The “Possible” Turn

By lex, on February 15th, 2010

Yesterday we briefly touched on the issue of the “impossible turn” back towards a runway after a low altitude engine failure. AOPA has pretty much endorsed the notion that turning back to the field from which you recently departed is, in fact, impossible using the sterling example of US Airways Flight 1549 and Captain Sully:

Pilots seeing images of US Airways Flight 1549 floating in the Hudson River probably shared three thoughts: those pilots did everything right; I hope I could do it right if I ever had to; I hope I never have to.

Bruce Landsberg, executive director of the AOPA Air Safety Foundation, says general aviation pilots can learn valuable lessons from the airliner’s amazing emergency landing and increase their odds of a successful conclusion to any emergency.

First, he says, pilots need to remember that the “impossible turn” back to the runway really is impossible.

“If you have an emergency shortly after takeoff, the call of the runway behind you can be extremely strong,” Landsberg said. “But time and again we see pilots desperately trying to make it back stall and spin into the ground instead. Look for the best option within a few degrees of your flight path.”

But David Rogers, a former Naval Academy professor of aerodynamics says that the impossible turn might just be possible after all, depending upon a myriad of factors. The keys, he says, are bank angle control, airspeed management and coordinated flight:

Fundamentally the turnback maneuver is an energy management problem. When the engine quits, the aircraft has a total energy equal to the sum of its kinetic energy (due to its velocity) and its potential energy (due to altitude). This energy must be continuously expended to overcome drag and maintain flying speed. The problem is to optimally expend the available energy while executing the turnback maneuver.

Using this approach a long time colleague Professor Bernard `Bud’ Carson developed a theoretical solution for a decelerating descending turn that showed that a bank angle of 45 degrees at stall velocity is the optimal turnback maneuver. Why 45 degrees at stall velocity? Simplistically, 45 degrees, because here equal amounts of the lift are being used to support the aircraft against the pull of gravity and to turn the aircraft. At stall velocity, because the lower the velocity the smaller the turn radius and consequently the less time spent in the turn. Coincidentally, the smaller turn radius keeps you closer to the field and turns you around faster. After completion of the turn the aircraft is accelerated to best glide speed by lowering the nose.

It’s easy to see why this approach has led to so many stall/spin fatalities when attempted by invariably surprised aviators who have suffered a loss of power soon after take-off. Flying an unpowered aircraft right at stall in a 45-degree angle of bank turn will generate some pretty eye-opening sink rates – up to 1000 FPM, according to Dr. Rogers. You were never very high to begin with, and now the earth is coming up at more than twice its accustomed rate. The temptation to further stretch the glide – and enter a spin prone angle of attack – will be severe. Just a little not enough rudder to keep the inboard wing flying and you’ll be belly-side up.

I’m not currently a Certificated Flight Instructor, and even if I were, I don’t think I’d teach this to any of my students. But I may just climb a rental up to safe altitude some leisurely Sunday afternoon to see what this feels like.

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Filed under Best of Neptunus Lex, Carroll "Lex" LeFon, Carroll LeFon, Flying

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