APU Battery Restart Limit / max altitude?

APU Battery Restart Limit / max altitude?

• A. 15,000' MSL
• B. 25,000' MSL
• C. 28,000' MSL
• D. 30,000' MSL

Answer: (B)

The reason this limit exists is that starting the APU with battery power depends on having enough electrical capability and air density to spin the APU up to speed. As you climb, electrical load and cooling become more challenging and the air gets thinner, making a reliable battery-start harder. To keep the restart chances realistic, manufacturers set a maximum altitude for attempting a battery restart: 25,000 feet MSL. Sticking to this limit gives a practical chance that the APU will start using only the onboard battery if other power sources aren’t available. If you’re above that altitude, a battery restart is unlikely to succeed, so you’d rely on other power sources or appropriate procedures. The other altitudes either fall short of or exceed the tested limit, which is why 25,000 feet is the correct value.

The reason this limit exists is that starting the APU with battery power depends on having enough electrical capability and air density to spin the APU up to speed. As you climb, electrical load and cooling become more challenging and the air gets thinner, making a reliable battery-start harder. To keep the restart chances realistic, manufacturers set a maximum altitude for attempting a battery restart: 25,000 feet MSL. Sticking to this limit gives a practical chance that the APU will start using only the onboard battery if other power sources aren’t available. If you’re above that altitude, a battery restart is unlikely to succeed, so you’d rely on other power sources or appropriate procedures. The other altitudes either fall short of or exceed the tested limit, which is why 25,000 feet is the correct value.