Pilots of fixed wing transport aircraft rarely encounter passenger-carrying hot air balloons as a potential collision risk, leaving some vulnerable to complacency about this threat. Typically (though not exclusively) these encounters happen just before or soon after dawn, during the fixed-wing airplane's descent to the destination aerodrome — often at the conclusion of long-haul, night cruise flight segments. Hot air balloons also have been encountered in other scenarios. The purpose of this article is to increase flight crews' awareness of hot air balloons with the benefit of current guidance developed by the U.K. Civil Aviation Authority, which updated the guidance in the context of increased issuance of air operator's certificates to balloon operators that meet public transport standards for commercial ballooning.
There are a number of differences between balloons and other aircraft, the most notable of them being:
Size - Although typically carrying relatively few occupants (usually 20 or less) in a basket/gondola, balloons tend to have a spheric shape with diameter similar to most airliners (e.g. BOEING 737-800).
Place of operation - Balloon flights usually tend to avoid steep and rocky terrain and immediate coastal areas. The flights take place mostly outside controlled airspace.
Time of operation - In order to fly in the most stable conditions and to avoid thermal activity, pilots normally operate balloons early in the morning and during the three hours prior to sunset in the summer period. It is possible for balloons to fly at any time during daylight hours on a calm winter's day. Very rarely, sporting and recreational balloon flights are conducted at night, but these are invariably planned to land after sunrise. Activity is usually higher at weekends and on public holidays.
Groups - Organised events may amass large number of balloons (e.g. 100 or more). These event times, locations and altitudes usually are communicated by NOTAM.
Flight rules - Balloon flights must be conducted under VFR.
Levels - Commercial balloons often fly below 2000 ft above ground level, affording the best view for their passengers, but some commercial balloons can be found up to FL 100. During flight, a balloon may also descend toward points of interest, for example lakes. If a balloon is seen to be close to touching the water it is probably not in any difficulty.
Duration - Flights commonly last for about one hour, but this can be extended considerably if a suitable landing site cannot be found on track. Fuel duration varies with the balloon size and flight conditions, but is normally between 1.5 and 2 hours. The pilot will usually descend to low altitude for the last 15 to 20 minutes of the planned flight in order to assess the low level winds and obstacles, and to manoeuvre for landing.
Vertical movement - The pilot of a hot air balloon controls descent either by letting the hot air inside the balloon envelope cool naturally or by opening the parachute vent in the top of the envelope (the vent reseals once the control line is released). Operation of the vent will cause the balloon to descend rapidly until more hot air is added with the burner controls to arrest the descent. Normal descent rate is less than 600 ft per minute and a large passenger balloon will rarely exceed this rate below 2000 ft AGL because it may not be possible to recover even with the use of all burners. The manufacturer's stated limit for climbing and descending is between 800 and 1000 ft per minute - surprisingly quick - and a balloon in this mode of flight will usually be unable to reverse this climb or descent quickly and should be given a wide berth.
Directional control - The guidance notes that all non-tethered balloons track downwind. A pilot can ‘steer’ a balloon using the wind directions between the surface and the gradient wind direction, which may vary by 30° or more. This often occurs during early morning flights and the balloon may be seen to climb or descend rapidly to achieve such steerage.
Communication - Normally, balloonists communicate with their retrieve teams and with other balloons using hand-held radios. The same radios are used for ATC communications, when required. Some commercial balloons that are regularly operated in and around controlled airspace carry a lightweight Mode-S transponder. However, SSR equipment is not yet common in balloons.