Incident Overview
At 1520 Pacific daylight time, a Helijet Sikorsky S-61N/SP helicopter (serial number 61711) lifted into a hover at the Vancouver Harbour helipad for a 35-minute visual flight rules flight to Victoria. The aircraft carried three crew members and 15 passengers. As the pilot applied take-off power and the helicopter climbed vertically through about 30 feet above the deck, a sudden and immediate loss of power occurred from the No. 2 engine, accompanied by a significant torque split indication—No. 1 engine high and No. 2 engine low. The pilot rejected the take-off, turned the helicopter left, and landed with one engine inoperative on the northern edge of the helipad. No damage or injury resulted from the initial landing.
Sequence of Events After Landing
After the helicopter landed and stabilized, the crew secured the No. 2 engine, which had shut itself down. The pilot determined that passengers could not easily disembark in the parked position and began to taxi forward to improve access to the airstair door. During the short taxi, the tail wheel ran off the edge of the deck, allowing the rear fuselage to fall and strike the deck edge, resulting in damage to the aft section of the hull. The tail wheel became caught in the deck-edge net, preventing the pilot from pulling it back onto the helipad. The pilot instructed the first officer and cabin attendant to deplane the passengers while he kept the main rotor running, as the airstair door alignment was now suitable. After passengers moved into the nearby terminal building, the first officer cut the netting cable, freeing the tail wheel. The pilot then taxied to the eastern side of the helipad and shut down the helicopter.
Investigation Findings
Records indicated that both pilots were trained and qualified in accordance with Transport Canada regulations, and the helicopter was maintained per those regulations. The No. 2 engine was removed, disassembled, and examined; its fuel control unit and flow divider were bench-tested and disassembled at an approved overhaul facility. No defects were found in the engine or its components that could have caused or contributed to the incident. The main-rotor gearbox was also removed and examined. Except for the No. 2 (right) input freewheel unit (IFWU), no defects were found. The No. 2 IFWU gear housing exhibited wear well beyond tolerance—eccentric wear with a depth greater than normally seen in a component with similar in-service life. The maximum permitted internal dimension at overhaul is 3.7535 inches; the No. 2 housing measured 3.7595 inches in places, exceeding the rejection limit by 0.0059 inch. Additionally, the wear was 0.0045 inch out of round. In contrast, the No. 1 IFWU showed less than 0.001 inch of uniform wear.
The IFWU acts as a clutch mechanism to engage or disengage engines from the main-rotor transmission. When an IFWU slips, the associated engine is instantly off-loaded, often reaching an overspeed condition and automatically shutting down. Both IFWUs were installed in January 1999, eight months before the incident, and had accumulated 464 hours of service. The normal service life is 1250 hours.
Wear in an IFWU is typically caused by overrunning—when the main-rotor gearbox input turns faster than the driving input speed from its engine. This occurs during rotor engagement when one engine operates at lower rpm than the other. The severity of wear can be influenced by operational variables and pilot technique.
Contributing Factors
Sikorsky Aircraft had identified premature IFWU removals in S-61 helicopters in early 1980 and issued a customer service letter (CSL-P-80-011) recommending operational techniques to reduce wear: alternating rotor engagements between engines, moderating engine acceleration when matching torques, monitoring free power turbine overshoot, and reducing IFWU freewheeling during ground operations. Helijet had not implemented the alternating starting procedures from the letter. The Transport Canada-approved rotorcraft flight manual (RFM) did not include reference to alternate starting procedures. Helijet's starting procedures involved starting the No. 1 engine first, engaging the rotor, then starting the No. 2 engine—always exposing the No. 2 IFWU to an overrun condition. Additionally, it was common to shut down the No. 2 engine first, stopping the main rotor, then shutting down the No. 1 engine, further overrunning the No. 2 IFWU. The helicopter had also been used for type-training, including single-engine flight operations with the No. 2 engine retarded, exposing that IFWU to additional overrunning.
The review of flight crew actions revealed no indication that their actions contributed to the chain of events. However, the company's standard operating procedures likely contributed to accelerated wear. Accelerated wear in the IFWU was probably caused by a combination of frequent overrunning of the same IFWU, frequent single-engine training, and rotor engagement techniques. It was not determined why the wear was eccentric.
Safety Actions
Following the incident, Helijet implemented an improved starting and engagement procedure: starting both engines and engaging the rotors together, significantly reducing overrun. They also reduced the maintenance inspection cycle for IFWUs to 300 hours. An inspection at 300 hours in service revealed virtually no wear on the IFWU housing, though the cam and rollers showed some wear. Helijet replaced the IFWU and scheduled another inspection after 450 hours. Sikorsky submitted the improvements from CSL-P-80-011 to the FAA for approval and incorporation into the RFM. The Transportation Safety Board released this report on 3 October 2001.