especially in ballooning, conditions can change frequently in boundary layer air. Since the balloon cannot “hold” a straight course of flight it may be necessary to execute a go-around. That will nessitate beginning a climb back to “cruise” altitude. During that climb a new suitable landing area may avail itself and the approach to landing will be initiated again.

The situation can develop with any of the styles of approaches mentioned. While I have not seen this argument tested in a certificate action case to date, it does provide a defense using powered flight rules, rules that the FAA wants to apply to balloon flight.

The important points here are that the balloon must be making a legitmate approach to a suitable landing area, that the balloon does not sustain level flight at a low altitude for prolonged period of time, and that the pilot be able to demonstrate later that the flight path met the requirements for an approach to a suitable landing area.

The AC states, “While the descent path can be varied and sometimes may be quite shallow, it is important to avoid level flight segments below minimum safe altitudes. Such level flight periods could lead observers to believe that the pilot has discontinued the approach and established level cruising flight at less than a minimum safe altitude.”

Flying a good approach in a balloon is more an art than a science. In addition to the articles mentioned above read How to Fly a Balloon, Chapter 16 published by Balloon Publishing Company. (See MarketPlace for more information.)

Wind components

The AC discusses rules-of-thumb for interpreting what will happen as the balloon passes through a horizontal wind shear. The information is presented to eliminate the statement, “the balloon encountered an unexpected wind gust that pushed it into an obstruction before a pilot could react.”

The AC states, “[This] is an unfortunate corollary to an accident. Application of general rules-of-thumb may provide advance warning and assist pilots to recognize and correct impending problems.”

Final Thoughts

This Advisory Circular first appeared in draft form in 1989. At that time much of what was in the document was challenged by the balloon community. The document was withdrawn and has been collecting dust since that time. Although there have been some changes from the original it does not fully address balloon flight. Rather it reinforces rules designed for powered flight.

While the FARs are not necessarily unfair to ballooning, their strict interpretation at times ignores the unique characteristics of balloon flight.

Balloon Life spoke with Lynn Harris, Balloon Federation of America Government Liason Chair, to find out if the balloon community had an opportunity to comment on the AC before it was issued. Harris told Balloon Life that indeed a number of prominent balloonists had an opportunity to review the document and make suggestions. Besides Harris, Beth Wright-Smith, BFA Board member and Operation Director for World Balloon Corporation in Albuquerque, Matt Weidehker in Minneapolis, among others made numerous suggestions. These suggestions were given to the FAA in Oklahoma City. While the FAA staff in Oklahoma City was receptive to the suggestions the author of the AC in Washington, DC apparently was not. The final document is almost unchanged from the original draft.

It is unfortunate that the FAA did not choose to work with the balloon community in developing this Advisory Circular. The FAA missed a real opportunity to build a relationship and create an understanding that would take into account how balloons fly while maintaining a safe environment for the public.


a. Aeronaut. A pilot or passenger of a balloon or airship.

b. Airborne Heater. A device carried in the balloon used to generate heat to maintain the temperature of the air inside the balloon envelope. Normally, a propane- powered burner assembly.

c. Ascent Profile. A climbpath over horizontal distance traveled, as applied to obstacle clearance.

d. Basket. A compartment constructed of wicker, occupied by the pilot and passengers.

e. Cruise Flight. A steady state level flight condition without climbing or descending.

f. Deflation Port. A panel in the top or side of a balloon envelope that, when opened/activated (some times referred to as “ripping out”), allows a large volume of air to escape from the balloon. It is normally used during landing and/or securing to eliminate the lift of the hot air and deflate the envelope.

g. Gondola. The compartment occupied by the pilot and passengers. It is usually constructed of wicker, metal, or fiberglass.

h. Ground Crew (also Chase Crew or Retrieve Crew). Individuals who track and follow the balloon’s flight and assist in launch and recovery operations.

i. Hot Air Balloon. A lighter-than-air aircraft that is not engine-driven and uses hot air for lift.

j. Lighter-than-Air Aircraft. An aircraft that can rise and remain suspended by using contained gas weighing less than the air that is displaced by the gas.

k. Pibal (Pilot Balloon). A small, helium-filled balloon launched before a flight to determine wind direction and velocity and identify windshear. It is used as an aid in selecting a launch site and/or landing site.

1. Weight Off. A command used prior to launch when a rapid initial rate of ascent is needed because of wind and obstacles. The basket/gondola is held to the surface by tether or by the additional weight of crewmembers until sufficient heat can be generated by the burner assembly into the envelope to attain the desired ascent rate. When the necessary heat has been generated, the tether is released or the crewmembers release their hold, allowing the balloon to ascend more rapidly than it would during a normal launch.

m. Windshear. The rate of change of wind velocity (direction and/or speed) per unit of distance, conventionally expressed as vertical or horizontal windshear.

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