Background
On Wednesday, 13 August 2003, the occurrence pilot completed the daily flights at the Bonaparte Lake site and returned to Kamloops, British Columbia, where the engine and transmission were to be changed; both were time expired and due for an overhaul. The total airframe time was 11 538.8 hours. On 13 and 14 August, the engine and transmission assemblies were changed by company maintenance personnel. A Honeywell (Lycoming) engine assembly model T5311B, part number 1-000-080-11, serial number LE-08228, was installed. This engine had a total time since new of 7398.1 hours and had been overhauled to a zero-time condition. A Bell Helicopter Textron transmission assembly part number 204-040-009-087, serial number B12-601, was also installed. This transmission had a total time since new of 6808.2 hours and 396.5 hours since overhaul.
The new components were run for the first time on the evening of 14 August, and the following morning, an engine on-ground power check (partial power) was performed. The power plant was operating at 1.4 per cent below the calculated N1 rpm required, and N1 take-off trim adjustment of 3/8 of a turn was required to increase N1 to the calculated figure. An entry was made in the aircraft journey log indicating that all work done was certified, pending a satisfactory test flight, including an N1 topping check and a power check. A test flight was performed and engine operating parameters relevant to the power check were recorded in the aircraft journey log on 15 August 2003. The data was plotted against the engine operation and functional-check charts, and results were also entered in the journey log. It was recorded that the engine was operating at an N1 speed of 2.3 per cent below the allowable limit of 93.3 per cent and an exhaust gas temperature (EGT) of 60°C below the allowable limit of 590°C. When the power-check calculations were reviewed during the investigation, it was determined that both were correct. Maintenance logs or personal notes did not record any results to verify the N1 topping speed after the engine was installed into the helicopter before or after the N1 adjustments. Such a record is not required by regulation. The pilot signed off the journey log, approving the aircraft for further flight.
An engine vibration check was completed at the overhaul facility; however, a vibration check was not accomplished prior to returning the aircraft to service following the engine's installation in the helicopter. The Honeywell Lycoming engine maintenance manual specifies that a vibration check shall be performed following an engine installation.
The helicopter resumed work at the Bonaparte Lake site. After about 20 minutes of working, the pilot relayed a message to the maintenance crew in Kamloops, requesting that the engine vibration test equipment be kept. Another 30 minutes later, the pilot reported to the British Columbia Forest Service office that the helicopter was unserviceable and he returned to Kamloops. The pilot reported a buzzing cowling and that the engine lacked sufficient power to lift the load, resulting in the main-rotor rpm drooping. The vibration test kit (Chadwick-Helmuth 7460A) was then installed on the engine and a flight test was carried out. The vibration check indicated that the engine was operating within specifications. The failure to perform a vibration check at initial installation, therefore, had no bearing on the subject failure, since a vibration check was conducted with no problems being indicated approximately 50 flying minutes after engine installation. Subsequent to the test flight, the power and drooping issues were addressed by a further small increase in the N1 take-off trim setting. No record of an N1 topping check could be provided. The next morning (16 August), the helicopter returned to the staging site and the pilot recorded 8.8 hours of air time for the day, during which the pilot made several reports to indicate that operations were normal. No further adjustments were made to the engine or transmission when the helicopter returned to Kamloops in the evening. The helicopter was refuelled at Kamloops with 563 litres of Jet A (maximum capacity of 915 litres).
Accident Sequence
On 16 August 2003, at about 1105 Pacific daylight time, the helicopter departed a staging site east of Bonaparte Lake, British Columbia, slinging an empty water bucket on a 100-foot longline for the first mission of the day. Shortly after take-off, the helicopter emitted a high-pitched, oscillating sound. The flight path and behaviour of the helicopter were normal as it went out of view over some trees. Immediately thereafter, there was the pronounced sound of main-rotor blade slap, followed by the sounds of impact with the trees. The helicopter struck the ground just short of reaching a small clearing adjacent to a fire road about ¼ nautical mile southeast of the staging site. A post-impact fire ensued, which destroyed the helicopter cabin area and melted the transmission casing and pylon structure. The main driveshaft assembly remained attached to the engine and transmission input quill assembly. The water bucket was found in a tree, detached from the longline, on the approach to the accident site. The longline was wrapped around another tree and lay in a direct line to the helicopter; other than the metal parts, the top 15 feet of the artificial fabric line was burned away. British Columbia Forest Service crews attended the site and extinguished the fire.
