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Twins - engine failure / EFATO (merged)

Why would the yaw damper be off? Is the rule to leave it off in a twin until at a safe height so you don’t lose your “engine out indicator” (i.e. your feet)?

Last Edited by Rwy20 at 25 Jun 23:32

I think it is why recurrency and the annual check ride are so important. There is no doubt handling an engine out takes a while to grow accustom to, but in time the signs become familiar, the reaction controlled and precise, the experience a lot less frightening, and the event – well much less. Then you dont want a marginal twin – what is the point? There is no substitute for performance – and then performance. I am also not a fan of any lack of redundancy in vital systems. I am not sure why you would design a twin around power packs that only work off one engine for example? You dont need any more diversions, just concentration on the task in hand.

The only thing I KNOW about ME engine failures is that statistically, they occur TWICE as often as SE engine failures …

FAA A&P/IA
LFPN

Rwy20 typically yaw damper goes on at 400’ AGL , arguably still in the EFATO phase although you should be comfortably above most obstacles in the departure path.

Michael, actually statistically they occur more than twice as you have the scenario of a double engine failure in a twin, which obviously is impossible in a single.

Enstone (EGTN), Oxford (EGTK)

Michael, actually statistically they occur more than twice as you have the scenario of a double engine failure in a twin, which obviously is impossible in a single.

I am still trying to get my head around that one Stats was never my strong point.

There is actually a valid discussion on whether an engine alone failure is more likely in a twin. Maybe a new thread? Views on this include

  • longer control runs
  • longer fuel system runs
  • more vibration (the wing is less rigid than the firewall on a SEP)
  • mission profile differences (more exposure to fuel system / air system icing)
  • this one
Administrator
Shoreham EGKA, United Kingdom

RobertL18C wrote:

Michael, actually statistically they occur more than twice as you have the scenario of a double engine failure in a twin, which obviously is impossible in a single.

I am not sure this is correct. A double engine failure is just two single engine failures happening at the same time. Should be caught in doubling the se failure rate no?

EGTK Oxford

If I understand the argument correctly, then I would say a twin’s engine failure rate is slightly less than double that of a SEP, all things equal and assuming that an engine failure is an independent risk of equal likelihood in a SEP and a MEP (which probably isn’t true). Because after the first engine fails, the SEP won’t fly for very long, while the MEP continues on one engine. But during this single-engine time, its risk of an additional engine failure is actually half the risk as before (and equal to that of a SEP).

In other words, you should always fly a MEP on one engine to reduce the risk of engine failure. Then after it fails, fire up the other engine. Or did I get something wrong again?

Robert is correct although I think he means “less” not “more”. You cannot simply add together the probabilities unless they are mutual exclusive. You have to add the probabilities of each and then deduct the probability of both.

But it is easier to understand if you multiply the opposite probabilities.

Let’s say you have two engines with a 10% chance of failure. Then chance of each one not failing is 90%.

Therefore the chance of them both not failing is 0.9*0.9 equals 81%.

And therefore the chance of either engine failing is 19%, not 20% as you might first think.

Equally, if you have a 4 engine aircraft with engines that each have a 25% chance of failure, you do not have a 100% change of a failure!

Last Edited by at 26 Jun 10:51

ortac wrote:

Robert is correct although I think he means “less” not “more”.

That’s a pretty important difference!

ESKC (Uppsala/Sundbro), Sweden

A nice Sunday so will not work out the probabilities, but no, Michael is correct, a twin has twice the probability of an engine failure than a single by virtue of having two engines, all things been equal (Peter’s comments apply, which suggest that twin engines may be subject to a higher probability by virtue of their design or useage – for example Peter’s Lyc 540 is boosted to 425 HP on the Pressurised Navajo). You would then add the small probability of a double engine failure which could be due to a common cause, eg fuel contamination in all tanks, or the remaining engine dying on you due to being operated at max continuous while it takes you back to the airport, hopefully.

These scenarios are additive.

Enstone (EGTN), Oxford (EGTK)
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