Robinson Helicopter R22 BETA N313DA accident near Parlier, California

Probable cause

The pilot inadvertently entered a low-G condition during cruise flight, which resulted in a momentary loss of control of the helicopter and mast bumping.

NTSB analysis

The helicopter pilot was on the first leg of a ferry flight. The helicopter was 3.2 hours out of its 2,200-hour overhaul. The pilot reported that he was in straight-and-level flight and he initiated a right turn. The helicopter immediately rolled to the right and entered a low-G condition. The pilot responded appropriately and righted the helicopter. He then conducted a power-on landing. As the main rotor system came to a stop, one of the blades impacted the tailboom. Post landing examination of the rotorhead revealed that the pitch change link on the red main rotor blade had fractured, the spindle tusk had sheared off, and damage to both elastomeric teeter stops was evident. Metallurgical examination of the spindle tusk determined that it failed in a downward direction relative to the rotor blade due to shear overstress. Examination of the pitch change link determined that it failed due to overstress characterized by reverse bending numerous times to failure. Witness marks on the pitch link and pitch change arm indicated that the angle of the blade at the time of the pitch link failure was 45-degrees downward, an angle that could be achieved if the spindle tusk had already been fractured. According to Robinson Helicopter Company, this angle could also be obtained during extreme teeter of the hub/blade during a mast bumping event. A Robinson Helicopter Company publication notes, "Pushing the cyclic forward following a pull-up or rapid climb, or even from level flight, produces a low-G (weightless) flight condition. If the helicopter is still pitching forward when the pilot applies aft cyclic to reload the rotor, the rotor disk may tilt aft relative to the fuselage before it is reloaded. The main rotor torque reaction will then combine with tail rotor thrust to produce a powerful right rolling moment to the fuselage. With no lift from the rotor, there is no lateral control to stop the rapid right roll and mast bumping can occur."

Text from the NTSB. Open the NTSB docket for WPR10LA138 for the full record.