Pilot Information
The pilot, experienced and familiar with the type of operation being conducted, had been hired by Gemini Helicopters Inc. on a temporary basis to provide relief for the pilot normally assigned to the helicopter. He possessed a valid Canadian airline transport pilot licence - helicopter, and had completed a pilot proficiency check on the Bell 204 in June 2003. Records indicate that the pilot had accumulated about 7500 hours of total flight time, of which 200 hours had been flown on Bell 204 helicopters in the previous 90 days. He had flown C-GEAP during the last two weeks of June and then again beginning on 13 August 2003.
Meteorological Conditions
The staging site was located at an elevation of about 4500 feet above sea level (asl) in an area of rolling hills. The vegetation consisted of undergrowth in a primarily spruce forest. The prevailing weather conditions were sunny, hot and dry. At the time of the accident, a British Columbia Forest Service weather site, 14 nautical miles to the southwest at an elevation of 3839 feet asl, recorded a temperature of 20°C with variable winds that reached a maximum speed of 8 knots throughout the day. A normal lapse rate would yield a temperature at the elevation of the staging site of about 19°C. An altimeter setting of 30.04 inches of mercury was recorded at the Kamloops Airport, 33 nautical miles to the south (elevation 1133 feet asl). At an average working altitude of 4500 feet asl, the weather conditions existing at the time equate to a density altitude of about 5900 feet and a pressure altitude of about 4620 feet. The previous day was about 5° warmer, with an additional 3 knots of wind.
Aircraft Information
The Bell 204B helicopter (registration C-GEAP, serial number 2063) was equipped with a single Honeywell (Lycoming) T5311B free-turbine engine, with a design limit of 1100 shaft horsepower. At the time of the accident, the helicopter had flown for about 10.9 hours since the engine and transmission were installed. The helicopter was not equipped with a flight data recorder or a cockpit voice recorder, nor were they required by regulation.
The Bell 204B flight manual indicates that, under the ambient conditions that existed, the helicopter would hover out-of-ground effect at a gross weight of 3484 kg. The estimated weight of the helicopter at the time of the accident was 3548 kg. The collapsible Bambi water bucket had a maximum capacity of 1225 litres and a maximum gross weight of 1280 kg. To restrict the capacity, the bucket was equipped with a circumferential strap, which could be manually cinched. The bucket, as found, was cinched to 80 per cent capacity (980 litres or 980 kg). The helicopter gross take-off weight for external load operations was 4309 kg. The centre of gravity was not considered to have been a factor.
The helicopter was equipped with a vertical reference kit that allowed for single-pilot operation from the left seat for longlining operations. Part of the vertical reference kit was a third foot pedal that provided mechanical release capability for the longline.
Injuries
The pilot was fatally injured.
Investigation
The investigation noted that an N1 topping check was not performed after adjustments to the take-off trim setting. The N1 speed was established to be 97.6 per cent at the overhaul facility, and this was the placard setting at which the engine produced its full-rated horsepower under International Standard Atmosphere conditions at sea level. By providing a physical restriction to fuel flow, the take-off trim setting is the N1 overspeed protection device. It is possible to operate the engine at higher N1 speeds, approaching the compressor overspeed limit, and it appears to produce all desirable effects; that is, more power, while apparently remaining within the allowable engine torque and EGT parameters. Without the confirmation of an N1 topping check after an adjustment, it is not known whether the N1 speed is within specifications. After the N1 was adjusted the last time, the pilot operated the helicopter the following day and made several reports indicating that operations were normal, which would suggest that the change appeared to be beneficial. However, if the N1 was above specification, excessive T5 temperatures could result in a degradation of the hot section components.