How do you recognize Low Rotor RPM?

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There is a different sound and a different vibration in the aircraft. The feel is the quickest way to identify almost everything in a helicopter, and this should be your ultimate goal. However this takes experience. Robinson wanted to make it as safe as possible so at 97% The horn and light will go off. This alerts the pilot to the fact that the RPM is below the normal operating range

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What is the corrective action?

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Promptly lower the collective and roll the throttle on. This is roll thumbs up away from you.Thumbs down or towards you is removing throttle.

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Note: If you do motorcycles, dirt bikes, or ATVs the throttle roll is backwards compare to the helicopter.