Gliding
Gliding (or soaring) is a recreational activity and competitive sport where pilots fly un-powered aircraft known as gliders or sailplanes. Properly, the term gliding refers to descending flight of a heavier-than-air craft, whereas soaring is the correct term to use when the craft gains altitude or speed from rising air.[1]
The most common methods of launching gliders are winch or tow by powered aircraft. Pilots then search for rising air to gain height. If conditions are good enough, experienced pilots can fly many hundreds, or even thousands, of kilometers before returning to their home airfields. But if the weather deteriorates, they must land elsewhere.
While many glider pilots merely enjoy the sense of achievement, some competitive pilots fly in races as quickly as possible round a pre-defined course. These competitions test the pilots' abilities to make best use of local weather conditions as well as their flying skills. Local and national competitions are organized in many countries and there are also biennial World Gliding Championships.[2]
All methods of launching gliders (apart from self-launching motor-gliders) require assistance from other participants. Gliding clubs have thus been established to share airfields and equipment, train new pilots and maintain high safety standards.
History
The development of heavier-than-air flight in the half-century between Sir George Cayley's coachman in 1853 and the Wright brothers mainly involved gliders (see History of Aviation). However, the sport of gliding only emerged after the First World War as a result of the Treaty of Versailles,[3] which imposed severe restrictions on the manufacture and use of single-seater powered aircraft in Germany. Thus, in the 1920s and 1930s, while aviators and aircraft makers in the rest of the world were working to improve the performance of powered aircraft, the Germans were designing, developing and flying ever more efficient gliders and discovering ways of using the natural forces in the atmosphere to make them fly further and faster.
The first German gliding competition was held at the Wasserkuppe[4] in 1920,[5] organised by Oskar Ursinus. The best flight lasted two minutes and set a world distance record of 2 km.[5] Ten years later it had become an international event in which the achieved durations and distances had increased greatly. In 1931 Gunther Grönhoff flew 272 km (169 miles) from Munich to Czechoslovakia, further than had been thought possible.[5]
In the 1930s gliding spread to many other countries. In the 1936 Summer Olympics in Berlin gliding was a demonstration sport and was scheduled to be a full Olympic sport in the 1940 games. A glider, the Olympia, was developed in Germany for the event, but the Second World War intervened. By 1939 the major gliding records were held by Russians, including a distance record of 748 km (465 miles).[5]
During the war, civilian gliding in Europe was largely suspended. Although some military operations in WWII involved military gliders, they did not soar and so are unrelated to the sport of gliding. Nonetheless, several German fighter aces in the conflict, including Erich Hartmann, began their flight training in gliders.
Gliding did not return to the Olympics after the war, for two reasons: first, the shortage of gliders following the war; and second, the failure to agree on a single model of competition glider. (Some in the community feared doing so would hinder development of new designs).[5] The re-introduction of air sports such as gliding to the Olympics has been occasionally proposed by the world governing body, the FAI, but this has been rejected on the grounds of lack of public interest.[6]
Still, in many countries during the 1950s there were a large number of trained pilots who wanted to continue flying. Many were also aeronautical engineers. They started both clubs and manufacturers, many of which still exist. This stimulated the development of both gliding and gliders — for example, the Soaring Society of America grew from 1,000 members then to its present total of 16,000. The increased numbers of pilots, greater knowledge and improving technology helped set new records, so that the pre-war altitude record was doubled by 1950, and the first 1,000 km (621 statute miles) flight was done in 1964.[5] New materials such as glass fiber and carbon fiber, advances in wing-shapes, electronic instruments, GPS and improved weather forecasting have since allowed many pilots to make flights that were once extraordinary. Today almost 500 pilots now have made flights over 1,000 km.[7]
Instead of Olympic competition there are the World Gliding Championships. The first event was held at the Wasserkuppe in 1937.[5] Since the war it has been held every two years. There are now six classes open to both sexes, plus three classes for women and two junior classes. Germany, the sport's birthplace, is still a center of the gliding world: it accounts for 30% of the world's glider pilots,[8] and the three major glider manufacturers are still based there. However the sport has been taken up in many countries and there are now over 116,000 active glider pilots,[9] plus an unknown number of military cadets. Each year many other people experience their first glider flight. It does not matter whether the countries are flat or mountainous, hot or temperate, because gliders can soar in most places.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/f/f5/Bluesky.jpg/220px-Bluesky.jpg)
Soaring
Glider pilots can stay airborne for hours by flying through air that is ascending as fast or faster than the glider itself is descending, thus gaining potential energy.[10] The most commonly used sources of rising air are thermals (updrafts of warm air), ridge lift (found where the wind blows against the face of a hill and is forced to rise), and wave lift (standing waves in the atmosphere, analogous to the ripples on the surface of a stream). Ridge lift rarely allows pilots to climb much higher than about 600 m (2,000 ft) above the terrain; thermals, depending on the climate and terrain, can allow climbs in excess of 3,000 m (10,000 ft) in flat country and much higher above mountains;[11] wave lift has allowed a glider to reach an altitude of 15,447 m (50,671 ft).[12] In a few countries gliders can continue to climb into the clouds in uncontrolled airspace but in many countries the pilot must stop climbing before reaching cloud-base (see Visual Flight Rules).
