A propulsion engine, using the speed control circuit shown in the illustration, fails to function at speeds lower than the low end of the critical speed range. Which of the following statements describes what should be done to correct this malfunction? Illustration MO-0114

Correct Answer: Option B — Device 17A needs to be replaced, repaired, or reset to the setpoint coinciding with the RPM value for the low end of the critical speed range.

Critical speed avoidance circuits utilize specific setpoints to bypass torsional vibration nodes. When a propulsion plant fails to operate below the low end range, device 17A must be recalibrated or repaired to match the exact RPM threshold defining that specific harmonic critical speed boundary.