What will happen when all engines of a commercial aircraft fail in flight?

All modern commercial aircraft engines are very reliable and situation where all two or four engines fail completely is very remote. Aircraft will not roll over and crash when all engines fail in flight. It will continues to fly at an optimum gliding speed, a speed much lower than its cruising speed to a safe landing. It may not be able to maintain its cruising altitude but will continues to lose height at a rate of approximate 4000 feet per minute.

All flight crews are trained to restart/relight the engines whenever they encounter engine failures in flight. If engines fail to restart, they would have no choice but to carry out a forced landing.

What will happen in the passenger cabin when all engines fail?

When both engines failed simultaneously (normally one after another), Cabin lights will flicker and may become dimmer. If aircraft’s Auxiliary Power Unit (APU) fail to start automatically, Cabin pressure will decrease and Oxygen masks will drop from the ceiling panel. Automatic emergency announcement will be made through the Public Address System. Cabin crew will carry out briefing on emergency procedures to all passengers onboard for a possible Forced Landing or Ditching into sea.

Is it safe to force land an aircraft?

Aircraft in general is still controllable even if most of its normal hydraulic system pressures are gone. If an aircraft is designed with emergency hydraulic pressures system generated by the Ram Air Turbine (RAT), RAT will automatically extended when it senses both engines failed.

The air flowing past the aircraft will drives and turn the Fan blades on RAT during descend. It generate sufficient hydraulic and electric power as the aircraft glide forward at a speed of about 250 mph. The landing gear will be extended by the emergency alternate system, and the aircraft has sufficient brake pressure to bring the aircraft to a complete stop using the emergency accumulator pressure. 

Therefore if an airplane encounter all engines failures, it is still capable of gliding to a safe landing provided there is a suitable landing area. The landing gears would then extend and brakes still functioning to stop the aircraft safely.

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