Thermals
Thermals are streams of rising air that are formed on the ground through the warming of the surface by sunlight.[13] If the air contains enough moisture, the water will condense from the rising air and form cumulus clouds. Once a thermal is encountered, the pilot usually flies in circles to keep the glider within the thermal, so gaining altitude before flying off to the next thermal and towards the destination. This is known as 'thermalling'. Climb rates depend on conditions, but several meters per second is common. Thermals can also be formed in a line usually because of the wind or the terrain, creating cloud streets. These can allow the pilot to fly straight while climbing in continuous lift.
When the air has little moisture or when an inversion stops the warm air from rising high enough for the moisture to condense, thermals do not create cumulus clouds. Without clouds or dust devils to mark the thermals, the pilot must use his skill and luck to find them using a sensitive vertical speed indicator called a variometer that quickly indicates climbs or descents. Typical locations to find thermals are over towns, freshly ploughed fields and asphalt roads, but thermals are often hard to associate with any feature on the ground. Occasionally thermals are caused by the exhaust gases from power stations or by fires.
As it requires rising heated air, thermalling is only effective in mid-latitudes from spring through into late summer. During winter the solar heat can only create weak thermals, but ridge and wave lift are still be used during this period.
Ridge lift
A ridge soaring pilot uses air lifted up the sides of hills. It can also be augmented by thermals when the slopes also face the sun.[14] In places where a steady wind blows, a ridge may allow virtually unlimited time aloft, though records for duration are no longer recognized because of the danger of exhaustion.[15]
Wave lift
![](http://upload.wikimedia.org/wikipedia/commons/thumb/4/4b/Lenticular4.jpeg/220px-Lenticular4.jpeg)
The powerfully rising and sinking air in mountain waves was discovered by a glider pilot, Wolf Hirth, in 1933.[16] Gliders can sometimes climb in these waves to great altitudes, if pilots use supplementary oxygen to avoid hypoxia. This lift is often marked by long, stationary lenticular (lens-shaped) clouds lying perpendicular to the wind.[17] Most altitude records for gliders have been set this way, though thunderstorms have also accounted for a share. The current world distance record of 3,008 km (1,869 statute miles) by Klaus Ohlmann (set on 21 January 2003)[18] was flown using mountain waves in South America.
A rare wave phenomenon is known as Morning Glory, a roll cloud producing strong lift. Pilots near Australia's Gulf of Carpentaria make use of it in springtime.[19]
Other sources of lift
The boundaries between two air masses meet are known as convergence zones.[20] These can occur in sea breezes or in desert regions. Glider pilots can gain altitude by flying along the intersection as if it were a ridge of land. Convergence may occur over considerable distances and so may permit virtually straight flight while climbing.
Glider pilots have been able to use a technique called "dynamic soaring",[21] where a glider can gain kinetic energy by repeatedly crossing the boundary between air masses of different horizontal velocity. However, such zones of high "wind gradient" are usually much too low to be used safely by gliders.
Launch methods
Aerotowing
Aerotows normally use single-engined light aircraft, although motor gliders have also been permitted to tow gliders. The tow-plane takes the glider to the desired height and place and the glider pilot releases the rope.[22] At the tow-plane end, a weak link is often fitted to the rope to ensure that any sudden loads do not damage the airframe of the tow-plane.
