N74317

HISTORY OF FLIGHT

On December 15, 2015, about 1723 mountain standard time, an Airbus, AS350 B3 helicopter, N74317, was substantially damaged when it impacted terrain while maneuvering near Superior, Arizona. The helicopter was registered to and operated by Air Methods Corporation, doing business as Native Air Ambulance, under the provisions of Title 14 Code of Federal Regulations (CFR) Part 135. The commercial pilot and the flight nurse sustained fatal injuries, and the flight paramedic sustained serious injuries. Visual meteorological conditions prevailed, and a company visual flight rules (VFR) flight plan was filed for the repositioning flight. The cross-country flight originated about 1708 from the Phoenix-Mesa Gateway Airport (IWA), Mesa, Arizona, with an intended destination of Globe, Arizona.

According to the operator, the helicopter was based in Globe and had transported a patient from the Cobre Valley Community Hospital in Globe to the Baywood Heart Hospital in Mesa. After transporting the patient, the pilot flew the helicopter to IWA for refueling before the return flight to Globe. (See figure 1)

Figure 1-Overview of FlightsThe flight paramedic stated that after refueling, they departed IWA and headed east toward the Superstition Mountains. Local radar and flight data obtained from an onboard Appareo GAU2000 "data logger" device showed that the helicopter departed IWA about 1708 and headed east-northeast maintaining an altitude of about 500 ft above ground level (agl). The helicopter made a 360° right hand turn over the small community of Gold Canyon at 1715. According to the flight paramedic, the flight nurse's daughter was outside her house in the small community, waving as they flew by at 400-500 ft agl. Over the next 4 minutes, the flight track continued east along the south side of the Superstition Mountains at or below 500 ft agl.

About 1719, the helicopter entered mountainous terrain, and the height of the helicopter above the terrain began to vary as the terrain elevation rose and fell. Between 1718 and 1720, the altitude varied between 240 ft agl and 1,150 ft agl. Between 1720 and the end of the flight, the altitude varied between 30 ft agl and 770 ft agl. About 1721, the helicopter turned from a heading of about 80° to a heading of about 45° and followed a canyon beneath its ridgelines. (See figure 2)

Figure 2-Aerial View of Accident FlightThe helicopter flew nearly perpendicularly over the north-south oriented Rogers Canyon, and at 1723:07, it continued through a saddle on the canyon's east wall, clearing the terrain by about 30 ft. As it passed over the eastern ridgeline, the helicopter banked to the right, changing from a ground track of about 43° to 76°, and reached a ground speed of about 120 knots. After the helicopter cleared the ridge, it started to descend and accelerate. The ground speed reached a maximum of 148 knots at 1723:21. The helicopter banked right (about 5° to 10° of roll), and its heading changed from 76° at 1723:18 to about 90° at 1723:32. (See figure 3)

Figure 3-Aerial View of the Accident Flight and Accident FlightAt 1723:32, the GAU2000 recorded an abrupt increase in the helicopter's pitch rate and right roll rate, consistent with right and aft cyclic inputs. According to the paramedic, around this time, the pilot said an expletive in a panicked voice. The paramedic looked up and saw a ridgeline immediately in their flight path and terrain filling up the view. The paramedic described the subsequent motions of the helicopter as a violent hard right bank, and he stated that the pilot did not say anything else but was making jerky fast hand movements. The helicopter impacted terrain on the northwest facing slope of a ridgeline, near a saddle, at an elevation of about 5,035 ft mean sea level.

PERSONAL INFORMATION

The pilot, age 51, held commercial pilot and flight instructor certificates, both with a rotorcraft-helicopter rating. His most recent Federal Aviation Administration (FAA) second-class airman medical certificate was issued on December 8, 2015, with the limitation that he must have available glasses for near vision.

A review of company documentation revealed that he had accumulated about 5,670 hours of flight experience of which about 2,117 hours were in the same make and model as the accident helicopter.

The pilot completed his initial company training in September 2014. He received his most recent annual 14 CFR 135.293 and 135.299 airman competency/proficiency check on August 22, 2015.

The paramedic stated that the pilot was the safety officer at the Globe base and took the job very seriously. He stated that the pilot flew lower than the other pilots but was never dangerously low. The pilot was one of his favorites to fly with because he was very helpful and would aid the medical crew with duties such as cleaning out the stretchers. He did not think that the pilot took risks or operated dangerously.

