Managing Flying Risk – Aerotow launching
It is important that all pilots who aerotow launch are aware of the detail contained on the ‘Safe Aerotowing’ webpage.
The dominant hazard, potentially fatal for the tug pilot, is a vertical tug upset initiated by the glider pilot moving up too quickly from a (safe) low position and inadvertently accelerating upwards and tipping the tug nose steeply down, or by the glider pilot turning at release height without having confirmed visually that the rope has separated.
Five tug pilots were killed in 24 reported tug upsets between 1978 and 1985. Following a long period with a low tug upset rate, the rate has increased dramatically in recent years. There were 7 tug upsets in the 12 months to Sept 2019.
ALL tugs are vulnerable to an upset, but light tugs are more vulnerable.
The following factors may cumulatively contribute to a hazardous situation. Where more than one item is present, advice should be sought before launching:
- Low experience of glider and/or tug pilot
- Gliders fitted with C of G hook only
- Glider’s C of G towards the aft limit
- Turbulent air in the take-off area
- Rough ground in the take-off area
- Significant crosswind component
- Short rope
- Light-weight glider, low wing loading
If you are the GLIDER PILOT;
If you are inexperienced, do not aerotow on a belly hook and do not aerotow in turbulent
Fly the glider and stay focussed on the tug. Maintain the correct vertical position of the tug in the canopy. Do not allow the glider to get too high.
If you are too low behind the tug shortly after the tug take off, or at any other time, move back into position. Being lower than the tug is not dangerous, but an upset can follow if you pull up quickly.
Release immediately if the glider is going high and the tendency cannot be controlled, or you lose sight of the tug.
Fly the glider and stay focussed on the tug. Leave any potentially distracting problems with instrumentation or ventilation until after release. Leave the undercarriage down.
At release height, is it clear? Locate and pull the release, visually ensure the rope has separated from the glider, and raise the nose slightly before making a turn.
If you are the INSTRUCTOR;
Avoid a potential upset by retaining adequate safety margins when teaching boxing the tow or recovery from being out of position, especially with lightweight tugs.
Be prepared to take control before the situation becomes dangerous. Stay focussed on the tug.
If you are the TUG PILOT;
Anticipate a possible upset from the glider changing position. Release the glider IMMEDIATELY if your nose is forced
Before launching, look for cumulative hazardous factors as listed above. If in doubt, do not launch.
Aerotow Rope and Weak Link
A long aerotow rope is safer for the tug pilot. 180 feet is a reasonable compromise. A shorter rope may be used if required. A weak link must be fitted to protect the aircraft from excessive loads. The maximum load allowed for a tug end weak link is specified in the tug operating manual or towing supplement. The BGA Aerotowing notes provide more guidance for clubs and pilots.
An aerotow requires teamwork between the glider pilot and the tug pilot. Although the tug pilot is in command of the combination, it is incumbent on both the tug pilot and glider pilot to ensure their aircraft will have adequate performance for the proposed flight. More details are available in the Aerotow Performance safety leaflet.
Cartwheeling accidents – predominantly to experienced pilots – happen as a result of not releasing the cable if the wing drops during the ground run. Cartwheel accidents have occurred during aerotow launches. If the wings cannot be kept level before take-off, release before the wing touches the ground.
Glider handling on tow
The handling of many gliders on aerotow is inferior to the handling of the same glider in free flight. Gliders with a large span may be particularly susceptible. Inferior handling on tow arises because the downwash behind the tug wing strikes a similar span of glider wing and reduces its angle of attack. The glider pilot has to increase the angle of attack of the whole wing in order to generate adequate lift. This action increases the angle of attack of the outboard part of the glider wing, with the result that the stability advantages of washout are diminished or lost, and the glider wing tip can stall before the root.
The severity of the effect varies between different glider-tug combinations and that needs to be considered during the eventualities brief; the achievable speed could initially be limited by the available take-off run, headwind and grass or runway conditions. And there are sometimes very few options if the glider becomes uncontrollable in the early part of the climb-out. The solution is to fly faster – at least the minimum recommended towing speed for the type of glider and weight – and on a suitably long rope.