During the aerotow, the glider pilot keeps the glider in one of two positions behind the tow-plane.[23] This position can either be the "low tow" position, just below the slipstream from the tow-plane's propeller, or the "high tow" position just above the slipstream. In Australia the convention is to fly in low tow, whereas in the United States and Europe the high tow prevails.[24] One aerotow variation is to attach two gliders to one tow-plane, using a short rope for the high towed glider and the long rope for the low tow.
Winch-launching
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/9a/Schempp-Hirth_Ventus_2b_glider_being_launched_at_Lasham_Airfield_in_UK.jpg/220px-Schempp-Hirth_Ventus_2b_glider_being_launched_at_Lasham_Airfield_in_UK.jpg)
Gliders are often launched using a stationary ground-based winch mounted on a heavy vehicle.[25] This method is widely used at many European clubs, often in addition to aerotowing. The engine is usually a large diesel, though hydraulic fluid engines and electrical motors are also used. The winch pulls in a 1,000 to 1,600 m (3,000 to 5,500-foot) long cable, made of steel wire or a synthetic fiber, attached to the glider. The cable is released at a height of about 400 to 500 m (1,300 to 1,600 feet) after a short and steep ride.
The main advantage of a winch launch is its lower cost. However, the launch height is usually lower than an aerotow, so flights are shorter unless the pilot can quickly make contact with a source of lift within a few minutes of releasing the cable. Although there is a risk of the cable breaking during this type of launch, pilots are trained to deal with this.
Other methods of launching
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/dd/Bungeelaunch1.jpg/220px-Bungeelaunch1.jpg)
In a few places gliders are launched from the top of a hill into a strong breeze using a rubber band, or "bungee".[26] For this launch method, the glider's main wheel rests in a small concrete trough. The hook normally used for winch-launching is instead attached to the middle of the bungee. Each end is then pulled by three or four people. One group runs slightly to the left, the other to the right. Once the tension in the bungee is high enough, the pilot releases the wheel brake and the glider's wheel pops out of the trough. The glider gains just enough energy to leave the ground and fly away from the hill.
Another launch method, the "autotow", was used more frequently in the past.[27] It requires a long runway, a pick-up truck and the cable. After gently taking up slack in the cable, the driver accelerates hard and the glider rises like a kite to as much as 400 m (1300 feet) if there is a good headwind and a runway of 1.5 km (1 mile) or more. A variation on this is the "reverse pulley" method in which the truck drives towards the glider that it is launching; the cable passing around a pulley at the far end of the airfield.
Glider pilots who want to use the different types of launch methods must be in current practice in each. Licensing rules in some countries differentiate between aerotows and ground launch methods, due to the widely different techniques.
Cross-country
Gliders can stay airborne for hours in good conditions. This enables gliders to fly long distances at high speeds.[28] The record speed for 1,000 km is 169.7 km/h (621 statute miles at 105 miles/h).[29] Even in places with less favorable conditions (such as Northern Europe) most good pilots have flights over 500 km (310 statute miles) every year.[30] As the performance of gliders improved in the 1960s, the concept of flying as far away as possible became unpopular with the crews who had to retrieve the gliders. Pilots now aim to fly around a course (called a task) via turn-points and back to the starting point.
In addition to just trying to fly further, glider pilots also race each other in competitions.[31] The winner is the fastest, or, if the weather conditions are poor, the furthest round the course. Tasks of up to 1,000 km have been set[32] and speeds of 120 km/h are not unusual.
In the sport's infancy, ground observers confirmed that pilots had passed the turn-points. Later, the glider pilots photographed these places. Today, gliders carry secure devices that record the position every few seconds from GPS satellites.[33] These recording devices now provide the proof that the turn-points have been reached.
