What action should be taken if propellers rotate unexpectedly during bleed air load?

When propellers rotate unexpectedly during a bleed air load, pressing the APU bleed air valve switch to closed is the appropriate action to take. This immediate step helps isolate the APU from the bleed air system, which could be causing the unexpected propeller movement. Closing the bleed air valve stops the flow of high-pressure air from the APU, preventing further operation of the propellers in an unintended manner and safeguarding both the aircraft and crew.

It is crucial in such situations to act quickly to prevent any potential damage to the aircraft or injury to personnel. Closing the bleed air valve effectively addresses the issue of unexpected propeller movement by cutting off the source of the bleed air load that may be influencing their operation.