Originally Posted By: Hokiefyd
Originally Posted By: Blaze
Astro,,, lets assume all on board are dead and the plane is at 35,000ft on auto-pilot and runs out of fuel...In what profile does this plane hit the water? Thanks
From a very limited knowledge of flight physics, I'd imagine that as the plane loses forward momentum, the auto pilot would try to pitch the aircraft higher and higher to maintain altitude. After a short while, it would stall and probably flat spin or otherwise lose altitude uncontrollably until it hit the water.
That's exactly what I think would happen initially...but it's what happens next that is unclear...
Near stall, the autopilot will either disconnect, or change to another mode. If it disconnects, a fly by wire plane will stay in the attitude at which the disconnect occurs. It's not a return to trim like older planes, the FCS will keep pitch rate zero, which means the airplane will fly near that disconnect speed. It should also keep yaw rate zero, meaning no slip and it should keep roll rate zero, meaning steady bank and a turn of unknown radius. The FCS on the A-320 will reduce bank to 33 degrees or less without pilot input. The 777 will use a multitude of surfaces to reduce bank to 30 degrees if it is exceeded.
But if, instead of a disconnect, the autopilot goes through a mode reversion to another mode, say airspeed, then what speed will it choose? Current Mach? TAS? IAS? Mode reversion in heading usually results in heading hold, so it stays in that heading....maybe.
This stuff isn't In the flight manual...and fly by wire (F/A-18, A-320, and 777) airplanes don't respond the way that older planes would.
The FCS on the 777 is prioritized for electric power. It would be energized on batteries on on the RAT, so it would be working after engine flameout...but the autopilot response is the key to answering the original question...but flat spin, spin, and even high speed spiral are all unlikely. Steady descent to impact is what I think would happen...but the precise nature? I don't know.
Originally Posted By: Blaze
Astro,,, lets assume all on board are dead and the plane is at 35,000ft on auto-pilot and runs out of fuel...In what profile does this plane hit the water? Thanks
From a very limited knowledge of flight physics, I'd imagine that as the plane loses forward momentum, the auto pilot would try to pitch the aircraft higher and higher to maintain altitude. After a short while, it would stall and probably flat spin or otherwise lose altitude uncontrollably until it hit the water.
That's exactly what I think would happen initially...but it's what happens next that is unclear...
Near stall, the autopilot will either disconnect, or change to another mode. If it disconnects, a fly by wire plane will stay in the attitude at which the disconnect occurs. It's not a return to trim like older planes, the FCS will keep pitch rate zero, which means the airplane will fly near that disconnect speed. It should also keep yaw rate zero, meaning no slip and it should keep roll rate zero, meaning steady bank and a turn of unknown radius. The FCS on the A-320 will reduce bank to 33 degrees or less without pilot input. The 777 will use a multitude of surfaces to reduce bank to 30 degrees if it is exceeded.
But if, instead of a disconnect, the autopilot goes through a mode reversion to another mode, say airspeed, then what speed will it choose? Current Mach? TAS? IAS? Mode reversion in heading usually results in heading hold, so it stays in that heading....maybe.
This stuff isn't In the flight manual...and fly by wire (F/A-18, A-320, and 777) airplanes don't respond the way that older planes would.
The FCS on the 777 is prioritized for electric power. It would be energized on batteries on on the RAT, so it would be working after engine flameout...but the autopilot response is the key to answering the original question...but flat spin, spin, and even high speed spiral are all unlikely. Steady descent to impact is what I think would happen...but the precise nature? I don't know.
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