Propeller/rotor modulation error can affect which instrument reading?

• A. CDI
• B. Attitude indicator
• C. Airspeed indicator
• D. Altimeter

Answer: (A)

Propeller/rotor modulation error refers to how the spinning blades and the associated electrical/ RF environment can inject ripple or interference into radio navigation signals. The instrument readings that depend on those signals are most affected. The CDI shows course deviation based on the VOR/LOC signals received by the navigation radio. If the propeller/rotor environment modulates or disturbs that radio signal, the CDI can display erroneous or unstable deviation indications. The attitude indicator, airspeed indicator, and altimeter rely on gyros or pitot-static pressures rather than radio signals, so they aren’t affected by this type of modulation in the same way.