What happened
A Eurocopter AS 350 B2 helicopter, registration C-GOGN, was conducting a moose survey near Sault Ste. Marie, Ontario, when it experienced a hydraulic system failure. The aircraft was carrying a pilot and three passengers, all employees of the Ministry of Natural Resources. After the failure, the pilot notified ground personnel of the emergency and attempted to reach a logging site at Mekatina to perform an emergency landing.
As the helicopter approached the landing area, it entered a left turn and subsequently lost controlled flight. The aircraft crashed into rising, wooded terrain east of the logging site, landing in an inverted position. The crash resulted in four fatalities. There was no fire following the impact.
The investigation
The investigation focused on the cause of the hydraulic failure, the aircraft's flight characteristics during the emergency, and the pilot's actions. Investigators examined the hydraulic pump drive belt and found evidence of extensive cracking, suggesting a potential design flaw that could lead to predictable failures. Laboratory testing of the failed belt revealed stress concentrations at the same location across multiple samples.
Investigators also looked into the state of the hydraulic circuit breaker, which appeared to have tripped in flight. This tripped breaker likely rendered the hydraulic CUTOFF and HYD TEST switches inoperative, preventing a smooth transition to manual controls and causing an asymmetric depletion of pressure in the main-rotor servos. Furthermore, the investigation explored the pilot's decision to fly at higher speeds and the physical forces encountered during the low-altitude maneuver.
Findings
- The primary cause of the accident was the helicopter departing controlled flight while maneuvering for landing.
- The hydraulic pump drive belt failure was the initiating event that caused the hydraulic system failure.
- The hydraulic circuit breaker was likely in the tripped position during flight, which prevented the pilot from using the hydraulic CUTOFF switch to balance pressure, leading to asymmetric hydraulic pressure depletion.
- The pilot may have encountered extreme control forces during the left turn that were too great to overcome at a low altitude.
Safety action
Following the investigation, Transport Canada issued several Airworthiness Directives. These included mandates for the installation of an improved Poly-V belt to replace the problematic design and requirements for pilots to perform thorough pre-flight checks of the hydraulic accumulator pressure. Additionally, modifications were required for the hydraulic bypass system to reduce residual pressure levels.