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In cases like the DHL shoot-down in 2003 or the Sioux City crash, both aircraft lost all of their hydraulics because of a catastrophic failure of aircraft structure.

  • What would happen if the aircraft just suddenly lost hydraulic power mid-flight?
  • Would the flap goes flat, or would in droop down, increasing lift, and force the aircraft into a phugoid motion?

I am talking about the same aircraft types presented in these disasters (big civilian transports) like Boeing 747-400.

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Assuming some sort of catastrophic failure of all three or more hydraulic systems in flight, and no emergency hydraulic power, it would all be based on the design of the systems on the specific aircraft, but, most likely, the following would happen:

Flaps would remain in their current configuration. Flaps are driven by screw drives with your powered from a torque tube driven by a hydraulic motor. This would not move position as a result of a hydraulic failure.

Landing gear will remain retracted. Large airliners have an uplock system, which holds the gear in the retracted position during flight. If the landing gear were down and locked prior to the hydraulic failure, they would remain down and locked afterwards. The downlocks are held in an over center position by spring bungees which require hydraulic pressure to release.

Flight controls would not function, nor could they be commanded into positions by the pilot, unless there was a mechanical backup of some sort.

Other secondary flight control systems, such as spoilers or speed brakes also could not be deployed due to the failed hydraulic systems.

Keep in mind, the sort of a scenario would be virtually impossible as there are multiple redundant failsafes on these systems.

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