cwatters wrote:
Basically the message appears to be not to float around the turn slowly trying to preserve height but the get the turn done positively. A slow turn has a larger radius, longer distance to fly therefore potentially a greater loss of height. I don't believe this contradict the advice to fly the turn "just above the stall" because the stall speed will vary with rate of turn.
The BGA training (backed up with the maths and empirical investigation, see e.g. Longland,
Gliding (A&C Black 2002) pp 105-9, though this discusses turning generally and not this specific case) is that the minimum height loss in a gliding turn is at 45 degrees of bank. This is the best compromise between time in the turn and sink rate, for gliders at least, and produces the minimum loss of height in the turn.
So far as speed is concerned, such a turn would normally be flown at at least approach speed (1.3 vs), some draggy types maybe faster. This would allow a (for gliders) comfortable margin above the turning stall speed of around 5-7 kt, so it might be "close to the stall" as discussed in this thread.
I only fly gliders, so can't say how far this translates to a powered a/c with engine failure. My sink rate in such a turn is less than 3 ft per second, so at 50kt I'd expect to lose no more than 50ft if I fly it properly. My guess is that the 45 degree turn is optimal for all a/c forced to become gliders, because the maths is the same, but this is only a guess.
Glider turnbacks would be from a failed aerotow, which would likely be caused by engine failure of the tug aircraft, and this is something which we consider and in some countries train for (though not the UK). The UK briefing is to set a decision height below which you will not turn back, even if you might theoretically make it. For experienced glider pilots that height will be between 200 and 300 ft - this gives a margin of, say 100ft for a hurried turn and 100 to 200 ft to make it back to the airfield. As the climb rate on an aerotow ought to be at least twice the sink rate of a glider, this leaves an adequate margin.
I'd say do the calculations on the ground - a failed engine a/c which has a sink rate higher than its climb rate with the engine working would never be in a position to turn back, even if it could make a no loss turn!