The Accident
A Bell 206L helicopter (serial number 45096) was returning from a forest fire mapping flight north of Grand Lake, Labrador. The pilot and two forestry employees were on board. The helicopter was flying east at 400 feet above ground level (AGL) at a cruising speed of 130 mph. The pilot felt a slight, high-frequency vibration but saw no abnormal instrument indications. Shortly after, a loud bang was heard and the helicopter yawed sharply left. The pilot warned passengers, lowered the collective, reduced airspeed to 70 mph, and began an autorotation toward a spit of land. During descent, the pilot saw the Engine Out caution light briefly illuminate. Near the ground, at about 50 feet AGL, he initiated a flare and raised collective. He saw boulders on the landing spot and warned passengers. At about 5 feet AGL, full collective was applied, main rotor rpm decreased, and the helicopter landed hard. It yawed left about 45 degrees and came to rest upright with a 5-degree right list. The engine was still running with hissing sounds; the pilot shut it down. All occupants exited uninjured.
Damage and Examination
The helicopter sustained substantial damage: downward bending deformation of the tail boom and tail rotor drive shaft, and structural deformation of the aft fuselage. The helicopter was airlifted to the operator's base. The engine, main rotor, and transmission were removed. Removal of the mast shaft was difficult; gear fragments were found inside the transmission case. The engine was disassembled and ground-run in a test cell; no internal damage was found and it operated satisfactorily.
The transmission examination revealed damaged sun gear splines in the engagement area with the main ring gear shaft internal spline. The sun gear had been operating with about 0.30 inches of engagement. A wear pattern indicated a previous sun gear with greater engagement. The damaged sun gear (part number 206-040-122-103, serial number AFS 004575) was designed exclusively for the Bell 206B helicopter transmission. The correct sun gear for the Bell 206L is part number 206-040-122-005, which is 6.010 inches long; the installed gear was 4.8200 inches.
Transmission Function
Engine power enters the transmission via a spiral bevel gear that drives the first reduction stage ring gear. The ring gear's inner splines drive a sun gear that engages four planetary gears. The second stage ring gear is fixed, so planetary rotation turns the planetary gear case, which turns the mast shaft. The ring gear also drives the main rotor tach generator, transmission oil pump, and hydraulic pump. The low rotor rpm warning light and horn activate at 90±2% rpm.
Maintenance History
The transmission had been removed on 6 October 1996 for a 1,500-hour inspection, and the sun gear was replaced. The helicopter flew 391.0 hours after that replacement.
Ordering and Installation of Incorrect Part
The operator ordered the sun gear on 6 October 1996 outside normal working hours by calling Bell Helicopter's Calgary parts warehouse. The part was shipped the next day. The purchase order form recorded the part number the operator wanted, but the invoice quoted the part and serial numbers of the gear later found in the transmission. It could not be determined how the incorrect part number was introduced: whether the operator quoted the wrong number, the supplier's employee relayed it incorrectly, or Calgary personnel entered the wrong number.
When the gear arrived at the operator's main base, a records clerk noticed the last three digits of the part number on the purchase order (-005) did not match the invoice (-103). The clerk overwrote the purchase order to match the invoice without consulting maintenance personnel. The reason is unknown.
The gear was shipped to the satellite base in Goose Bay with a release certification tag showing the correct part number of the shipped gear. An aircraft maintenance engineer (AME) and his supervisor installed it without physically comparing it to the removed gear or consulting the Illustrated Parts Catalogue. They apparently relied on the main base to send the correct part. The shorter sun gear resulted in minimal engagement (0.30 inches) and contact with the ring gear shaft face.
Failure Mechanism
The incorrect sun gear was installed over 10 months prior. When the splines of the sun gear and ring gear failed, the engine became uncoupled from the main rotor. Main rotor rpm decreased; the pilot's autorotation checked the descent. Since the ring gear was still driven by the engine, the hydraulic pump and rotor tachometer generator continued to operate. The rotor rpm indicator may have briefly shown above 100% then returned to 100% and stayed there as long as the throttle was full open. Thus, the low rotor rpm warning did not activate.
Contributing Factors Identified by Investigation
The investigation identified two critical points where the error could have been caught: (1) when the records clerk amended the purchase order without consulting maintenance, and (2) when the AME and supervisor installed the gear without verifying its identity. The specific reasons for these lapses could not be determined. The breakdown in communication during ordering and shipping could not be pinpointed.