National competitions generally last one week, with international championships running over two. The winner is the pilot who has amassed the greatest number of points over all the contest days. However, these competitions have as yet failed to draw much interest outside the gliding community for several reasons. Because it would be unsafe for many gliders to cross a start line at the same time, pilots can choose their own start time. Furthermore, gliders are not visible to the spectators for long periods during each day's contest and the scoring is complex, so gliding competitions have been difficult to televise. In an attempt to widen the sport's appeal, a new format, the Grand Prix, has been introduced.[34] Innovations introduced in the Grand Prix format include simultaneous starts for a small number of gliders, tasks consisting of multiple circuits, and simplified scoring. An informal Internet competition called the On-Line Contest[35] where pilots upload their GPS data files and are automatically scored based on distance flown. 7,800 pilots worldwide participated in this contest in 2006.[36]
Maximizing speed
Soaring pioneer Paul MacCready is usually credited with developing a mathematical theory for optimizing the speed to fly when cross-country soaring,[37] though it was first described by Wolfgang Späte in 1938.[38] The speed to fly theory allows the optimal cruising speed between thermals to be computed, using thermal strength, glider performance and other variables. It accounts for the fact that if a pilot flies faster between thermals, the next thermal is reached sooner. However at higher speeds the glider also sinks faster, requiring the pilot to spend more time circling to regain the altitude. The MacCready speed represents the optimal trade-off between cruising and circling. Most competition pilots use MacCready theory to optimize their flight speeds, and have the calculations programmed in their flight computers. The greatest factor in maximizing speed, however, remains the ability of the pilot to choose the strongest lift.[39]
On cross-country flights where strong thermals are forecast, pilots fly with water ballast, which is stored in tanks in the wings and tail fin.[40] The first pilot to do this was George Tabery.[41] Ballast makes the glider fly faster, but slows its climb rate in thermals. However, if the thermals are strong, the disadvantage of slower climbs are outweighed by the higher cruising speeds between them. Thus, the pilot can improve the speed over the course by several percent.[40] To prevent over-stressing the glider, the flight manuals of gliders require that the pilot dump the water before landing.
Badges
Achievements in gliding has been marked by the awarding of badges since the 1920s.[42] For the lower badges, such as the first solo flight, national gliding federations set their own criteria. Typically, a bronze badge shows preparation for cross-country flight, including precise landings and a pair of two-hour flights. Higher badges follow the standards set down by the Federation Aeronautique Internationale (FAI).[43] The FAI's Sporting Code[44] defines the rules for observers and recording devices to validate the claims for badges, which are defined by kilometers of distance and meters of altitude gained. The Silver-C badge was introduced in 1930.[42] Earning the Silver Badge shows that a glider pilot has achieved an altitude gain of at least 1,000 m, made a five-hour duration flight, and has flown cross-country for a straight-line distance of at least 50 km: usually, but not invariably, in separate flights. The Gold and Diamond Badges require pilots to fly higher and further. A pilot who has completed the three parts of the Diamond Badge has flown 300 km to a pre-defined goal, has flown 500 km in one flight (but not necessarily to a pre-defined goal) and gained 5,000 m in height. The FAI also issues a diploma for a flight of 1,000 km and further diplomas for increments of 250 km.
Landing out
If lift is not found during a cross-country flight, for example because of deteriorating weather, the pilot must choose a field and 'land out'.[45] Although inconvenient and often mistaken for "emergency landings", landing out (or "outlanding") is a routine event in cross-country gliding. The pilot has to choose a field where the glider can be landed safely, without damaging property such as crops or livestock.[46]
The glider and the pilot(s) can be retrieved from the field using a purpose-built trailer. Alternatively, if the glider has landed in a suitable field, a tow-plane can be summoned to re-launch the aircraft (as long as the property owner gives permission). The glider pilot typically pays for the time the tow-plane is in the air, both to and from the field, so this alternative can become expensive.
Use of engines
![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/cc/Ash-25.jpg/220px-Ash-25.jpg)
To avoid the inconvenience of landing out some gliders have a small engine and a retractable propeller, but these add to the weight and expense of a glider. "Self-launching" gliders have engines that are powerful enough to launch the glider unaided. In "self-sustaining gliders" these engines are not powerful enough for launch, but can provide enough power to climb slowly and return to the home airfield. However, engines have to be started at a height that includes a margin that would still allow a safe landing-out to be made, if the engine were to fail to start.[47] In a competition, starting the engine is regarded as ending the flight. Gliders without a retractable engine are lighter and, as they do not need a safety margin for an engine-start, they can safely thermal much lower in weaker conditions. Consequently unpowered gliders may complete competition flights when some powered competitors cannot.[48]
Touring motor gliders have a non-retractable propeller. Since the additional drag reduces their performance, they are seldom used in competition. They can, however, be useful in training for cross-country flights.[49] After take-off, the engine is switched off and the pupil pilot flies the aircraft as a glider, while learning to navigate. Landings in unfamiliar fields can be practiced while the motor idles. If the pupil chooses an inappropriate field or misjudges the approach, the instructor can wait until the last moment before applying power and climbing away.
