What is the maximum duration for dry motoring on the ground before a cool-off period is required?

The correct duration for dry motoring on the ground before a cool-off period is required is 90 seconds. This limitation is important for the maintenance of engine integrity and to prevent overheating. During dry motoring, the engines are being turned without fuel, which can generate significant heat due to friction. Allowing the engine to run for 90 seconds provides enough time for necessary checks or system tests while minimizing the risk of damaging the engine components due to excessive heat.

When the 90 seconds has elapsed, a cool-off period is mandated to allow the engine temperatures to return to a safe level before any further dry motoring. This practice ensures that the engines remain operational and safe for flight.

In contrast, the options representing shorter durations, such as 30 seconds, 60 seconds, and 120 seconds, either do not provide enough time for necessary checks or could risk overheating, leading to potential engine damage if the limits are exceeded. The 90-second limit strikes a balance by ensuring operational tests can be conducted without compromising the integrity of the engine while adhering to safe operational procedures.