Under which condition is retreating blade stall more likely to occur?

Retreating blade stall is more likely to occur at high density altitudes primarily due to the reduced air density. As altitude increases, the performance of the rotor blades diminishes, leading to a decrease in lift. This is particularly critical in helicopters where the rotor blades are designed to work effectively within specific air density conditions. At high density altitudes, the helicopter may experience a reduced margin of error during maneuvers, especially when the rotor blades are partially unloaded, raising the risk of stall.

In conditions of high density altitude, the effectiveness of the rotor's retreating blades diminishes, and if the helicopter is still required to produce significant lift (for instance during climbs), the likelihood of stalling those retreating blades increases dramatically. Hence, understanding the effects of environment and physics on helicopter performance and the mechanics of rotor blade function is crucial for safe operations.