Aerobatics
Aerobatic competitions are held regularly.[50] In this type of competition, the pilots fly a program of maneuvers (such as inverted flight, loop, roll, and various combinations). Each maneuver has a rating called the "K-Factor".[51] Efficient maneuvers also enable the program to be completed with the height available. This number of points is given if the maneuver is flown perfectly, otherwise a number of points is subtracted. The winner is the pilot with the most points.
Hazards
Gliders, unlike hang-gliders and paragliders, surround the pilot with a strong structure, so most accidents cause no injuries,[52] but there are some hazards.[53] Even though training and safe procedures are central to the ethos of the sport, a small number of fatal accidents occur every year, almost all caused by pilot error.[52] In particular there is a risk[54] of mid-air collisions between gliders because the pilots tend to fly to the same areas of lift. To avoid other gliders and general aviation traffic, pilots must comply with the rules of the air and keep a good lookout. They also usually wear parachutes. In Switzerland, Austria and Germany, the FLARM warning system is used to avoid mid-air collisions between gliders.[55] Collisions with commercial aircraft are unlikely because glider access to the controlled airspace used by airliners is tightly restricted.
Challenges for the gliding movement
Gliding as a sport faces challenges in the years ahead.[56] These include:
- Time pressures on participants: gliding typically takes whole days, which many people today find harder to devote. As a result the average age of glider pilots is getting older
- Airspace: in many European countries the growth of civil aviation is reducing the amount of uncontrolled airspace
- Competition from other activities: there is now a greater variety of similar sports such as hang gliding and paragliding that may attract potential glider pilots.
- Lack of publicity: without coverage by television, many people are unaware of competitive gliding.
- Increasing bureaucracy.
Learning to glide
![](http://upload.wikimedia.org/wikipedia/commons/thumb/0/07/Blanik_3_a.jpg/220px-Blanik_3_a.jpg)
Most clubs offer trial lessons to people interested in learning to glide. National gliding associations have contact details for their member clubs. The pupil flies with an instructor in a two-seat glider fitted with dual controls.[57] The instructor does the first launches and landings but otherwise the pupil uses the controls. Some clubs offer courses over several days, though, with a mixture of winch and aerotow launches, it often takes ab initios at least 50 training flights before they are allowed to fly solo.[58]
If winches are used, the cost of learning to glide is much less than that of learning to fly powered aircraft. Training using aerotow costs more, even though fewer launches (as few as 30) might be needed. Simulators are also beginning to be used in training, especially during poor weather.
Early solo flights are restricted to within gliding range of the airfield. Further training continues after the first solo until the pupil is judged capable of taking a glider cross-country. Pilots must also familiarize themselves with the regulations, use of the radio, weather and navigation.
Related sports
Hang-gliding uses simpler and cheaper aircraft in which pilots exercise control by shifting body weight, whereas glider-pilots use conventional control columns. Hang-gliders use fabric wings, the shape of which are determined by frames. The lower aerodynamic efficiency of these wings means that shorter distances are flown than in gliders. Unlike the hang-gliders' wings, paragliders' wings have no frames and their shape is entirely formed by the pressure of the air. The aerodynamic efficiency of paragliders is lower still and so cross-country flights are even shorter. Radio-controlled gliding uses scale-models of gliders mainly for ridge soaring.
References
- ^ "Frequently asked questions about gliding" (PDF). Retrieved 2006-08-24.
- ^ "Information about gliding competitions" (PDF). Retrieved 2006-08-24.
- ^ "History of gliding" (PDF). Retrieved 2006-08-24.
- ^ "Wasserkuppe, gliding and model gliding". Retrieved 2006-09-28.
- ^ a b c d e f g Welch, Ann (1980). The Story of Gliding 2nd edition. John Murray. ISBN 0-7195-3659-6.
- ^ "FAI the Olympics". Retrieved 2006-08-24.
- ^ "List of pilots who have flown over 1,000km". Retrieved 2006-09-28.
- ^ "FAI membership summary". Retrieved 2006-08-24.
- ^ "FAI membership summary". Retrieved 2006-08-24.
- ^ "Visual explanation of soaring". Retrieved 2006-08-24.
- ^ "Mountain flying". Retrieved 2006-09-14.
- ^ "Altitude record". Retrieved 2006-09-01.
- ^ "Diagram of thermals". Retrieved 2006-09-05.
- ^ "Diagram of ridge lift". Retrieved 2006-09-05.
- ^ "Duration record". Retrieved 2006-08-24.
