Propeller/rotor modulation error can cause VOR CDI fluctuations up to ±6°. Which action normally smooths out this roughness?

• A. Enable autopilot
• B. Slight changes to the rpm setting
• C. Increase altitude
• D. Use a different VOR

Answer: (B)

Propeller and rotor modulation of the VOR signal creates fluctuations seen on the CDI because the rotating parts imprint a modulation pattern on the radio signal. Because that modulation is tied to engine RPM, making a small change in RPM shifts the modulation characteristics just enough to reduce the amplitude of the CDI roughness. In practice, a tiny tweak in RPM alters how the receiver demodulates the signal, smoothing out the ±6° fluctuations.

Autopilot won’t inherently fix the signal path and just follows the displayed course, so it doesn’t remove the modulation. Increasing altitude doesn’t address the mechanical modulation either, though it may help with some other signal quality issues. Using a different VOR could avoid the specific station, but the normal, quick fix for this modulation-based roughness is a slight RPM change.