What feature prevents an APU from shutting down while it is still hot?

What feature prevents an APU from shutting down while it is still hot?

• A. The APU fuel control incorporates an automatic time-delay feature that closes the bleed air valve to remove most of the load and reduce the APU temperature before it is shut down.
• B. The APU has an emergency shutdown switch.
• C. A manual cooling cycle by the maintenance crew.
• D. An automatic fuel cutoff when temperature rises.

Answer: (A)

When an APU is still hot, shutting it down abruptly can cause thermal stress and potential damage to rotating parts and bearings. The feature being tested is an automatic time-delay in the fuel control that, before final shutdown, closes the bleed air valve to remove most of the load. By reducing the load, the turbine and exhaust temperatures drop gradually, allowing the unit to cool safely before it is shut down. This cooling sequence helps prevent overheating and thermal shock. Other options won’t protect as effectively: an emergency shutdown switch stops the unit quickly without a controlled cool-down; a manual cooling cycle depends on maintenance actions and isn’t automatic; automatic fuel cutoff by temperature would stop fuel immediately but wouldn’t manage the cooling process to prevent heat-related damage.

When an APU is still hot, shutting it down abruptly can cause thermal stress and potential damage to rotating parts and bearings. The feature being tested is an automatic time-delay in the fuel control that, before final shutdown, closes the bleed air valve to remove most of the load. By reducing the load, the turbine and exhaust temperatures drop gradually, allowing the unit to cool safely before it is shut down. This cooling sequence helps prevent overheating and thermal shock.

Other options won’t protect as effectively: an emergency shutdown switch stops the unit quickly without a controlled cool-down; a manual cooling cycle depends on maintenance actions and isn’t automatic; automatic fuel cutoff by temperature would stop fuel immediately but wouldn’t manage the cooling process to prevent heat-related damage.