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N971AE accident description

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Tail numberN971AE
Accident dateDecember 14, 2004
Aircraft typeEurocopter AS-350-B3
LocationApache Junction, AZ
Near 33.408333 N, -111.4525 W
Additional details: None

NTSB description


On December 14, 2004, at 2237 mountain standard time, a Eurocopter AS-350-B3, N971AE, operating under the call sign Air-Evac II, collided with terrain while attempting a landing in a shopping center parking lot at Apache Junction, Arizona. The helicopter was operated by Petroleum Helicopters, Inc., (PHI) as a positioning flight under the provisions of 14 CFR Part 91. The commercial pilot and a medical crewmember both received serious injuries; a second medical crewmember received fatal injuries. The helicopter was destroyed. The air medical flight originated at a local base of operation and was positioning for a patient recovery at the landing site and transport to a hospital. Visual meteorological conditions prevailed and a company flight plan had been filed. The wreckage was at 33 degrees 24.50 minutes north latitude and 111 degrees 27.15 minutes west longitude.

The emergency landing zone was located in a store parking lot. The area for landing was secured by Apache Junction police and fire department personnel. The pilot was in communication with the ground personnel for a briefing on possible landing obstructions. The pilot confirmed the final landing zone and light pole obstructions that were pointed out to her.

Witnesses reported that the helicopter overflew the landing zone in right turns, then approached from the northeast. The witnesses' observations of the final approach altitude were estimated at 100 to 300 feet above ground level. None of the ground witnesses reported seeing a landing light showing on the front of the helicopter.

During the final approach the helicopter was observed by ground witnesses to become unstable. It rolled right on the longitudinal axis about 30 degrees, then left about the same, then right about 45 degrees, pitched nose up to the left, and descended while spinning to the left. The helicopter impacted the parking lot nose down on the left side. The left skid was destroyed and the right skid was intact. The engine continued to run after impact and was subsequently stopped by the fire department personnel spraying fire suppressant foam into the engine air inlet. Two witnesses heard the helicopter make a hissing sound while descending and spinning; the witnesses likened the sound to an "18 wheeler's (truck) air brakes bleeding off."

The pilot was interviewed in the hospital on December 18 by a Federal Aviation Administration (FAA) inspector and management representatives of PHI. The pilot reported that it was her 5th day of flying nights. She stated that she was not fatigued, nor under any stress, and had eaten regular meals during the day. She said she was not on any prescription or OTC medications other than birth control.

At 2200, her crew was paged for a scene response to Apache Junction for a trauma patient. The landing zone was a secured area in a strip mall parking lot secured by police and fire units from Apache Junction. As the medical crew arrived, she had the helicopter started and running, with all preflight checks completed. Prior to liftoff, she turned the exterior lights on to include the steerable search light, strobes, and rotating beacon; she did not turn these off while en route to the landing site. The night sun and taxi lights were off. She was wearing her own non-noise canceling headset and the volume control was normal (not all the way down).

The pilot said the scene location was a parking lot, with the designated landing area surrounded by four tall light poles. There was a fire engine on each edge of the LZ. The winds were calm. She performed one full circle around the landing zone and setup to land to the southwest. On final approach, and on top of the power lines along the road, she remembers saying "clear of power lines." She was at about 100 feet above ground level (agl) and had slowed to 20 - 25 knots. At that time she felt the aircraft nose come up to the right gently, but not as a yaw or a roll. She said she corrected left with the cyclic and the helicopter responded with a significant roll to the left. She remembered the aircraft starting to spin (at least once) and saw the buildings of the strip mall. She then put the cyclic to the full left to avoid the building. The helicopter came close to the ground with a 90-degree nose down attitude. She said she saw a red light she thought was the hydraulic caution light on just before impact, but did not hear the warning gong. She then grabbed the cyclic with both hands and pulled back and right but it didn't move. The anti-torque pedals appeared to work and she thinks that is what stopped the spin. The helicopter then impacted the ground.

The director of PHI Flight Safety provided a narrative history of flight on the Safety Board form 6120.1/2, Pilot/Operator Aircraft Accident Report. According to the statement: "Aircraft lifted off IWA at 2229 hours en route to a scene response in Apache Junction which was 9nm away. The LZ was in a strip mall parking lot and was identified by police and fire units on the ground. Upon arrival at the scene location the pilot made a high recon of the area to assess hazards and obstructions. She initiated her approach from the NNE and landing SSW. At approx. 100' and 20-25 knots and just after crossing wires on the approach the pilot states she felt the aircraft pull gently to the right and up. She moved the cyclic left and aircraft rolled violently left and started spinning. (At least once or twice). She saw a building. She pushed pedals to try and avoid buildings and aircraft was close to the ground and nose was approximately 90 degrees down. She grabbed the cyclic with both hands and couldn't move it. She remembers seeing a red caution light she thinks was the hydraulic caution light but does not remember hearing a gong."


