What is the required delay between APU start attempts to allow for starter cooling?

The correct answer emphasizes the importance of allowing sufficient time for the APU starter to cool down after an attempt has been made to start it. The specific required delay of 2 minutes is in place to prevent overheating of the starter, which could lead to damage or failure of the starter system. APU starters are designed for limited use, and excessive attempts without appropriate cooling time can result in reduced reliability or operational capability.

The 2-minute interval allows the starter to dissipate heat generated during the starting process. If a pilot were to attempt another start prematurely, it might not only risk the starter's integrity but could also delay operations if a failure occurs, potentially leading to further complications during the flight or ground handling.

The other choices suggest longer or shorter cooling times, which wouldn't align with the manufacturer's recommendations and safety protocols, ultimately creating risks during APU operations.