What indicates a drive system failure during helicopter operation?

A decrease in rotor RPM is a crucial indicator of a potential drive system failure during helicopter operation. The rotor system's RPM needs to remain within a specific range for effective performance and safety. If there is a failure in the drive system, which might include issues such as a severed or slipping connection between the engine and the rotor system, the rotor RPM can start to drop. This decrease could signal that the engine is not delivering the necessary power to maintain rotor speed, which can adversely affect lift and control of the helicopter.

In contrast, stable engine RPM does not necessarily indicate any problems, as it can occur under normal operations. An increase in rotor RPM can also signal other issues or conditions, such as excessive power, but it does not directly indicate a drive system failure. Consistent flight altitude would generally suggest stable flight conditions, but it, too, does not specifically point to a drive system failure. Thus, monitoring rotor RPM is essential for pilots to quickly identify any faults in the drive system and take corrective action.