Retreating blade stall onset is recognized by either a four per revolution vibration or illumination of which light?

• A. Limit light
• B. Rpm warning light
• C. Both A and B
• D. Neither

Answer: (C)

Retreating blade stall onset shows up in two practical ways: a distinctive four-per-revolution vibration due to the retreating blade stalling and the corresponding change in rotor dynamics, and cockpit warnings that illuminate when operating conditions push the rotor toward its limits. The limit light becomes visible as rotor speed nears the allowable limit, while the rpm warning light triggers for abnormal rotor RPM. Because either the vibration signature or one of these lights can alert the crew to the onset of retreating blade stall, both indicators are valid cues. When you observe either cue, it signals you to take corrective action to reduce rotor loading and restore safe airflow over the blades.