Between APU starts there must be a minimum of how many seconds?

• A. 60 seconds
• B. 120 seconds
• C. 180 seconds
• D. 90 seconds

Answer: (D)

The key idea is the required cooldown/off-time between APU start attempts. Waiting at least 90 seconds gives the APU and its starter motor a chance to cool and for lubrication and oil pressure to stabilize, reducing wear from a rapid second start. It also allows the purge and fuel vapors from the previous start to clear and prevents a surge of electrical load on the aircraft’s systems when the next start is attempted. Shorter intervals can stress the starter, increase the risk of overheating, and raise the chance of faults or trips during the next start. Longer intervals aren’t required as the standard practice; they’re simply more conservative than the minimum. So the minimum clinically used interval is 90 seconds.