The APU will not start if the APU rpm is rotating above what speed on the ground vs in flight?

The APU will not start if the APU rpm is rotating above what speed on the ground vs in flight?

• A. Ground: 5%, Flight: 12%
• B. Ground: 6%, Flight: 12%
• C. Ground: 5%, Flight: 10%
• D. Ground: 4%, Flight: 12%

Answer: (A)

The question tests understanding of when the APU start sequence is inhibited by existing spool speed. The start system requires the APU compressor spool to be essentially at rest before engaging the starter and initiating ignition. If the rotor is already turning faster than a small threshold, the starter may not be able to synchronise properly, and starting could cause mechanical stress or an improper ignition sequence. That’s why there are two limits: on the ground, the APU must be below 5% RPM to allow a start; in flight, it can be below 12% RPM. If the spool is above these values, the start will be inhibited. The higher in-flight limit accounts for residual motion from windmilling or air-driven rotation that can occur in flight, which is not considered dangerous as long as it stays under the 12% threshold.

The question tests understanding of when the APU start sequence is inhibited by existing spool speed. The start system requires the APU compressor spool to be essentially at rest before engaging the starter and initiating ignition. If the rotor is already turning faster than a small threshold, the starter may not be able to synchronise properly, and starting could cause mechanical stress or an improper ignition sequence. That’s why there are two limits: on the ground, the APU must be below 5% RPM to allow a start; in flight, it can be below 12% RPM. If the spool is above these values, the start will be inhibited. The higher in-flight limit accounts for residual motion from windmilling or air-driven rotation that can occur in flight, which is not considered dangerous as long as it stays under the 12% threshold.