Everyone has their favorite altitude number for "X", which is a factor of many variables, including aircraft engine off glide ratio and density altitude. Usually it's a comfortably conservative number, say 1000 feet minimum AGL.
There's a good reason for that cautionary buffer zone of course: many pilots - and passengers - have died trying to make the killer turn back to the airport from too low an altitude.
In an attempt to demystify the infamous "Impossible turn", AOPA online managing editor Alyssa J. Miller goes about the worthy business of investigating firsthand just how high one should be above launch airport altitude to feel safe about turning back for that oasis of engine-out safety: the runway.
It's all based on aviation journalist Barry Schiff's Impossible Turn Maneuver Checklist, also replicated in the article. Miller's goal is to "find out how much altitude you need to turn around safely—not to try to turn the aircraft around in a pre-set amount of altitude."
It's an important distinction, that difference between knowing the absolute minimum altitude you'll need vs. having a mindset of "must turn in 500 feet" to wrestle with.
And she does a service thereby for all of us by accumulating some real world numbers.
Taking wing, she climbed to a safe altitude with CFI Sandy Geer, then recorded several repetitions of:
<> simulated engine failure on takeoff
<> stabilizing to best glide speed
<> turning 270 degrees (a turn left or right dictates you will need more than just 180 degrees to line back up with the runway)
<> flaring, as if performing a landing
<> then recording the total altitude lost from pulling the power.
Miller's best altitude loss number was 300 feet in a Cessna 172! The average altitude lost for the entire group of simulations was between 300 and 500 feet. I'm itching to try this in an LSA...especially a motorglider.
After discussing their efforts, they each settle on a minimum above ground comfort altitude: CFI Reed's is 1000 feet AGL, while Miller says she might consider 750 feet her personal minimum.
Both note that in a true emergency situation any number of distractions will lead to greater altitude loss, or as we say Webside: YRMV (Your Results May Vary).
It's a thought-provoking read, with a helpful accompanying video.
At Sebring next month, I'm going to add this maneuver to my pilot report flight list, which should also give me some interesting comparison figures between different models of LSA, since all flights will take place from the same airport.
Meanwhile, you can check out the article here.