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We talked today in our flight school about the following scenario: if I climb in my 737-800 to FL100 and then have a dual engine failure, how many seconds would it take me before I reach ground?

I'm assuming:

  • I want maximum glide time (not distance)
  • I'm within 10NM of the departure airport
  • Flaps 15
  • Gear extended
  • No speedbrakes
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    $\begingroup$ It depends. Do you want to be on the ground as soon as possible, or stay in the air as long as possible? "I want to return to the airport". How far away is it? What bearing are you flying relative to it? $\endgroup$ – Simon Jul 10 '17 at 20:41
  • $\begingroup$ Stay in the air as long as possible, 10NM out and flying standard traffic circuit pattern $\endgroup$ – sesc360 Jul 10 '17 at 21:08
  • $\begingroup$ Your question contradicts your assumptions (and vice versa). If you want to know how long you can stay in the air, why does it matter how close you are to an airport? OTH, If you want to know if you can make it to an airport 10NM away from an altitude of ten thousand feet in this configuration, that's a different question with a different answer. Please clarify what you are asking. $\endgroup$ – bclarkreston Jul 11 '17 at 2:37
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    $\begingroup$ RalpJ's answer addresses these concerns and variables, what's the problem with the question? Why the down votes? Are we accountants here or aviators? $\endgroup$ – Koyovis Jul 11 '17 at 4:13
  • $\begingroup$ @sesc360 If you want to stay aloft for as long as possible, you should fly at the maximum endurance speed for that configuration of flaps 15º and landing gear down. You can find that speed by observing the variometer at different glide airspeeds. The maximum endurance airspeed is the one that gives the minimum descent speed... $\endgroup$ – xxavier Jul 11 '17 at 6:52
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For reference consider TACA Air Flight 110, a 737-300 that lost both engines at FL165, on approach to New Orleans in a very heavy hail/thunderstorm.

No exact listing of how long 110 stayed in the air with no power, but the Mayday/Air Disasters show on this incident hints that it was in the vicinity of 13-15 minutes, which indicates a drop rate of around 1100-1300fpm.

A heavier example was BA flight 9 a 747 that lost all four engines due to flying into a cloud of volcanic dust at night at FL370. This led to the classic announcement:

Ladies and gentlemen, this is your captain speaking. We have a small problem. All four engines have stopped. We are doing our damnedest to get them going again. I trust you are not in too much distress.

Fourteen minutes later at FL135, they got a restart. At that descent rate, they would have had maybe another nine minutes before hitting the ocean.

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Your assumptions are incompatible with your stated goal of maximum duration.

For the longest time in the air, you want minimum drag, so gear down and flaps 15 isn't what you'd want. It's a very rare day to have that configuration at 10,000' under any circumstances. To be configured that high would mean that you wanted to come down quickly.

In a clean configuration, a 737 NG will fly at flaps up maneuvering speed & idle thrust at about 1100-1200 feet per minute. Engines off & windmilling will add some drag, but the effect of that is probably considerably less than the difference between a nearly empty 737-800 with 10k of fuel, and a full 737-800 with lots of fuel. You didn't specify a weight, and I don't have a simulator handy to get an exact number anyway, so let's say 1200 feet per minute from 10,000' down to about 2000'. That's about 6 to 7 minutes there.

Then a level decel segment to slow and configure. With no thrust, from flaps-up maneuvering speed down to a flaps 15 v-Target, that's at least one minute, probably a bit more.

Then the final segment, which would be the trickiest part, since your glide angle would be steeper than what you'd typically fly, with no ability to correct if you get shallow & slow. I'm guessing here, but probably a vertical speed of 1500 feet per minute with flaps 15 & gear down and zero thrust.

Add all that up, you have ballpark of 8 to 10 minutes from 10,000 to a sea level landing, which includes time to slow and configure at a sensible point.

If you insist on starting already configured at 10,000 feet, then it's probably more like 6 minutes.

Starting at 10,000' and 10 miles from your departure airport, you have ample time and distance to set up for a landing -- the NG climbs much steeper than it glides.

"Good luck, we're all counting on you!"

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