Accident Overview
At 0830 mountain daylight time at Grande Prairie Airport, Alberta, a Highland Helicopters Limited Eurocopter AS 350 B2 helicopter (registration C-GBHH, serial number 3180) was standing on a prepared hard surface outside the company hangar. The helicopter was running at flying rotor rpm. The pilot was conducting the pre-flight hydraulic servo accumulator test sequence. During the lateral servo check, the helicopter tilted back sharply and became airborne.
Sequence of Events
The pilot could not push the collective lever down and was unable to prevent the helicopter from lifting off to about 15 feet above the ground. The helicopter then turned left, descended tail down, and rolled to the right. The main rotor blades struck the ground and broke off, and the fuselage fell onto its right side. The pilot was not injured but the helicopter was substantially damaged; there was no fire. Minor damage occurred to an adjacent helicopter, but no persons were injured on the ground. The accident occurred during daylight and in calm, visual meteorological conditions.
History of the Flight
Following an uneventful start-up, the pilot began conducting his normal pre-flight hydraulic test sequence. In preparation, the pilot engaged the collective lock in the cockpit and established the rotor rpm (Nr) at 100 per cent, as required by approved procedure.
Circumstances Leading to Uncommanded Collective Movement
The original design of the locking button on the pilot's collective stick permitted incorrect engagement of the locking plate. This created a situation where the downward force on the collective stick necessary to release the lock was considerably less than had it been engaged correctly. Once the lock was released, the collective stick was free to move up.
In this accident, the pilot exhausted the forward servo accumulator as part of his usual pre-flight checks, but stopped his test and centralized the flight controls to allow the AME under the rotor disk. This action likely included light downward pressure on the collective stick as a matter of normal reflexive action. Such pressure was sufficient to release the improperly engaged lock and free the collective; at this stage, the pilot was unaware that the lock had disengaged.
Once the pilot continued testing, it was only a matter of time before the collective stick rose up as a result of the lateral accumulator(s) exhausting and causing the flight controls to move to the non-hydraulic-powered position.
Weight and Balance
The helicopter was well below maximum certificated gross weight and the longitudinal CG was near the aft limit. As a result of the relative light weight, a lesser amount of collective pitch application was necessary for the helicopter to become airborne. The aft CG condition required the pilot to maintain more forward cyclic input in flight than normal, reducing available forward cyclic stick travel. Although not a significant factor until airborne, this reduction made the helicopter vulnerable to nose-high attitudes; the onset speed and magnitude of the nose-up force may have lessened the pilot's ability to prevent the nose from rapidly rising.
Hydraulic Test Procedures
The former hydraulic test procedures prescribed by the original Canadian AD incorporated a proactive approach toward detecting a failing accumulator, whereas the procedure in the RFM is reactive—it only identifies a failed accumulator after it cannot supply the required two or three cyclic control movements. There is doubt regarding the number of cyclic movements prescribed by Eurocopter for the hydraulic system ground test and in-flight loss of hydraulic pressure, due to inconsistent interpretation. The diagnosis of the final stage of the test relies on a small interval of one second or less to identify a defective accumulator, making the recharging element subjective and insensitive.
Another factor was the procedural requirement to establish full flying rotor rpm for the pre-flight hydraulic test. Coupled with low gross weight and exhausted accumulators, flying rotor rpm placed the helicopter in a condition to certainly become airborne if the collective stick lock released.
Servo Actuator Anomalies
The servo actuators were tested and found anomalous in two areas: inconsistent piston extension and retraction travel rates, and low accumulator pressures. Though it could not be proven that inconsistent servo travel rates directly contributed, there is doubt that such behaviour is always benign. Eurocopter tests have shown that asymmetric lateral servo accumulator depletion can cause uncommanded servo movement. Following pertinent Airworthiness Notices and Airworthiness Directives, modifications to the hydraulic system were mandated by both Canada and France, allowing pilots to shut off hydraulic system pressure and exhaust accumulators simultaneously.
Hydraulic Accumulator Pressure Monitoring
The hydraulic accumulators are crucial to flight control system function. The pre-flight test is the last opportunity for the pilot to ascertain accumulator functionality. The test methodology is not particularly effective—many instances of undetected low accumulator pressures have been found (35 to 175 psi, where 218 psi was expected). These low-pressure accumulators were in service without indication to pilots or maintenance personnel and all apparently passed the pre-flight test. The revised hydraulic accumulator test is not effective in identifying accumulators lacking prescribed pressure. There is a risk that flight could commence with one or more marginally serviceable accumulators.
Findings as to Causes and Contributing Factors
The design of the lock button on the collective stick allowed improper engagement with the locking plate and led to premature release during the hydraulic test. When the pilot interrupted his test, the collective stick lock released without his knowledge. When the lateral accumulator(s) exhausted, the unlocked collective stick rose up sharply, causing the helicopter to become airborne—a predictable characteristic of the AS 350 B2. The rapid feedback forces from exhausted accumulators prevented the pilot from lowering the collective or regaining control before ground impact. The center of gravity, near the aft limit, exacerbated the nose-up attitude. The required flying rotor rpm during the test enabled inadvertent takeoff when the accumulator exhausted with the lock disengaged.