- ^ "Article about wave lift". Retrieved 2006-09-28.
- ^ "Diagram of wave lift". Retrieved 2006-09-05.
- ^ "Distance record". Retrieved 2006-08-24.
- ^ "Morning Glory". Retrieved 2006-09-27.
- ^ Bradbury, Tom (2000). Meteorology and Flight: Pilot's Guide to Weather (Flying & Gliding). A & C Black. ISBN 0-7136-4226-2.
- ^ Reichmann, Helmut (2005). Streckensegelflug. Motorbuch Verlag. ISBN 3-6130-2479-9.
- ^ "Further information on launch methods". Retrieved 2006-09-03.
- ^ "Aerotowing explained". Retrieved 2006-09-03.
- ^ "On-line debate on the relative merits of high tow versus low tow and where each method is used". Retrieved 2006-10-09.
- ^ "Further information on launch methods". Retrieved 2006-09-03.
- ^ "Bungee launching explained". Retrieved 2006-09-28.
- ^ "Autotow launching information and discussion". Retrieved 2006-09-28.
- ^ "How gliders fly cross country". Retrieved 2006-09-28.
- ^ "FAI World records page". Retrieved 2006-09-06.
- ^ "On Line Contest". Retrieved 2006-08-24.
- ^ "Introduction to gliding competitions". Retrieved 2006-09-28.
- ^ "Typical competition results". Retrieved 2006-08-24.
- ^ "How competitions are monitored and scored". Retrieved 2006-09-28.
- ^ "Sailplane Grand Prix". Retrieved 2006-08-24.
- ^ "On Line Contest". Retrieved 2006-08-24.
- ^ "Listing of lowest ranked participants in the On Line Contest". Retrieved 2006-08-24.
- ^ "MacCready Theory". Retrieved 2006-08-24.
- ^ Pettersson, Åke (Oct–Nov 2006). "Letters". Sailplane & Gliding. 57 (5). British Gliding Association: 6.
{{cite journal}}
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(help)CS1 maint: date format (link) - ^ "How gliders fly cross country". Retrieved 2006-09-28.
- ^ a b "Water ballast". Retrieved 2006-09-28.
- ^ "Reference to Tabery's first use of water ballast" (PDF). Retrieved 2006-10-09.
- ^ a b >Eckschmiedt, George (Feb/Mar 2004). "A Modest Proposal". Free Flight. 2004 (1). Soaring Association of Canada: 8.
{{cite journal}}
:|access-date=
requires|url=
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ignored (|author=
suggested) (help) - ^ "FAI Badges page". Retrieved 2006-09-07.
- ^ "FAI Sporting Code". Retrieved 2006-08-24.
- ^ "Cross country flying and landing out". Retrieved 2006-09-04.
- ^ "Code of practice for field landings". Retrieved 2006-09-04.
- ^ "Information about self-sustaining gliders". Retrieved 2006-09-04.
- ^ "Guide to Self-launching Sailplane Operation". Retrieved 2006-09-04.
- ^ "Motor gliding training syllabus for instructors". Retrieved 2006-09-04.
- ^ "Information about gliding aerobatics". Retrieved 2006-08-24.
- ^ "FAI Aerobatics Catalogue" (PDF). Retrieved 2006-10-09.
- ^ a b Every, Douglas (Oct/Nov 2006). "Accident/incident Summaries". Sailplane & Gliding. 57 (5). British Gliding Association: 61.
{{cite journal}}
:|access-date=
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(help) - ^ "How safe is gliding?". Retrieved 2006-09-28.
- ^ "Analysis of serious and fatal gliding accidents in France". Retrieved 2006-09-28.
- ^ "Summary of collision avoidance techniques". Retrieved 2006-10-09.
- ^ "Challenges facing gliding reported to FAI". Retrieved 2006-09-18.
- ^ "Learning to glide". Retrieved 2006-09-18.
- ^ "Information about learning to glide". Retrieved 2006-08-24.
Further reading
- Longland, Steve (2001). Gliding: From Passenger to Pilot. The Crowood Press Ltd. ISBN 1-8612-6414-3.
- Piggott, Derek (2002). Gliding: A handbook on soaring flight. A & C Black. ISBN 0-7136-6148-8.
- Stewart, Ken (2003). The Glider Pilot's Manual. Air Pilot Publisher Ltd. ISBN 1-8433-6078-0.