Tuesday, September 20, 2011
Is the impossible turn, in fact, possible?
Have you heard this? "If your engine fails on early climbout, you have no choice: You can't turn around, and you must continue straight ahead." This is said with impressive seriousness: It's the stuff of life and death. I suggest all who are interested in a scholarly analysis consult The Possible 'Impossible' Turn by David F. Rogers.
Safe turnaround altitude forms a cornerstone of my analysis of the initial climb segment. Faced with a low-altitude engine failure, every combination of pilot and single-engine airplane can have such a number: an altitude below which there's insufficient height to turn around, but at and above which it's not only practical, but advisable, to turn back. Why do so many deny the possibility?
Turnaround rejection stems from a mixture of entrenched opinion, a long history of tragic turnaround failures and reluctance to engage in personal experimentation. Any engine failure requires accurate decision making and prompt action. Low-altitude failures are the most severe, and no one wants to be guilty of giving wrong advice and perhaps making things worse. In consequence, instructors have traditionally embraced the most conservative approach, a "do nothing—just crash straight ahead" policy.
Creation of a safe turnaround altitude is a matter of technique, not belief. In defense of entrenched opinion, two things are certainly true: If you climb at Vy (or even faster) and believe that the best way to conserve altitude in a gliding turn is to use very shallow bank angles, you don't have a safe turnaround altitude. In fact, it's fair to say that no matter how high you climb, 500 or 5,000 feet, you'll never be able to return to your departure runway without power. So, you're stuck: An early engine malfunction will inevitably find you searching anxiously ahead for a suitable place to put down. If you do succumb to temptation and start to turn, you'll quickly run out of altitude and possibly try to stretch the glide, then stall and crash.
Fortunately, straight ahead may not be your only option. But you do need to change two aspects of your technique to alter the outcome. Start climbing at Vx, not Vy. Make your turnaround with a minimum 45-degree bank (60 degrees would be even better).
The entire argument for Vx climbs in single-engine airplanes requires more space than available here. In a nutshell, it's safer to climb as close as possible to the airport (Vx) than to stretch your climb out over a greater distance (Vy). If you insist at climbing at Vy, as you were almost certainly taught in primary instruction, no amount of skill in turning around is going to do much good: Vy climbs maximize altitude gain, but cover too much ground in the process. Experimentation will quickly demonstrate that an increase in climb efficiency is of no benefit if your angle of climb takes you so far from the runway that you can't glide back after a successful turn.
It's obvious that turning by itself doesn't lead to stalls and spins. Failures proceed from attempts to stretch gliding performance beyond what's possible, whether in the turn itself or in the glide that follows. This much is generally acknowledged.
What angle of bank would you use in attempting a low-altitude emergency turn-around? My experience suggests that most pilots naturally assume that mild angles would be best, perhaps 15 degrees or 30 degrees at most, on the theory that steeper banks will consume the most altitude. But if you limit the bank to anything less than 45 degrees, your turnaround is doomed to failure, very likely a worse choice than a straight-ahead crash.
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