7
$\begingroup$

For reference, I fly a Cessna 172.

I've read about a couple of different strategies for the impossible turn (attempting to turn back to the airport after engine failure at low altitude).

The first strategy is the one I've heard most commonly on aviation websites and videos: that is, essentially, maintain a best glide and try to make the turn as quickly as possible without stalling. Unfortunately, a slow turn also means you'll have to turn a little more than 180 degrees to get back to the runway.

The second strategy I've heard is not generally mentioned in the typical safety videos (at least that I've seen). A former flight instructor had mentioned that you could complete the impossible turn by pushing the nose down and using the additional airspeed and downward angle to execute an extremely sharp turn that would keep you closer to the runway; when you come out of the turn, you won't have to worry as much about lining back up with the runway.

Strategy 2 is clearly more risky, at least for someone who has not practiced the maneuver, so that could explain why I don't see it mentioned in most safety videos. I was wondering if anyone here had an opinion on either maneuver (or other options) for executing the impossible turn and, if possible, links to literature explaining it.

Thanks in advance.

Improve this question
$\endgroup$
4
  • 3
    $\begingroup$ Such a maneuver is grossly dangerous. See this question for details. $\endgroup$ – raptortech97 Jan 26 '15 at 5:39
  • 1
    $\begingroup$ Welcome to Aviation.SE! As it's currently written I don't think this question is a good fit for our site. Can you be a little more specific about what sort of opinions you're looking for? Note that this question already has 10 answers, and perhaps might answer your question as well? $\endgroup$ – Steve V. Jan 26 '15 at 6:04
  • 2
    $\begingroup$ There's a reason it's called the impossible turn... the best strategy is to forget there's a runway behind you and find the safest place in front of you to put the plane down without killing yourself. $\endgroup$ – Jon Story Jan 27 '15 at 16:46
  • $\begingroup$ That sounds insane unless you have Tucker/Hoover genes. $\endgroup$ – copper.hat Feb 2 '15 at 5:09
3
$\begingroup$

The already linked question has a link to this study. Its authors seem to have done the calculations and concluded that turn with 45° bank flown on the edge of stall lets you do it with smallest loss of altitude. Which is the point here. Don't forget you still need to glide back to the runway.

As rbp mentions below, flying the turn at edge of stall is extremely dangerous, so in practice you should plan turning around only from altitude from which the turn can be made with limit above stall (and don't forget in 45° bank the stall speed is ~20% higher and quickly increases with more bank) you are confident you can manage to maintain. This increases the altitude above which you should only attempt it but the optimal bank angle remain 45°.

If you used higher bank angle, you'd have to fly faster so you'd still have enough vertical lift to balance your weight. That would mean you'd need more lateral lift (even for the same turn rate the lateral acceleration is proportional to speed) to complete the turn and this lift would come with it's additional induced drag. Drag equals energy loss and the only energy you have at that point is potential energy due to altitude. So such turn would spit you out lower, which is the last thing you need. And note, that this is exactly your second option.

Also note, that due to reasons mentioned by rbp below your margin between stall warning and severe stall is even lower in higher bank, so that's another reason against that option.

Improve this answer
$\endgroup$
9
  • $\begingroup$ I don't give that study credence because in a real airplane an accelerated stall caused by increasing angle of bank at the same airspeed doesn't reliably trigger the stall horn; the wing just drops. You can see the difference in this diagram I made. imgur.com/SCSg94r it shows two ways to stall at the velocity/load. $\endgroup$ – rbp Jan 26 '15 at 14:19
  • $\begingroup$ @rbp: That diagram does not explain why it should make a difference. Stall warning devices sense whether angle of attack exceeded certain value (either pressure port on the leading edge, vane on the leading edge or AoA vane on side of fuselage), not changes in speed and load. I do understand the warning might be biased to higher or lower AoA if the sensor is on one side only and depending whether it's on inner or outer side of the turn, but that effect is mentioned in that study as far as I remember. $\endgroup$ – Jan Hudec Jan 26 '15 at 14:33
  • $\begingroup$ because the load factor can go up in a split second and the stall horn sensor doesn't have time to react before the stall is fully developed. even if you're doing a max performance takeoff stall straight ahead, the amount of time it takes to lose the airspeed is enough to make the horn go off, so students mistakenly think the horn is the first indication. the easiest way to experience this is to do a Vne-5 dive, and pull up quickly into an accelerated stall. go out and try it. stay coordinated :) $\endgroup$ – rbp Jan 26 '15 at 14:37
  • $\begingroup$ Ok, I can understand that. It would mean the pilot has to learn to tighten the curve very gently to avoid changing the load too fast, right? Well, it does say it took some training to be able to make the turn without stalling. $\endgroup$ – Jan Hudec Jan 26 '15 at 14:42
  • 7
    $\begingroup$ my advice is to have a plan for engine failure, and always fly the plan. for example: on the runway, land on the runway. 0-1000 land straight ahead, 1000-2000, make a 180, 2000+ make a pattern. in the aircraft, make the altitude callouts. and fly the plan. $\endgroup$ – rbp Jan 26 '15 at 14:58

Not the answer you're looking for? Browse other questions tagged or ask your own question.