According to Wikipedia:

If no control inputs were made following flameout and the disengagement of autopilot, the aircraft would likely have entered a spiral dive and entered the ocean within 20 nmi (37 km; 23 mi) of the flameout and disengagement of autopilot.

Does this mean the autopilot automatically disengages when both engines experience flameouts? If so, how likely is it it would enter a spiral dive within 37km of the flameout?

Are there any other control inputs the pilot could have made before flameout that would cause the plane to glide until it impacted the water?

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  • $\begingroup$ Re: Helios - Actually the holding pattern exit was attributed by the investigators to a "person in the Captain's seat making an effort to control the aircraft." I've deleted the answer. $\endgroup$ – ymb1 Feb 21 '19 at 17:41
  • $\begingroup$ The pitch stability will be positive, otherways the 777 could not be certified. $\endgroup$ – Peter Kämpf Feb 21 '19 at 18:06

Assuming CG is within limits, the plane is straight, level, and both engines flame out simultaneously, lacking any control inputs, the plane will dive until it reaches its elevator trim speed, then glide at that speed until it impacts. As the directional and roll stability would be fairly good with the 777, without variable cross wind effects or any other yawing tendencies, it would probably be a straight in crash.

This scenario could be tested under controlled conditions.

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  • $\begingroup$ "Lacking any control inputs" would possibly result in a Phugoid oscillation. $\endgroup$ – Bianfable Feb 21 '19 at 16:09
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    $\begingroup$ @RobertDiGiovanni: A phygoid is very weakly damped, so if the trim point is at a markedly different speed than the starting point of the maneuver, a phygoid motion is a certain outcome. It will die down over time but certainly go through several cycles. Also, I expect the roll instability to appear sooner rather than later, so if there is more than a few 1000 feet of altitude to burn, the aircraft will start to enter a spiral dive. $\endgroup$ – Peter Kämpf Feb 21 '19 at 18:01
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    $\begingroup$ @PeterKämpf For a double flameout in cruise, though, I‘d expect a trimmed condition to start with. 777 is roll direct, so not much roll damping from control laws I guess. I‘d also be very surprised if it made it straight in without any control inputs; spiral (even a slow entry one) is much more likely. $\endgroup$ – Cpt Reynolds Feb 21 '19 at 18:05
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    $\begingroup$ @CptReynolds: Roll damping is provided by the wings. Roll stability, less so. And I agree with you that 1) a trimmed starting position is most likely, and then no phygoid would be noticeable and 2) a slowly developing spiral is most likely. $\endgroup$ – Peter Kämpf Feb 21 '19 at 18:10
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    $\begingroup$ @PeterKämpf Thank you. It’s been a while since I actively worked in flight controls, but isn’t roll damping also provided in the control laws e.g. for roll rate command laws as in the FBW Airbusses and 787? $\endgroup$ – Cpt Reynolds Feb 21 '19 at 18:31

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