Accident Overview
A Bell 212 HP helicopter (registration C-FWDV, serial number 30973) was conducting heli-skiing operations near Blue River, British Columbia. After departing from a glacier at approximately 8,000 feet above sea level, the pilot initiated a downwind approach to land at a pick-up area at the toe of another glacier. At about 150 feet above ground level and 30 knots airspeed, the pilot increased collective pitch to reduce the rate of descent, but the engines—Pratt & Whitney Canada PT6T-3DF—did not respond. The low rotor rpm warning activated, and rotor rpm decreased. The pilot lowered the collective and confirmed the rpm beep was full up and the engine throttles were fully open.
Emergency Landing and Damage
The pilot directed the helicopter toward a snow-covered frozen lake. The sink rate could not be arrested as rotor rpm did not recover. The helicopter landed hard, yawed about 90 degrees to the right, and remained upright. Deep snow absorbed some impact forces, but the helicopter sustained substantial damage. After landing, rotor rpm appeared to accelerate, and the pilot shut down the engines immediately. The pilot, the sole occupant, was not injured.
Environmental and Operational Conditions
Outside air temperature ranged from 0°C to −5°C, and the emergency landing altitude was about 6,100 feet above sea level. Sky conditions were overcast with good visibility. The descent profile was shallow because the intended landing area was one valley over and only about 2,000 feet lower. The helicopter's gross weight was approximately 8,200 pounds, which is 3,000 pounds below maximum gross weight.
Investigation Findings
The helicopter incorporated Supplemental Type Certificates related to the PT6T-3DF engine installation; notably, the Twin-Pac was not fitted with Pg air accumulators. Post-accident inspections found fuel clean and appropriate. Engine controls were intact and functioned properly, with the rpm trim beeped full up. The torque control unit (TCU) was removed; a simple test indicated the TCU ports appeared closed. Oil magnetic chip plugs from the engines and combining gearbox (C-box) were inspected; only the C-box centre plug contained metal chips (silver and iron particles). The C-box oil filter also contained such particles.
When the Twin-Pac was tested in an approved engine test facility, the engines did not operate normally: both engines' rpm oscillated and they would not accelerate on demand. In manual governor mode, they operated normally. The TCU was disconnected, but the problem persisted. Replacement of the power turbine (Nf) governors with similar units (about 400 hours in-service use) restored normal operation.
Engine Control Anomalies
The original Nf governors had control arms statically positioned at 74° and 73°, whereas standard positions are 85° or 90°. When reinstalled at 90°, the engines operated normally. When the substitute governors were rigged at 74° and 73°, oscillations and acceleration issues reappeared. Bench testing of the accident governors revealed they bled Pg air at 11.9 inches Hg when specification calls for 6.8–7.8 inches Hg. Disassembly showed in-service wear on bushings and uneven seating of port valves. The governors had only 823 hours in service versus an overhaul life of 4,500 hours.
A review of maintenance records indicated that maintenance personnel had replaced the TCU to address torque fluctuations. A replacement TCU was not readily available; Pratt & Whitney Canada suggested a new version approved for -3B engines. This new TCU had its Pg constant bleed ports capped (blocked), which affected turbine speed (Nf) on both engines, necessitating adjustment of the control arms. This rigging change was not incorporated in approved maintenance manuals. The new TCU operated for about 150 flight hours before the accident.
The Nf governors had been installed for about 823 flight hours. A well-known overhaul facility reported that these type of governors are typically received for repair due to wear at an average of 1,600 hours. Pratt & Whitney Canada and Bell Helicopters Textron worked in autumn 2002 to resolve TCU issues, finding that Nf governors performed better without TCUs. Bell did not approve removal but approved a TCU with blocked constant bleeds; limited use began January 2003 for specific PT6-3B engines.
Conclusions
The installation of a non-standard TCU required abnormal rigging of the Nf governors. This non-approved rigging amplified normal-type wear, causing governor malfunction and inadequate power from both engines upon pilot demand. Rpm and torque oscillations likely aggravated the governors' weaknesses due to wear, causing simultaneous malfunctions. The loss of power occurred at a critical phase, resulting in a hard landing. The investigation also noted that in-service wear causes governors to malfunction before reaching their 4,500-hour overhaul life; the average removal time for repair is about 1,600 hours.