What happened
On a flight departing from Winnipeg/St. Andrews Airport, Manitoba, a Cessna 207A (registration C-FBHP) experienced significant engine performance issues shortly after takeoff. The aircraft, operated by Northway Aviation Ltd., was carrying one pilot and three passengers toward Bloodvein River when the engine began backfiring and performance deteriorated.
As the aircraft climbed, cylinder head temperatures rose rapidly beyond allowable limits, and the engine began to vibrate. Despite the pilot's attempts to maintain altitude using full throttle and a rich mixture, the aircraft could not stay airborne. The pilot executed a forced landing on a two-lane section of Provincial Highway 8. While the aircraft occupants escaped without injury, one driver in an oncoming vehicle sustained minor injuries after swerving to avoid the plane and entering a ditch.
The investigation
Investigators examined the engine and the recent maintenance history of the aircraft. The aircraft had recently undergone a 50-hour inspection, which included the removal and reinstallation of the magnetos following a 500-hour inspection at an overhaul facility.
It was determined that the maintenance engineer, who possessed a red/green colour vision deficiency, had used an incorrect reference point to set the magneto timing. During the engine cleaning process, a scratch embedded with debris had accumulated on the alternator drive pulley. Because the engineer could not clearly discern the red paint on the actual timing mark, he mistakenly used this scratch as the timing reference. This resulted in the magnetos being timed to approximately 50 to 60° BTDC, rather than the required 22° BTDC.
Findings
- The incorrect magneto timing caused pre-ignition and detonation of combustion gases, leading to high cylinder head temperatures and a loss of engine power.
- The maintenance engineer used a scratch on the alternator drive pulley as a timing reference instead of the correct, red-painted mark.
- The engineer's red/green colour vision deficiency hindered his ability to identify the correct coloured reference mark.
- The engine ground run following the maintenance was not long enough to detect the timing anomaly.
- The operator had evaluated, but decided not to implement, a Mandatory Service Bulletin (MSB94-8C) that recommended preferred magneto timing methods, and failed to complete the required maintenance evaluation documentation.