AIRCRAFT INFORMATION

The helicopter, serial number 4317, was manufactured in March 2007. At the time of the accident, the helicopter had accumulated about 4,236 flight hours. The helicopter was equipped with a Turbomeca Arriel 2B1 turboshaft engine, which had accumulated about 2,491 hours. The helicopter's weight at the time of the accident was about 4,801 pounds, which was less than the maximum gross weight of 4,961 pounds.

According to the operator, the helicopter was maintained under an FAA-approved aircraft inspection program. Helicopter logbook records showed the following maintenance events during the days before the accident:

• December 15, 2015: daily check of the tail rotor laminated half bearings for deterioration

• December 14, 2015: 10-hour inspection

• December 10, 2015: 10-, 15-hour/7-day, 25- and 30-hour inspections

The helicopter was equipped with an Artex Aircraft Supplies, INC., (now ACR Electronics Inc.), C406-N HM Emergency Locator Transmitter (ELT), part number 453-5061 (serial number 04326). According to the manufacturer's original documents for that serial number, the ELT was manufactured in October 2007. The helicopter records indicated that the ELT was installed in May 2008 by Texas Aviation Services. The ELT battery, part number 452-0133 (serial number 359028-018), was recorded as being installed in May 2015. The last maintenance that occurred was recorded as consisting of a check per 14 CFR 91.207 (d) on October 29, 2015.

The Airbus AS350 B3 is equipped with a single hydraulic system, which provides 600-psi hydraulic boost to the cyclic, collective, and tail rotor controls. The main rotor control system consists of a series of rigid rods interconnected by bell cranks and reversing levers. The respective control linkages interface with the swash plate through three hydraulic servo actuators, which are designed to exert the necessary control force. If the required control force exceeds the maximum force that can be provided by the available servo pressure, the hydraulic system reaches its limitation, and the remaining required force must be supplied by the pilot via the flight controls. This can be felt by an apparent stiffening of the controls, which become gradually heavier to operate. The phenomenon that then arises is called servo transparency. Servo transparency is also known as hydraulic transparency, servo reversibility, and jack stall. In short, servo transparency begins when the aerodynamic forces acting to change the pitch of the rotor blades exceed the hydraulic servo actuators' capability to resist those forces and maintain the commanded blade pitch angles. The NTSB has examined the servo transparency phenomenon before, during the investigation of a September 30, 2003 accident involving an Aerospatiale (Eurocopter, now Airbus Helicopters) AS350BA helicopter in the Grand Canyon (NTSB # LAX03MA292), and produced a "Servo Transparency Study." That Study located in the public docket for LAX03MA292, describes servo transparency as follows:

According to [Airbus], servo transparency is a condition when the forces exerted from the rotor system overcome the force handling capability (output) of the flight control hydraulic actuators. The condition manifests itself when the aerodynamic forces of the main rotor system in flight are higher than that of the hydraulic servo control force. The main rotor system forces are transmitted (feedback) back through the flight control pushrod/bellcrank system through all three main servos of the AS350 helicopter to the pilot's controls. The feedback forces usually occur only during extreme maneuvering. The servo transparency is also known as hydraulic transparency, servo reversibility, and jack stall. ...

According to [Airbus], servo transparency begins when the aerodynamic forces generated by the main rotor system exceed the hydraulic forces from the control system and the difference between the forces is transmitted back to the pilot's cyclic and collective controls. On clockwise turning main rotor systems, the right servo receives the highest load when maneuvering, so when servo transparency condition occurs, it results in an uncommanded right and aft cyclic motion accompanied by down collective movement. The force transmitted through the controls tends to be progressive and the feed back forces through the controls could give an unaware pilot the impression that the controls are very hard to move or are jammed. The amplitude of the induced control feedback loads is proportional to the severity of the maneuver, but the phenomenon normally lasts less than 2 seconds when the pilot is aware of the condition and relaxes the pressure on the flight controls. ...

On December 4, 2003, Eurocopter published Service Letter No. 1648-29-03 concerning servo transparency. In the service letter, pilots were advised about the servo transparency phenomenon, what happens during the event, how it manifests itself, factors that increase the likelihood of encountering the condition, what to do i...