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Let's say that you're in a 737, mid-cruise in a non-ETOPS situation, and you lose one engine. Are there any FARs governing how long you need to be able to stay airborne? Or is all that's required the ability to continue safely and land at nearest appropriate airport? Are you required by any regulation to be able to fly for an X number of hours before landing?

I couldn't find anything in the FARs about this, only regulations regarding ETOPS and engine out climb. Just want to make sure I'm not missing anything. Thanks!

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    $\begingroup$ None that I know of, most FAR's deal with being able to climb out on a single engine if one fails during takeoff (for twins, more engines have different requirements). Most twin engine airliners can fly just fine on a single engine, but probably not at cruise altitude and not for an extended flight. It is unlikely though that the pilot would elect to continue the flight unless the destination was closer than an alternate. $\endgroup$
    – Ron Beyer
    Feb 27, 2017 at 21:33
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    $\begingroup$ Note that loss of pressurisation is always worse (regarding diversion options) than loss of engine, because loss of pressurisation forces you down to 10,000 ft and that is way less efficient flying—and there is much more terrain that limits your routing there—than at single engine drift-down altitude, which is much higher. $\endgroup$
    – Jan Hudec
    Mar 1, 2017 at 21:42

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No, nothing addresses that case.

Let's say you're 20 minutes out from your destination, and because it's a clear VFR day you have minimum normal reserves -- roughly an hour of flying (on 2 engines, at medium-to-low altitude). Those reserves are already prescribed for you by FAR's. Now you lose the engine. How long can you fly? Until you run out of fuel or the other engine quits -- so in all probability, about that same hour, plus or minus.

Different case, you're over at Atlantic heading to Bermuda or the Azores or Iceland or NYC to the Caribbean, or over the Gulf of Mexico -- not ETOPS but a long ways from anywhere. The fuel on board will be lots more, because you were planning for a much longer flight. (And again, those fuel requirements are defined elsewhere.) You now have the added twist of drifting down on one engine to a single-engine ceiling -- you set max continuous thrust on the good motor, and descend at a speed that gives you max range, which probably comes out to a few hundred feet per minute of descent. Back in the flight planning, the check had to be performed that at the worst case, you could lose the motor, drift down, and continue to reach a suitable airfield without needing more gas than you'd have. But in something like a 737, the gas you'll have on-board anyway is almost certainly more than sufficient.

But in this latter case, the time you can stay aloft is going to be hours -- way beyond what you had in the first case.

So no, the regulations don't prescribe a time that you have to be able to keep flying after losing one engine. You DO have to be able to keep flying -- that's important -- and you have to have enough fuel to get to a destination with the one motor shut down. But in terms of a prescribed time, that isn't given in the FAR's.

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    $\begingroup$ To further clarify, if nothing prevents continuing to fly a twin with an engine out, is there any altitude requirement -- e.g. FAR or airworthiness limitation -- to fly at some minimum altitude, perhaps giving possibility to glide to a runway if the second engine fails? $\endgroup$
    – mins
    Feb 28, 2017 at 8:13
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    $\begingroup$ @mins No, the same rules govern altitude as with everything operating normally. As a matter of fuel economy, the aircraft will tend to remain high until it's time to start down for a landing, but it's not really about being able to glide to a runway in case the other engine quits. Engines on modern jets fail so rarely that you're already in a very rare case if 1 quits; the second motor quitting is a rare^2 case; if you glide to a runway, that's great, but dual engine failure isn't really a trained scenario, the way losing 1 motor is. $\endgroup$
    – Ralph J ♦
    Feb 28, 2017 at 19:59
  • $\begingroup$ @RalphJ: A double engine failure isn't quite a rare^2 case; given that a lot of engine failures are from things that affect both engines equally (fuel exhaustion/starvation/contamination, volcanic ash ingestion), the chance of having the second engine fail given that you've already an engine failure is much greater than the chance of having an engine failure in the first place (because, if one engine fails, there's a good chance that whatever did it will also take out the second engine in a minute or two). $\endgroup$
    – Vikki
    Sep 28, 2019 at 0:31

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