What is the prohibited range for Np stabilized operation while on the ground to prevent ground resonance?

The prohibited range for Np (propeller RPM) stabilized operation while on the ground is between 62% and 80%. This range is critical because operating within these RPMs can lead to ground resonance, a phenomenon that can occur due to the interaction between the vibration frequencies of the aircraft structure and the rotor system.

This can be particularly dangerous during ground operations, as it may lead to excessive vibrations that can compromise structural integrity or result in a loss of control. By avoiding this RPM range, pilots and operators ensure that the aircraft remains stable and safe on the ground, mitigating the risk of resonance which could result in severe operational consequences.

Understanding the dynamics of the aircraft system in relation to RPM helps ensure safety and effectiveness while on the ground. The other options fall outside this range, and while they may not pose the same level of risk for ground resonance, they do not address the specific requirement to avoid the critical range identified in the training materials.