A review of the PHI company training records and FAA airman and medical record files revealed that the pilot held a commercial pilot certificate, with rotorcraft ratings for helicopters and instrument-helicopter, and a private pilot certificate for airplanes single engine land at the time of the accident. The pilot obtained her first pilot's certificate in her native land of Spain, a private certificate rated for single engine land airplanes. The FAA issued her a U.S. private certificate on October 11, 1994, at Oakland, California, on the basis of the certificate issued by Spain.

The pilot was hired by PHI, on October 8, 2001. At that time the pilot reported a total helicopter flight time of 3,583 hours and 277 hours of fixed wing time. Of the helicopter hours, 3,084 were accrued in the Robinson R-22. At the time of the accident, the pilot had accrued a total helicopter time of 4,604 with 80 hours in the Eurocopter AS-350-B3. The pilot's flight instructor time was noted as 2,631 hours of R-22 time. With PHI, the pilot had accumulated about 300 hours PIC in the BO-105 helicopter and 300 hours in the Bell 206L. The pilot's most recent documented second-class flight physical occurred on May 10, 2004, with restrictions.

A review of the pilot training records revealed the most recent Airman Competency/Proficiency Check occurred on September 16, 2004. The type of check was the 14 CFR 135.293 and 135.299, a 12-month check.

The pilot's complete training file appears in the docket for this accident. In summary, the pilot completed three initial and multiple recurrent company 14 CFR Part 135 training programs on three different helicopters (Bell 206L, MBB BO-105, and the AS350) that have critical hydraulic systems. Emergency procedures for the hydraulic system are part of all the training programs, and include the criticality of the hydraulic cutoff and test switches for the AS350. Except for the BO-105 where complete loss of the hydraulic systems is catastrophic, actual flight training was conducted with simulated hydraulic system failures (by the check pilots turning off the systems).

According to PHI, once the pilots report for duty at their assigned base, no extra duties are given to the pilots beyond flying. Rest facilities, including beds, are provided. The pilots are at the facilities for 12-hour shifts. Examination of the flight and duty time logs for the Williams Gateway base disclosed that the pilot was off duty from December 5 to December 9. On December 10 through the accident date the pilot reported for duty each night at 1950 and signed off duty at 0800 the following morning. A one 28-minute flight was conducted on December 10. No flight time was recorded for December 11, 12, or 13. The accident flight was the first one on December 14.


1.3.1 General Maintenance History

The Eurocopter AS-350-B3 helicopter, serial number 3230, was manufactured in 1999. A review of the aircraft records disclosed that it had accumulated a total time in service of 2,496 hours. A Turbomeca Arriel 2B engine, serial number 22069, was installed in the airframe and had accumulated a total time in service of 1,826 hours.

The helicopter was maintained under an FAA approved continuous airworthiness inspection program. The most recent inspection, a 100-hour, occurred on November 14, 2004, 23 hours prior to the accident, at a total airframe time of 2,472.

1.3.2 Discrepancy History

Review of the PHI engineering report flight log sheets for this helicopter revealed that in the 3 months prior to the accident, two write-ups reported stiff flight controls, the last one recorded on November 15, and one write-up reported an increased amount of cyclic required to maintain straight and level forward flight. One additional write-up on September 3, 2004, noted that the hydraulic warning horn and light momentarily activated in flight.

The records list the corrective action for the September 3 horn and light discrepancy as "repaired damaged wire at the right-hand servo solenoid valve and cleaned cannon plug."

For the "stiff flight control" write-ups, a October 20, 2004, entry noted "flight controls stiff." The corrective action recorded by the maintenance department stated: "Disconnected servos from stationary swashplate. Isolated stiffness to lower controls. Cleaned bearing surfaces on lower controls. Reattached servos."

In a October 29, 2004, write-up, a pilot reported that an increased amount of cyclic input was required to maintain straight and level forward flight. The corrective action recorded by maintenance was: "Performed flight control rigging check. No Adjustments required."

On 15 November 2004 a pilot reported stiffness in the cyclic control. The corrective action listed by maintenance was: "Replaced L/H and Collective servos due to rough and ratchety piston. Performed M/R rigging."

