What is propeller 'low speed' mode and its limitations?

• A. Increases RPM to 1900 and is suitable for icing
• B. Reduces RPM from 1700 to 1550 for reduced cabin noise; NOT approved for icing
• C. Maintains 1700 RPM but reduces route distance
• D. Disables the propeller while maintaining engine power

Answer: (B)

Low speed mode lowers the propeller RPM to reduce noise and cabin vibration, trading a bit of thrust for a quieter flight. On the NGX, this mode reduces RPM from about 1700 down to roughly 1550. That lower RPM means the prop produces less noise, which can be desirable during cruise when you don’t need full power.

However, this setting has a clear limitation: it is not approved for icing conditions. In icing, you rely on the propeller and its anti-ice system to maintain performance and prevent ice buildup. Running at a reduced RPM can compromise ice protection effectiveness and overall controllability, so you must not use low speed mode when icing is present. The other options describe actions that a low speed mode does not perform—raising RPM, keeping RPM constant while shortening range, or disabling the propeller—so they don’t fit the actual function and restriction of this mode.