Abstract: The blades of a helicopter propeller are supervised by pressure-sensitive transducers operating a resonance circuit rotating therewith, for determining its frequency. The circuit includes a rotating secondary of a transformer, intermittently coupled to a stationary primary which is energized by a voltage having a frequency equal to the resonance frequency of the resonance circuit when there is no blade defect. An impedance senses the amplitude of these pulses which varies on detuning due to a blade defect. The pulses are processed to yield a warning. Scanning through a range of source frequencies permits identification of the defective blade. The circuit is supplemented for operational testing during flight as well as on the ground.