What happened
During a before takeoff check, a commercial pilot was conducting a hydraulic accumulator test. As part of this procedure, the hydraulic pressure had been bled off. During the process, the helicopter inadvertently lifted off the ground. Because the hydraulic pressure was depleted, the pilot was unable to regain control of the aircraft before it impacted the ground, striking the tail rotor first.
The investigation
Investigators examined the mechanics of the collective lock and the pilot's actions during the test. It was determined that the collective had been locked in the down position for the duration of the accumulator test. However, as the pilot was repositioning his hands to restore hydraulic power following the test, the collective became unlocked.
Findings
FAA inspectors found that the design of the collective lock allowed the collective to be released when the cyclic was moved while the hydraulic pressure was depleted.