Valdez, AK, USA
N917JT
Eurocopter AS-350-B2
The pilot of the helicopter was returning to a remote lodge with six skiers aboard. While in cruising 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 above the road. After coming to a hover the helicopter rolled hard left, and impacted the terrain inverted. According to the helicopter's operator 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. 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. The belt is the subject of repeated maintenance inspection and has a life limit of 600 hours of operation. The failed belt had been in operation on the accident helicopter about 419 hours.
HISTORY OF FLIGHT On April 11, 2002, about 1415 Alaska daylight time, a Eurocopter AS-350-B2 helicopter, N917JT, sustained substantial damage when it impacted terrain following a loss of control during an emergency landing, about 30 miles east of Valdez, Alaska. The helicopter was being operated as a visual flight rules (VFR) on demand charter flight under Title 14, CFR Part 135, when the accident occurred. The helicopter was being operated by Evergreen Helicopters of Alaska, Inc., of Anchorage, Alaska. The airline transport pilot and the six passengers were not injured. Visual meteorological conditions prevailed, and a VFR company flight plan was filed. The flight originated at the Tsina Lodge helipad, with an offsite landing at Thompson Pass, about 35 miles east of Valdez, to pickup skiers, and then return to Tsina Lodge. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on April 12, the pilot said he was en route to the Tsina lodge from Thompson Pass, when the hydraulic pressure warning horn activated, followed by the illumination of the hydraulic (HYD) warning light. He said he was descending west bound from about 4,000 feet msl to a roadway about 1,600 feet msl. The pilot said he knew he could not make it back to the lodge helipad, so he setup a landing pattern for the road. He said as he flew the landing pattern the cyclic control became stiff and hard to move. He brought the helicopter to a hover about four feet above the road. According to the pilot, the cyclic was frozen in the full aft left position when he lost control. The helicopter rolled left, and struck the ground inverted. In his written statement to the IIC dated April 12, the pilot wrote; "Turned into the wind and set up a shallow approach at about 30 knots airspeed. At approximately a 4 foot hover, the aircraft became uncontrollable and banked left and contacted the ground inverted." During an in-person interview with the IIC on April 16, a mechanic for the operator said he was at the helipad near Tsina lodge when the pilot of the accident helicopter radioed that he had a hydraulic problem. He said he drove to the area of the roadway where the pilot intended to land. He blocked the road with his pickup truck, and watched as the pilot made an approach to the road. He said the helicopter seemed to make the base leg of the approach a little slower then normal, but appeared to be under control. During the final approach leg the helicopter was coming straight at him, and he could see that the sink rate of the helicopter appeared normal, but he could not estimate the helicopter's forward speed. He said the helicopter seemed to come to a hover about 10 feet above the road. He said the helicopter then started to move forward and banked steeply to the helicopter's left, and he thought the pilot was "going around." The helicopter continued banking hard and rolled inverted. The helicopter disappeared below the edge of the elevated roadway. He said he drove to where the helicopter disappeared, and found the helicopter crashed inverted in the snow, in a wetlands area below the level of the road. He helped the passengers and pilot out of the wreckage. PERSONNEL INFORMATION The pilot held an airline transport pilot certificate with airplane single-engine land, and helicopter ratings. The most recent second-class medical certificate was issued to the pilot on October 3, 2001, and contains the limitation for corrective lenses. According to records supplied by the pilot, his total aeronautical experience consisted of about 11,000 hours, of which about 760 hours were accrued in the accident helicopter make and model. In the preceding 90 and 30 days prior to the accident, records indicate a total of 85.1 and 75.6 hours of flight time respectively. In the 24 hours preceding the accident the pilot flew 6.1 hours. Training records provided by the operator indicate that the last FAR Part 135 proficiency check performed by the pilot in the accident helicopter make and model was accomplished on May 13, 2001. According to the records, the pilot accomplished a FAR Part 135 proficiency check in a Bell 206 helicopter on January 10, 2002. FAA form 8410-3 AIRMAN COMPETENCY/PROFICIENCY CHECK dated May 13, 2001, indicates the pilot received a satisfactory rating in emergency procedures. The records further indicate the pilot completed and passed training and written testing on the accident helicopter make and model hydraulic system on May 2, 2001. AIRCRAFT INFORMATION The accident helicopter had accrued a total airframe and engine time of 1424.8 hours at the time of the accident. The last 100 hour inspection was completed on April 5, 2002, at a total airframe and engine time of 1402.3 hours. The helicopter is equipped with a hydraulic actuated flight control system. It is comprised of a hydraulic pump driven by a belt connected to the tail rotor drive shaft, four hydraulic servo/actuators, and four nitrogen precharged accumulators. The pump supplies hydraulic pressure to the hydraulic servo/actuators, which in turn assist the pilot with flight control inputs. If a mechanical loss of hydraulic pressure occurs, such as the failure of the hydraulic pump or the hydraulic pump drive belt, the four nitrogen precharged accumulators attached to the hydraulic actuator/servos provide pressure to assist the pilot while slowing the helicopter's airspeed. The airspeed reduction is required to reduce the dynamic loading of the rotor system to a level where the pilot can fly the helicopter without the hydraulic assistance. The precharged accumulators will not provide sufficient pressure to operate the system for a protracted period. The number of control inputs available depends on the nature and duration of the inputs. In order to operate the flight controls