What signs indicate that retreating blade stall is occurring?

The correct choice is indicative of the physical phenomena associated with retreating blade stall in helicopters. When retreating blade stall occurs, it is characterized by a low frequency vibration that happens as the retreating blade experiences a stall due to insufficient lift being generated as the rotor speed increases and the angle of attack becomes too steep.

This stall can lead to a tendency for the helicopter to roll towards the retreating blade side, which is a critical situation since the performance of that blade is compromised. The low frequency vibration is often felt in the helicopter's controls and airframe, which can warn the pilot of this dangerous condition.

In contrast, other choices highlight different scenarios or indicators that aren't directly related to retreating blade stall. For instance, a loud humming noise may indicate some other mechanical or aerodynamic issue, while loss of altitude may stem from a variety of other factors unrelated to blade stall. The reference to a sudden drop in engine power suggests an issue with engine performance rather than blade stall dynamics. A rapid increase in altitude and the nose pitching down do not correlate with the characteristics of retreating blade stall, as they suggest an opposite flight dynamic. Understanding these signs is crucial for maintaining safe helicopter operations.