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Sounds like a zombie plane (AFIAK all jet zombies have been oxygen/pressurization failures) but why the extreme descent at the end rather than simply gliding in

Sounds like a zombie plane (AFIAK all jet zombies have been oxygen/pressurization failures) but why the extreme descent at the end rather than simply gliding in once the engines ran out of fuel?

justsomehnguy 3y ago
Because autopilot tries to correct the plane, it doesn't have any logic for 'run out of fuel'. And without the engine power any action only destabilise the plane further. Also even without the autopilot the planes aren't stable at flight (besides brobably An-2?) and are flying in gigantic sea of air which has its own tides, waves, turbulences, zones of higher/lower pressure... Look at that wiki article, probably a similar situation. https://en.m.wikipedia.org/wiki/1999_South_Dakota_Learjet_crash#Crash https://en.m.wikipedia.org/wiki/1999_South_Dakota_Learjet_cr... Edit: and even if everything is 'good' the unconscious pilot's body can brush against the stick (bump from turbulence eg) and force autopilot to disengage (or it was already disengaged by itself by being not able to correct the plane course) and push (quite literally) the plane to the downward spiral.
- LorenPechtel 3y ago
  While the autopilot doesn't understand out of fuel it does understand angle of attack and shouldn't depart controlled flight in an effort to maintain altitude. Or perhaps the autopilot doesn't know what to do and turns off. I do think you might have a point about the body causing a disengagement.
  - justsomehnguy 3y ago
    > shouldn't depart controlled flight in an effort to maintain altitude Aircraft isn't inherently stable in a level flight, otherwise there would be no need for autopilot. There are constant small corrections. And when the engines dies the plane starts to drop the nose but not with an exactly 0 degree roll, because nor the weight is distributed ideally nor the atmosphere is an ideal gas. And while I don't know how far the plane should be out of the flight parameters for the AP to disengage, you should remember what it is still tries to correct the plane (while not yet disengaged) but the dwindling engine power (and hence airspeed) make the control surfaces less effective, so now the AP needs to over-correct and this how you end with rudder and elevators stuck in weird positions and sending the plane anywhere but the stable gliding path. Though in this case it's less probable becaue looks like the engines were still working, if I understood some sources right.

Sounds like a zombie plane (AFIAK all jet zombies have been oxygen/pressurization failures) but why the extreme descent at the end rather than simply gliding in once the engines ran out of fuel?

justsomehnguy 3y ago

Because autopilot tries to correct the plane, it doesn't have any logic for 'run out of fuel'. And without the engine power any action only destabilise the plane further. Also even without the autopilot the planes aren't stable at flight (besides brobably An-2?) and are flying in gigantic sea of air which has its own tides, waves, turbulences, zones of higher/lower pressure... Look at that wiki article, probably a similar situation. https://en.m.wikipedia.org/wiki/1999_South_Dakota_Learjet_crash#Crash https://en.m.wikipedia.org/wiki/1999_South_Dakota_Learjet_cr... Edit: and even if everything is 'good' the unconscious pilot's body can brush against the stick (bump from turbulence eg) and force autopilot to disengage (or it was already disengaged by itself by being not able to correct the plane course) and push (quite literally) the plane to the downward spiral.

LorenPechtel 3y ago
While the autopilot doesn't understand out of fuel it does understand angle of attack and shouldn't depart controlled flight in an effort to maintain altitude. Or perhaps the autopilot doesn't know what to do and turns off. I do think you might have a point about the body causing a disengagement.
- justsomehnguy 3y ago
  > shouldn't depart controlled flight in an effort to maintain altitude Aircraft isn't inherently stable in a level flight, otherwise there would be no need for autopilot. There are constant small corrections. And when the engines dies the plane starts to drop the nose but not with an exactly 0 degree roll, because nor the weight is distributed ideally nor the atmosphere is an ideal gas. And while I don't know how far the plane should be out of the flight parameters for the AP to disengage, you should remember what it is still tries to correct the plane (while not yet disengaged) but the dwindling engine power (and hence airspeed) make the control surfaces less effective, so now the AP needs to over-correct and this how you end with rudder and elevators stuck in weird positions and sending the plane anywhere but the stable gliding path. Though in this case it's less probable becaue looks like the engines were still working, if I understood some sources right.