following a loss of hydraulic pressure, the hydraulic system must be fully disengaged. When the system is disengaged, the servos attached to the hydraulic actuators open gate valves that allow hydraulic fluid to flow freely through the system. If hydraulic fluid cannot flow freely during a hydraulic failure, the system may suffer a hydraulic lock. The system is controlled by a cutoff switch located on the pilot's collective control lever. METEOROLOGICAL INFORMATION According to the pilot, weather at the time of the accident was VFR, and weather was not a factor in the accident. COMMUNICATIONS The only communications at the time of the accident was via the company radio. According to the pilot and the mechanic, the pilot clearly communicated that he had a hydraulic problem and was returning to land. The company radio is not recorded. WRECKAGE AND IMPACT INFORMATION Photographs of the accident site were provided to the IIC by an FAA airworthiness inspector who is assigned to the operator as the primary maintenance inspector (PMI). The inspector stated that due to the reported hydraulic failure, the first thing he looked for at the site was the hydraulic pump drive belt. He said the belt was not on the drive-shaft pulley and hydraulic pump. He located the broken belt inside the engine/transmission cowling. The inspector said he did not see anything at, or near, the drive-shaft pulley and hydraulic pump that would have cut the drive belt. The belt was given to the IIC. On April 16, the IIC examined the wreckage of the helicopter in a hangar at Anchorage. Continuity of all cyclic and collective controls was established. The tail rotor actuator/servo was visually inspected, and there were no signs of hydraulic fluid leakage or spray. The nitrogen precharge for this actuator/servo accumulator was not checked. The three actuator/servos integrated in the rotor head control linkage were visually inspected. All hydraulic and electrical lines were attached, and there were no signs of leakage or hydraulic fluid spray. The actuator/servo accumulator located forward on the right side had a nitrogen precharge of 91 psi. The actuator/servo accumulator located forward on the left side had a nitrogen precharge of 95 psi, and the actuator/servo accumulator located on the aft left side had a nitrogen precharge of 125 psi. The normal actuator/servo nitrogen precharge is about 220 psi and bleeds down to zero during emergency operation. The actuator/servos were energized during the examination, and they all operated correctly. The hydraulic pump, which mounts on the rear of the transmission housing, showed no visible damage. The hydraulic pump is driven by a flat synthetic belt. The belt is driven by a pulley machined into the first segment of the tail rotor drive shaft attached to the transmission tail rotor output quill. There was no visible damage to the output quill, drive shaft, or pulley. The drive belt (part number 704A33690004) is flat, about 5/8 inch wide, about 24 inches long, and the ends are bonded to form a continuous loop about 7 1/4 inches in diameter. The belt was broken at the point where the ends are bonded. The ends had uniform fraying laterally across the width of the belt. There were no visible signs of cutting at the point where the belt separated. There were no mechanical anomalies discovered with engine operation. The main rotor head was damaged, and the three main rotor blades were destroyed. The roof and windscreen of the pilot/passenger cabin was crushed inward along a diagonal plane from the nose to the top of the cabin's aft bulkhead. The forward section of the cabin floor was buckled accordion fashion reward, and the tail cone was buckled near its attachment to the cabin. ADDITIONAL INFORMATION Instructions for the emergency operation of the helicopter following a hydraulic failure are contained in the Owner/Operator's Manual, and read in part: Hydraulic Servo-control System Failure In-flight: Calmly reduce collective pitch and adjust the airspeed to between 40 and 60 knots in level flight. Disengage the collective pitch hydraulic pressure. Control loads are felt: on collective pitch increase on forward and L. H. cyclic If necessary, increase I.A.S., but the control load feedback will also increase. Make a flat approach over a clear landing area, and land with slight forward speed. There are no 'warnings' in the operator's manual with regard to re-energizing the hydraulic system via the collective lever mounted control switch. During a telephone conversation with the IIC on April 17, the pilot said during the hydraulic failure emergency procedure he reduced the helicopter airspeed, and "cycled" the hydraulic pressure cutoff switch located on the pilot's collective control lever. When asked what he meant by "cycled," he said turned off, and then back on. He said he left the switch in the on position for the remainder of the procedure/flight. According to the manufacturer the hydraulic pump drive belt must be inspected on a recurring basis and has a service life of 600 hours of operation. The belt on the accident helicopter had been in operation approximately 419 hours. A survey of Service Difficulty Reports (SDR) compiled from Federal Aviation Administration (FAA) revealed that during the period from February 22, 1988, until April 23, 2002, there have been 48 reported failures/replacements of the hydraulic drive belt, part number 704A336900004. Of these, 14 were replaced because of the belt's condition found during inspection. The remaining 34 failed in-flight resulting in a total hydraulic system failure. The earliest records indicate, additional failures from March 14, 1980, but records are incomplete from 1980 through 1988. According to American Eurocopter, about 540 helicopters are currently flying with the same hydraulic drive belt system as the accident helicopter. As of the model year 2000, all AS-350 helicopters are fitted with an improved 'V-belt' to drive the hydraulic pump. About 600 AS-350 helicopters have been produced, of those about 60 were produced with or converted to the new 'V-belt' drive system. Under service Bulletin 63.00.08, older helicopters can be retrofitted with the new style drive belt, but the conversion is not mandatory.
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 operations 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.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
