What happened
During a routine pre-flight hydraulic servo accumulator test at Grande Prairie Airport, Alberta, a Eurocopter AS 3CO B2 helicopter, registration C-GBHH, unexpectedly became airborne. The aircraft was stationary on a prepared hard surface with its rotors running at flight RPM. While the pilot was performing a lateral servo check, the helicopter tilted sharply backward and lifted approximately 15 feet off the ground.
As the aircraft ascended, the pilot was unable to push the collective lever down to counteract the movement. The helicopter then entered a left turn, pitched tail-down, and rolled to the right. The main rotor blades struck the ground, causing the fuselage to land on its right side. While the helicopter sustained substantial damage, there were no injuries to the pilot, and no persons on the ground were harmed. An adjacent helicopter also sustained minor damage.
The investigation
The investigation focused on why the collective stick moved upward uncommanded during a prescribed ground test. Investigators determined that the design of the collective stick's locking button allowed for improper engagement with the locking plate. This flaw meant that a much smaller force than expected was required to release the lock. During the test, the pilot applied light downward pressure on the collective while centralizing flight controls, which inadvertently disengaged the improperly seated lock.
Furthermore, the investigation examined the state of the hydraulic accumulators. It was discovered that the accumulators were operating at pressures significantly lower than the required 218 psi, yet there was no visual or mechanical way for the pilot to detect this during the pre-flight check. The investigation also noted that the helicopter's center of gravity was near the aft limit and the aircraft was well below its maximum weight, factors that made the aircraft more susceptible to becoming airborne once the lock failed.
Findings
- The improperly engaged collective stick lock allowed the collective to release without the pilot's knowledge.
- The exhaustion of the lateral accumulators caused the collective stick to rise sharply, a known characteristic of the AS 350 B2 rotor system when the lock is disengaged.
- The pilot was unable to regain control due to the high, unpredictable feedback forces resulting from the exhausted accumulators.
- The aircraft's aft center of gravity and low gross weight exacerbated the nose-up attitude and facilitated the unintended takeoff.
- Existing pre-flight test procedures were found to be ineffective at identifying accumulators with low internal pressure.