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Gremline Flight Safety Report: Fatal Winch Launch - Jantar Glider

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the gremline digest —  a fatal glider winch launch

Another Fatal Winch Launch

 The two commonest methods of launching a glider are aerotow and winch. Both require skill from the glider pilot and from either the pilot of the tow plane or the winch driver, if the launch is to be safe and successful. In my opinion, the aerotow launch demands less skill on the part of the glider pilot than does a winch launch. Perhaps this is because I spent most of my “professional” flying as a fighter pilot, so did a lot formation flying. During an aerotow the glider pilot maintains a constant position in relation to the tug and the tug pilot controls the flight up to the release point. During a winch launch the glider pilot has to control both the airspeed of his glider and its trajectory in a steep climb until the release point, almost vertically above the winch. The winch is not visible to the pilot during launch. If the glider pilot adopts a shallow climb during a winch launch he will get a slow launch to a low release height and may not be able to land back at the launch site. If the pilot adopts a very steep climb attitude he will probably overload the safety link in the cable, resulting in a cable break and a loss of all pull from the winch while the glider is at a steep pitch angle and losing inertia very quickly. Neither scenario is ideal. The temptation may be for the glider pilot to pull back on the pole as soon as the glider is off the ground and leave it to the winch driver to control the launch. Pilots who do this are very unpopular with winch drivers and will earn a few well-chosen words of advice from their CFI.

 


A recent fatality highlights the necessity for glider pilots to have an understanding of the forces involved during a winch launch and to follow the Safe Winch Launch Initiative instituted by the British Gliding Association (BGA).
      A Jantar Standard 2 glider was being launched by winch in “unremarkable” weather conditions. The pilot had been gliding for about a year and had recently gained his BGA Bronze qualification. The BGA Bronze award requires a pilot to have flown 25 solo flights including 2 flights that each remained airborne for either 60 minutes after an aerotow to 2000 ft, or remained airborne for 30 minutes after a winch launch. This was to be the pilot’s first flight in the Jantar.
      The winch launch began normally, although one witness saw a slight right wing drop which was quickly recovered. The glider then stayed low for a slightly longer period than normal before it pitched up very sharply when about 100 feet above ground. The left wing dropped and the glider continued rotating to the left, descending rapidly before striking the ground. The pilot was killed.
      The winch driver reported that he saw the glider in plan form earlier than expected and then saw the aircraft arc towards the west. He realised that the glider was not going to recover, applied the winch brake and shut off the winch before driving to the crash site.
      Examination of the wreckage found no reason to suggest that a technical fault was a causal factor. The investigation concluded that the pilot probably applied a larger control input than was appropriate as the glider rotated, resulting in the rapid rate of pitch rotation. The stall and loss of control was unrecoverable given the height available.

 


The AAIB Accident Report discussed stalls during rotation at the beginning of a winch launch. A stall during the transition from takeoff to the main climb on a winch launch may result in the glider rolling uncontrollably. In some cases, the glider has hit the ground inverted, with the cable still attached. A stall during pitch rotation can result in one wing losing lift marginally before the other, causing it to drop. The stalled wing has an increasing angle of attack as it drops, keeping it stalled. The rising wing has a reduced angle of attack, moving it away from the stall and allowing it to produce lift. This induces a rapid rolling moment and can lead to autorotation and a spin.
      The stall speed of a glider increases during rotation in pitch as a larger angle of attack is required to achieve more lift. More lift is required to balance the other forces on the glider and to provide a vertical acceleration into the climb. There are three reasons for this:


1. As the nose pitches up the lift force is inclined away from the vertical and must be increased if the vertical component of lift is to balance the weight of the glider.

2. The pull force on the cable is large, typically 80% of the weight of the glider. At takeoff this force is horizontal, providing the glider’s initial horizontal acceleration. As the nose pitches up during rotation the lift force becomes increasingly opposed to the pull force. The lift must therefore increase if it is to balance this pull force and stop the horizontal acceleration.
3. At the end of the rotation the glider is climbing at about 55 kt, which gives a vertical velocity of about 35 kt. The vertical velocity of the glider must therefore increase during rotation from zero at takeoff to about 35 kt. This requires a force which comes from an increase in lift generated by the wing.


The forces on a glider during rotation may be modelled and the load factor (g) estimated for different rotation rates, pull forces on the cable, climb angles and other variables. This modelling shows the stall speed during rotation is very dependent on the RATE of rotation, i.e. the higher the rate of rotation, the shorter the time in which the glider has to be accelerated vertically from zero to 35 kt. As this requires a greater force from the wings, there is an associated increase in the stall speed.
      The dependence of stall speed on rate of rotation for a Jantar Standard 2 ( with an unaccelerated stalling speed of 34.5 kt and a maximum lift to drag ratio of 39) at climb angles of 10 degrees and 25 degrees, with a cable pull of 80% of the weight of the glider is indicated in the graph below.

 

 The blue line represents the stalling speeds in a 25 degree climb. The red line represents the stalling speeds in a 10 degree climb. Note that the critical factor is the ROTATION RATE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

At the 10 degree climb angle the stalling speed increases from 41 kt at a rotation rate of 7 degrees per second to 56 kt at a rotation rate of 30 degrees per second. (Red line) The corresponding stall speeds at a 25 degree climb angle are 43 kt and 58 kt. (Blue line).
The facts in the above report on the Jantar accident are taken from AAIB Report EW/C2009/05/02 which source is gratefully acknowledged.
      The BGA, who assisted AAIB with this investigation, had previously conducted an analysis of their accident database. They found that a significant percentage of glider accidents occurred during winch launches. To address this they developed the Safe Winch Launch Initiative.
      The initiative consisted of an educational campaign within the BGA community to make glider pilots more aware of the hazards associated with winch launching and this, initially, resulted in a reduction in the accident rate.

 

 

 The Safe Winch Launch is an ongoing initiative by BGA. It is recommended that all glider pilots get a copy of the “Safe Winch Launching” leaflet (downloadable from the excellent BGA website) and study it carefully. Perhaps you already have copies around your gliding club. WELL READ IT! A visit to the BGA website will allow you to watch realistic simulations of what can go wrong during a winch launch. Don’t think it only applies to inexperienced pilots. Experienced pilots are MORE likely to have winch launch accidents.

 

 

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