Some Aircraft Have Rats

Ram air turbine on Airbus A320. Image courtesy Curimedia, CC BY 2.0, via Wikimedia Commons.

Ram air turbines that is, or RATs. If main power is lost, the small windmill-like turbine is lowered from the belly of the fuselage and uses ram pressure or force of the moving air to drive an electrical generator or hydraulic pump. This is a last resort scenario though. Usually an aircraft's engines generate electrical power as well as thrust for propulsion. If they fail, an auxiliary power unit (APU) provides power for electrical systems. This also provides system power before the main engines start up, when an aircraft is on the runway. An APU is a mini jet engine, usually mounted in the tail of the aircraft, independent of the main engines and not involved in propulsion.

References:

Naidu, A. (2024, December 29). Redundancy approaches to flight safety: The curious case of the b737’s absence of a ram air turbine compared to the A320". LinkedIn. https://www.linkedin.com/pulse/redundancy-approaches-flight-safety-curious-case-b737s-andrew-naidu

Lomas, C. (2024, March 11). What is the Auxiliary Power Unit (APU)?. Flightradar24 Blog. https://www.flightradar24.com/blog/auxiliary-power-unit/