The Airbus A320’s Auxiliary Power Unit (APU) is vital on the ground — powering electrics and starting engines — but it can’t be started above 25,000 feet. In thin air, the APU’s small turbine simply can’t spool up safely, so Airbus designed it primarily for ground use and low-altitude emergencies. Once running, it can provide backup power in flight, but at cruise, you’re relying on the main engines and redundancy systems. A clever compromise of weight, safety, and design. #avgeek #planespotting