APU starter limits: how many attempts and wait between attempts?

• A. Two attempts in an hour with a one-minute waiting period
• B. Three attempts in a one hour period, with a two minute waiting period between each attempt
• C. Four attempts in two hours, with a five-minute waiting period between each attempt
• D. One attempt per flight with no waiting

Answer: (B)

The main idea being tested is how to protect the APU starter by limiting how often you can try to start it and by enforcing a short cooling period between attempts. The right rule is three start attempts allowed within a one-hour window, with a two-minute wait between each attempt. This schedule provides enough opportunities to get the APU running when needed, but it also prevents excessive cycling that can overheat or wear the starter motor and related equipment. The two-minute interval gives a brief cooling period for the starter’s windings and electrical components, reducing the chance of failure on subsequent starts. Other schedules either restrict starts too much or allow too many starts in a short time, increasing the risk of damage, whereas three attempts per hour with two minutes between attempts strikes a practical balance.