AS350 Hydraulic Failures
On August 3, 1999, a pilot completing transition training in a Eurocopter AS350 helicopter with a flight instructor crashed while practicing a simulated hydraulics failure.
According to the NTSB, after flying the helicopter to the ground without incident, with hydraulics still disabled, the pilot was instructed to hover taxi approximately 100 ft., perform a left pedal turn and taxi back. The pilot successfully hovered forward; however, during the turn the pilot could not maintain control of the helicopter. The instructor attempted recovery, but the helicopter rolled and the main rotor blades struck the terrain. Orientation of the helicopter following the turn resulted in a 14-knot tailwind. Examination of the wreckage revealed no pre-impact anomalies and no operational deficiencies with the hydraulic servos. Both pilots sustained minor injuries.
The CFI reported that the commercial-rated pilot receiving instruction announced that she was experiencing unusual feedback in the flight controls. "I immediately got on the controls with her and the helicopter began an uncontrollable roll to the left while at a hover." He added that "as the helicopter continued its uncontrollable rolling motion to the left and reached an approximate angle of 30 degrees, I tried to level the helicopter by using both hands to attempt to pull the cyclic control to the neutral/level position. The helicopter continued to roll to the left and subsequently the main rotor blades struck the ground. The helicopter eventually came to rest on its right side."
The NTSB determined the probable cause of this accident was the pilot's loss of control during hydraulics-off hovering flight and the instructor's inadequate supervision of the maneuver. A factor was the tailwind.
In several other AS350 accidents involving hydraulic system failure or deactivation, pilots have reported an inability to move the controls. The NTSB reported that Eurocopter's Chief Pilot was questioned about the likelihood of a servo hard-over malfunction after the hydraulic system was disabled. He said, "It is impossible for the servo to have a hard-over once you isolate the hydraulic system. Once you dump the pressure, the pin drops, locks the pilot valve, and you have a mechanical linkage. The servo becomes a rigid part of the push-pull tube."
In July 2000, citing a desire to align their training syllabus with Eurocopter France, the helicopter manufacturer removed in-ground-effect maneuvers with the hydraulics off from their flight-training guide.
Then on April 11, 2002 the pilot of an AS350 helicopter returning to a remote lodge with six skiers aboard experienced an in-flight hydraulic failure. According to the NTSB, while in cruise flight the hydraulic pressure warning light on the enunciator panel illuminated and the aural warning horn sounded indicating a loss of hydraulic pressure. The pilot confirmed the loss of hydraulic pressure and slowed the forward airspeed of the helicopter. He cycled the hydraulic cut-off switch on the collective lever by turning the system off momentarily and then on again. The pilot made a landing pattern and approach to a road and brought the helicopter to a hover. The helicopter then rolled left and impacted the terrain inverted. The pilot stated that the cyclic control was frozen in the full aft position. Although the helicopter sustained substantial damage, the ATP rated pilot and six passengers were not injured.
An inspection of the helicopter revealed that the hydraulic pump drive belt separated at the point where the belt is bonded together. The separation of the belt resulted in a total loss of hydraulic pressure.
According to the helicopter's flight manual, upon loss of hydraulic pressure, the pilot should reduce collective pitch to reduce the airspeed, disengage the collective pitch hydraulic pressure switch, make a flat approach and land with forward airspeed.
The National Transportation Safety Board determined the probable cause of this accident was the pilot's failure to disengage the hydraulic system and maintain forward airspeed during landing as delineated in the emergency procedures section of the helicopter's flight manual, which resulted in a loss of control while hovering. Factors contributing to the accident were the separation of the hydraulic pump drive belt and the subsequent lockup of the flight control system.
A survey of Service Difficulty Reports compiled by the FAA revealed that during the period from February 22, 1988, until April 23, 2002, there have been 34 in flight hydraulic pump belt failures. Earlier records indicate additional failures as far back as March 14, 1980, but the details are incomplete from 1980 through 1988. According to American Eurocopter, as of the model year 2000, all AS-350 helicopters are fitted with an improved V-belt to drive the hydraulic pump.
The hydraulic pump belt has always been a source of contention with the AS350 helicopter. This helicopter requires ample physical strength to fly without hydraulic boost. Thus a complete understanding of the accident history, service issues and proper emergency techniques is extremely important to safe operations. Moreover, pilots should keep in mind that without hydraulic boost the collective control moves down. Letting go of that control in a hover will cause the helicopter to quickly hit the ground.