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Last week, I had my first lesson in a 737 sim. The instructor, when talking about risks with flying the 737, said the only time it would be "game over", would be if you had a double engine failure on takeoff. He had only ever heard of this happening once.

I asked what the minimum altitude you would need to glide back to the airport and land on a runway would be. He wasn't prepared to give an answer, but when I asked if 15k ft would be enough, he said probably not.

Could anyone answer this for me? Any pilots tried this on a sim and know first-hand?

Cheers!

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    $\begingroup$ Wouldn’t that depend on the distance to the nearest suitable airport and the glide ratio? Or is there something special that makes the B737 un-glideable? $\endgroup$
    – StephenS
    Sep 7, 2021 at 15:44
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    $\begingroup$ Hi @StephenS I'm talking about returning to the takeoff airport $\endgroup$
    – Cloud
    Sep 7, 2021 at 15:45
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    $\begingroup$ So you’re looking for a rule like 1000ft to make the “impossible turn” like we have for GA planes? $\endgroup$
    – StephenS
    Sep 7, 2021 at 15:51
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    $\begingroup$ 15,000 ft is definitely enough! I cannot answer what the minimum would be, but in this Mentour pilot video they are returning successfully in a full motion sim after dual engine failure at 6200 ft. $\endgroup$
    – Bianfable
    Sep 7, 2021 at 16:06
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    $\begingroup$ The instructor was pulling your leg. All depends on what the effective climb gradient is, but assuming you are able to climb anywhere near the gradient in JohnK's answer, 15k is pleeeeeeeenty. $\endgroup$
    – Jpe61
    Sep 8, 2021 at 8:07

2 Answers 2

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Most modern airliners have L/Ds of 18 to 22:1 (sorry I couldn't find a direct reference). So from one nautical mile, 6076 ft, in the air you can glide around 18 to 22 nm in still air.

Departure climb gradient at, say 4000 fpm, a typical both engines climb rate, which is about 40kt vertical speed, going 200 kts horizontally, is about 5:1.

So you are climbing at a gradient of 5:1, but can glide at a gradient of 4 times flatter, at 20:1.

So, being able to make it back depends. In the case of a departure and a immediate climb to enroute altitude, with no level off intervals, you should have no problem returning to the airport after a double flame out at, say, 18000 ft, because you'll only be about 15-20 nm from the airport, and from 3 nm up you can glide 50-70 nm, and you should have ample space to turn around and maneuver.

On the other hand, if ATC (or the SID you are flying) makes you level off for a period at, say 4000 ft, and keeps you there for several minutes, and then you have a double flame out, you will likely be out of range. If you're only at 4000 ft, your absolute gliding distance is only 13 nm, not allowing for manuevering, and by then you might be 15 miles out. You'll be heading for the river, like those other guys did.

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    $\begingroup$ The main problem will be that you're pointing the wrong way, so you also need energy for the turn (which involves pointing your lift slightly sideways, so you're going to lose height there). $\endgroup$
    – Simon Richter
    Sep 9, 2021 at 13:38
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    $\begingroup$ Like I said, "space to turn around and maneuver"... $\endgroup$
    – John K
    Sep 9, 2021 at 13:44
  • $\begingroup$ @JohnK Simon didn't say space, they said loss of height. A 25 degree bank means your lift drops your upward force by 10%. On top of that, a powered turn is like 24 nmi ( aviation.stackexchange.com/questions/8012/… ). And that doesn't get you pointed back at the airport, it gets you pointing parallel to it 15 nmi away. At 3nmi up and 20 nmi from airport, ignoring height loss from turn (!), it is 23 nmi to turn around and now you are 25 nmi away, so 48 nmi of distance. I think you aren't gonna make it? Or did I do a math error? $\endgroup$
    – Yakk
    Sep 9, 2021 at 17:23
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    $\begingroup$ @Yakk Handy calculator here: csgnetwork.com/aircraftturninfocalc.html A 45 deg banked turn (roughly what gives you the lowest altitude loss) done at 230kt has a 1.5 nm diameter, and will take about 40 seconds to turn 180 deg, so I don't know where you got all that. Say you're descending at 3000 fpm while crankin' and bankin'. You'll come out of a 270 deg turn in about a minute, and will find yourself at about 15000 ft, maybe 22 or 23 miles away, still with 60 miles of potential gliding distance. The problem is getting down, not making it back. Lots of flying around to lose alt. $\endgroup$
    – John K
    Sep 9, 2021 at 22:59
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First, the actual "impossible turn possible" altitude number is going to vary dramatically by factors such as hot, high, heavy, wind, how wind interacts with your departure path, and whatever the damage did to the airplane.

This question was extensively examined on the very similar Airbus A320, after Chelsey "Sully" Sullenberger had a live-fire exercise on double-engine failure, over New York City, and elected to put it in the Hudson River rather than attempt "the impossible turn" (which would left poor choices in the event of falling short).

Extensive post-mortem study of the accident revealed most pilots in Sully's situation could indeed have accomplished a >180 degree turn back to Laguardia, but only if they had precognition of the extremely unexpected double-engine failure, and started their turn the instant the birds struck. I won't discuss that further.

The point is: this gives us a pretty good frame of reference: Sully was at 2,818 feet at the time of the bird strike when a return to LaGuardia was possible.

For an A320 in NYC in January... the very opposite of "hot and high". With the wind Sully had that day. If Sully's departure had been in the other direction toward Boston, all bets are off.

Further, it would have been a very different picture if an aerodynamic cowling had been knocked off an engine.

enter image description here

That thing is pretty much a barn door. This would have impact on an aircraft's glide range.

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    $\begingroup$ I'm no expert, but that engine... that's Not Good™ $\endgroup$
    – FreeMan
    Sep 8, 2021 at 18:25
  • $\begingroup$ But nice to see an intact fence between fan bits and pax. Something worked, at least. $\endgroup$
    – user_1818839
    Sep 9, 2021 at 14:06
  • $\begingroup$ @user_1818839 oh, that fence seems to pretty much always work. The problem is, fan bits won't always throw themselves futilely against that line of defense. Sometimes they unexpectedly detour via the Ardennes Forest and Belgium. $\endgroup$
    – Harper - Reinstate Monica
    Sep 16, 2021 at 23:27

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