APU starting limitations specify how many starts per 30 minutes with what separation time?

• A. 2 starts per 30 minutes with 60 seconds in between
• B. 3 starts per 30 minutes with 90 seconds in between
• C. 4 starts per 30 minutes with 120 seconds in between
• D. 1 start per 30 minutes with 30 seconds in between

Answer: (B)

APU starting limits are about how many start attempts you can make in a 30-minute window and how long you must wait between each attempt to protect the unit. The reason three starts per 30 minutes with a 90-second separation is correct is that this spacing gives the APU enough cooldown and time for oil pressure to stabilize between starts, reducing wear from rapid cycling. If you try more than three starts in that window or don’t maintain the 90-second gap, you increase the risk of overheating, starter damage, or a failed start caused by insufficient system stabilization. The 90-second interval isn't arbitrary; it ensures the APU has a moment to recover before the next attempt.