What should be done if the START light does not turn off within 90 seconds?

When the START light does not extinguish within the specified time frame of 90 seconds, the appropriate action is to wait for the turbine to cease spinning before attempting a second start. This protocol is in place to ensure that the APU's starting system can safely and effectively operate. Allowing the turbine to stop completely helps to prevent damage to the turbine from multiple start attempts while it is still spinning, which can lead to overheating or mechanical failure.

Additionally, waiting ensures that any residual conditions or anomalies that may have prevented the successful start can be resolved, thereby increasing the chances of a successful second attempt. This practice emphasizes the importance of safety and the proper functioning of equipment during maintenance and operational procedures. Other options, such as attempting a second start immediately or rechecking systems before the turbine has stopped, might increase the risk of damage to the APU or not provide enough time for the system to reset properly. Notifying maintenance personnel can be an important step, but the paramount action is to ensure that the turbine is in a safe condition before trying to start the APU again.