Pilot Debrief

AIRBUS Helicopters AS350 near Superior, AZ — 2015-12-16

Final reportWPR16FA040
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Date
2015-12-16
Location
Superior, AZ, USA
Aircraft
AIRBUS Helicopters AS350
Registration
N74317
Category
Helicopter
Highest injury
Fatal
Fatalities
2
Phase of flight
Maneuvering

Probable cause

The pilot's loss of helicopter control in mountainous terrain as the result of operating the helicopter outside the performance envelope of its hydraulic system and encountering the servo transparency phenomenon. Contributing to the accident was the pilot's decision to perform low-level, high-speed maneuvers through mountainous terrain.

Contributing factors

Contributing to the accident was the pilot's decision to perform low-level, high-speed maneuvers through mountainous terrain.

NTSB narrative

The commercial pilot was repositioning the helicopter (with a flight nurse and flight paramedic on board) to its base following an air ambulance flight. The paramedic, who survived the accident, reported that after refueling, they departed and headed east towards mountainous terrain; peak elevations were 5,700 to 6,000 ft. About 10 minutes after takeoff, the helicopter entered the mountainous terrain, and the height of the helicopter above the terrain began to vary as the terrain elevation rose and fell. During the final few minutes of the flight, the helicopter's altitude above the ground varied between 30 ft and 770 ft. About 30 seconds before impact, the helicopter flew east over a north-south canyon and continued through a saddle on the canyon's east wall, clearing the terrain by about 30 ft. As the helicopter passed over the eastern ridgeline, it banked to the right 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, and about 10 seconds later, there was an abrupt increase in the helicopter's pitch and right roll rates, 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 flight characteristics seconds before impact, as described by the paramedic and shown in flight data, were consistent with a rapid onset of servo transparency. The helicopter impacted terrain on the northwest facing slope of a ridgeline, near a saddle, at an elevation of about 5,035 ft. 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 force deficit is then transmitted back to the pilot's cyclic and collective controls. On clockwise turning main rotor systems such as the AS350B3, the right servo receives the highest load when maneuvering (retreating blade), resulting in an uncommanded right and aft cyclic motion accompanied by down collective movement. The NTSB's Servo Transparency Study for accident No. LAX03MA292 notes that the pilot's control force required to counter this aerodynamically-induced phenomena "tends to be progressive" and is "proportional to the severity of the maneuver," and "may give a pilot who is not aware of this phenomenon an impression that the controls are jammed." If the pilot does not reduce the maneuver, the aircraft will roll right and pitch-up, but the phenomenon normally lasts less than 2 seconds. The general load on the main rotor increases under the following conditions: high speed, high torque (increase in collective pitch), high g-load, and increase in density altitude. Although the helicopter will self-correct and recover from the servo transparency, the potential exists for a significant flight path deviation. The onset of servo transparency is rapid and could conceivably lead to a helicopter in a right turn exceeding 90º of bank before the pilot was able to recognize what was happening and react accordingly. The associated transition from light and responsive controls to heavy controls that require considerable force to counter the uncommanded maneuver, could cause an unsuspecting pilot to believe that he was experiencing a malfunction, rather than a known characteristic of the helicopter when maneuvered at the published performance limits. The Operation Control Center mistakenly lost tracking of the helicopter about 2 hours and 10 minutes after the accident occurred. Another company helicopter was then sent to search for the accident helicopter and located the wreckage about 50 minutes later. Due to the mountainous terrain and limited access to the accident site, another helicopter responded to the area about 4 hours after the accident and was capable of hoisting medics to the accident site. The emergency locator transmitter (ELT) did not activate during the accident sequence, resulting in the delayed response of the search and rescue teams. Examination of the ELT revealed that the G-switches in the unit failed to activate due to a powdery residue from internal wear. The pilot's autopsy identified a lesion in his brain consistent with a cavernous hemangioma. No bleeding or other acute finding around the cavernous hemangioma was described by the autopsy report, and no other natural disease was identified. Given the paramedic's description of the flight indicating that the pilot was actively flying, it is unlikely that the cavernous hemangioma contributed to the accident. According to the paramedic, the flight nurse survived the impact and was initially awake and alert, but over time, he became increasingly short of breath and eventually stopped talking and breathing. An injury study was completed on the flight nurse to evaluate whether improved communication regarding the timing and location of the crash, such as through a functioning ELT, could have allowed him to survive. Given the flight nurse's injury severity, particularly to the chest, the amount of internal bleeding, and the fact that he was wet and exposed with minimal clothing in 20°F temperature, it is unlikely that he would have survived until help arrived even if the initial notification of the crash had occurred more rapidly. No evidence was found of any preimpact mechanical malfunctions or failures of the airframe or engine that would have precluded normal operation. Examination of the accident site and wreckage revealed that the helicopter impacted terrain in a right bank and level attitude.

Analysis

Primary failure mode
Loss of control
First missed decision gate
Pilot could have avoided aggressive maneuvers near terrain to prevent loss of control.

NTSB coding

Evidence available

  • Photos
  • 34 docket documents
View NTSB final reportView NTSB docket

Docket documents34

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