WEEK 11
H2F brings you the ‘helicopter mayday of the week’ with a short accident report summary. I tell you what happened. You think more about why it happened. We all learn from it. Because that’s what accident reporting is for.
TITLE
Low-level high-speed maneuvering in mountainous terrain leads to servo transparency.
WHAT?
An AS350 air ambulance helicopter was repositioning to its base in Globe, Arizona following a patient transport when it entered mountainous terrain east of Phoenix. The pilot flew at low altitude along ridge lines and through canyons, at times clearing terrain by about 30 feet. After crossing a ridge line near a saddle, the helicopter accelerated to about 148 knots. Seconds later, flight data recorded abrupt pitch-up and right roll rates consistent with right and aft cyclic inputs characteristic of retreating blade stall. The helicopter impacted terrain on the northwest-facing slope of a ridge line.
WHERE?
Near Superior, Arizona, in rugged mountainous terrain with surrounding peaks between 5,700 and 6,000 ft mean sea level.
WHEN?
December 15, 2015, at 17:23LT, shortly after sunset during dusk conditions. The NTSB final report was published on March 18, 2019.
HOW?
After refuelling in Mesa, the helicopter departed under VFR and proceeded east toward mountainous terrain. Flight data showed prolonged low-altitude, high-speed maneuvering, including a 360° turn and terrain-following flight through canyons. As the helicopter crossed a ridge line at high speed and banked right, aerodynamic loads increased. The helicopter entered servo transparency, where rotor aerodynamic forces exceed the hydraulic servo capability, resulting in uncommanded right roll and pitch-up. The rapid onset of control feedback forces, combined with low altitude and high airspeed, led to loss of control and impact with terrain.
CONDITIONS?
Weather was VMC with light winds and good visibility & density altitude at the accident elevation was over 5000ft. The helicopter was operating below maximum gross weight however, the flight profile involved low-altitude maneuvering near VNE, increasing load factor. These conditions are known to increase susceptibility to servo transparency, particularly when maneuvering at high speed close to the ground.
OUTCOME?
The helicopter was substantially damaged. The pilot and flight nurse sustained fatal injuries, while the flight paramedic sustained serious injuries and survived. The ELT failed to activate on impact, and satellite tracking loss was not immediately recognised, delaying search and rescue efforts.
WHY?
- The pilot’s (who was also the Base Safety Officer) decision to conduct low-level, high-speed maneuvering near VNE through mountainous terrain.
- Loss of helicopter control after operating outside the hydraulic system performance envelope and encountering servo transparency.
- Failure of the other crew members to question the flight profile and pilot’s behaviour.
- No mention is made in the report of organisational factors including company culture, supervision, or SOPs.
REFERENCE?
National Transportation Safety Board. (2019, March 18). Aviation investigation final report: Accident number WPR16FA040 (Airbus Helicopters AS350 B3, N74317). National Transportation Safety Board. https://data.ntsb.gov/Docket?ProjectID=92454
Note:
Accident reports selected from the following open source databases: US NTSB; UK AAIB; Flight Safety Foundation’s Aviation Safety Network; Australia’s ATSB. Ireland’s AAIU; Taiwan’s TTSB; France’s BEA; Spain’s CIAIAC. Germany’s BFU.
competentaviators.com 