RADALT fluctuations may induce undesired collective movements via the FD and/or AFCS when using which operating modes?

• A. T-HOV, HOV, and HOV AUG
• B. HOGE, TRNG, and HOV
• C. T-HOV only
• D. HOGE and TRNG

Answer: (A)

Radar altitude based hover modes rely on the radar altitude as the reference for maintaining a set height. When RADALT data wiggles, the flight control system sees a changing altitude error and will command corrective changes in rotor lift via the collective to hold the target height. That automatic corrective action can produce unintended collective movement, especially in hover modes where the FD/AFCS actively manages vertical position.

The modes that are susceptible are the ones that use RADALT for height reference during hover: T-HOV, HOV, and HOV AUG. In these modes, RADALT fluctuations directly influence the vertical control loop, so undesired collective inputs can occur through the FD or AFCS.

Other modes, like HOGE or TRNG, either don’t rely on RADALT in the same way for altitude hold or have AFCS logic that minimizes this coupling, so RADALT fluctuations are less likely to drive unwanted collective movement in those cases.

In practice, if you encounter RADALT-induced flutter, consider switching to a non-RADALT hover reference or disengaging automated altitude control to maintain manual control until the RADALT signal stabilizes.