1.3.2 Hydraulic Control System Transparency Phenomena

Discussions and correspondence were conducted between Safety Board investigators and Eurocopter engineers and flight test pilots, concerning the hydraulic control system transparency phenomena. While typical hydraulic systems in other helicopters and fixed wing aircraft operate at pressure ranges from 1,000 to 3,000 psi, the hydraulic system in the AS350 series helicopters operates at 600 psi. This system pressure was chosen during design as a balance between the loads generated by the rotor system that must be overcome and acceptable load limits on the rotor system dynamic components (control rods, links and fittings, etc.).

The "servo transparency" effect occurs in maneuvers that result in increased positive g-loading on the helicopter and rotor system. The threshold g-load values for onset of the phenomenon vary according to helicopter speed, gross weight, and the atmospheric density altitude. As explained by Eurocopter, when the helicopter reaches a threshold g-loading for the phenomenon onset, the hydraulic system does not have enough pressure available to move the main left lateral, right lateral, and fore/aft servos against the dynamic forces being fed back from the rotor system into the controls. At the onset of "servo transparency," the flight controls essentially go from boosted to manual reversion, where they remain until the g-loads decrease below the onset threshold values. According to the Eurocopter France Chief Test Pilot for the AS350 program, as the system enters the "transparency mode" the pilot would feel the collective lever moving down and the cyclic pitch channel moving to the right.

At the request of Safety Board investigators, Eurocopter France provided a copy of certification flight test report H/EV 17.530. The report documents a series of test flights conducted in 1985 to explore the points where the servo transparency effect (also called control reversibility) occurs. From the data collected, a series of graphic plots were developed and included in the flight test report, which predict the g-loading for phenomenon onset for a given weight, density altitude, and helicopter speed.

Eurocopter personnel stated that the "transparency" phenomenon is non-violent and transitory, lasting only 2 to 3 seconds at most due to the "self-correcting actions of the pilots" to reduce the g-loads and/or the natural static and dynamic stability "response of the helicopter." They also stated that the controls are fully operable throughout the entire "transparency" event; however, the force required to effect movement of the flight controls against the rotor system dynamic feed back loads would increase significantly. Eurocopter stated that the force feed back for each control channel would be dependent in part on the amount of g-loading experienced; however, they estimated that about 22 pounds of force would be required to move the collective in the UP or increased pitch direction, with the same amount to move the cyclic to the left.

Safety Board investigators reviewed Section 3.2 of the FAA/DGAC Approved Rotorcraft Flight Manual, which covers hydraulic system failures. This section lists the approximate force increase to move the flight controls following a hydraulic system failure. As examples, at a speed of 140 knots the force required for movement of the collective would go from minimal (boosted condition) to 44 pounds, while the force for lateral cyclic movement would go from minimal to 26 pounds, and the force for fore/aft cyclic movement would go from minimal to 9 pounds, depending on the direction of control input. According to Section 7.8 of the FAA/DGAC Approved Rotorcraft Flight Manual, the hydraulic system has accumulators for each control channel, which would "provide continued hydraulic assistance for a limited time in the event of a hydraulic pressure loss in the system. The limited time is sufficient to allow the pilot to achieve a flight regime (airspeed of 40 to 60 knots) under which the control feedback forces are acceptable without hydraulic assistance." Hydraulic System Shut-off Switch

A toggle switch is mounted on the end of the collective control lever, which allows the pilot to manually turn off the hydraulic system by depressurizing the pump output, system, and accumulators. Prior to 1990, the switch was guarded by perpendicular walls the height of, and on each side of, the toggle. In response to in-service reports (including one accident) where inadvertent deactivation of the hydraulic system occurred by objects (principally sleeve cuffs and straps) catching on the switch, Eurocopter changed the design of the switch guard, which incorporated a horizontal fixed plate cover over the toggle switch to preclude inadvertent movement of the toggle. With the new guard, two separate motions are required to first reach the toggle, then move it. Service Bulletin 67-17R2 was issued on October 25, 1990, and called for replacement of the old style guards with the new ones. A check of Eurocopter manufacturing documents and airworthiness conformity documents in the maintenance records disclosed that the accident helicopter was equipped with the new style guard, with SB 67-17 (through revision 2) incorporated on the production line. Hydraulic System Test Switch

The systems control panel (pedestal) has provisions for 36 backlighted push-on push-off switches. The switches h

(c) 2009-2011 Lee C. Baker. For informational purposes only.