Occurrence
On a flight from Winnipeg/St. Andrews Airport, Manitoba, to Bloodvein River, Manitoba, a Northway Aviation Ltd. Cessna 207A (registration C-FBHP, serial number 20700647) operating as Northway Flight 841 encountered engine performance deterioration shortly after take-off. The pilot noted several engine backfires and attempted to return to the airport, but the aircraft could not maintain altitude. A forced landing was executed on Provincial Highway 8, approximately 2 nautical miles north of the airport at 1356 central daylight time. The aircraft was not damaged and none of the three passengers or the pilot was injured. Two oncoming vehicles left the paved roadway to make way for the aircraft; one driver sustained minor injuries when his vehicle entered a ditch.
Pre-Flight Conditions
The aircraft carried one pilot and three passengers (two adults and one infant). The adults were restrained by lap belts; the infant was held in the arms of a passenger. Hand luggage and a stroller were restrained in the rear of the cabin. The aircraft weight and centre of gravity were within prescribed limits. The pilot completed a pre-flight inspection and engine run-up; all parameters were normal. During the take-off run on Runway 36, the aircraft did not accelerate as expected, a condition attributed to the high ambient temperature of 27°C.
Engine and Maintenance Findings
After lift-off, climb performance degraded and cylinder head temperature rose rapidly beyond the maximum allowable value. The engine began to vibrate. Despite full throttle, fine propeller pitch, and rich mixture, altitude could not be maintained.
Post-occurrence examination revealed that both engine magnetos were incorrectly timed. The required timing is 22° before top dead center (BTDC) on the compression stroke; the magnetos were timed to approximately 50–60° BTDC. Such advanced timing leads to pre-ignition or detonation, resulting in high cylinder head temperatures and engine power loss.
Recent maintenance included a 50-hour inspection and a 500-hour magneto inspection per Slick Service Bulletins SB2-08 and SB3-08. The magnetos were removed and sent to an overhaul facility; they were reinstalled eight days later. The maintenance engineer, who had approximately 15 years of experience (seven as a licensed AME), used the external engine timing mark method. The engineer has a red/green colour vision deficiency. During reinstallation, a scratch on the alternator drive pulley, caused by debris from engine washing, was mistaken for the correct timing mark, which was painted red. The engineer could not discern the red paint and aligned the scratch with the 22° BTDC position on the external timing plate. The correct timing mark was out of view.
A subsequent engine ground run was of insufficient duration to indicate the timing anomaly; the lower-than-normal rpm drop on single magneto operation and higher after-run engine temperature were attributed to the high ambient temperature.
Contributing Factors
The Transportation Safety Board found that during recent maintenance, both engine magnetos were incorrectly timed. This condition was not detected during the subsequent engine ground run or before the flight. The incorrect magneto timing led to pre-ignition or detonation, causing high cylinder head temperatures and engine power loss after take-off.
Risk and Other Findings
The operator evaluated but did not incorporate Service Bulletin M84-8 and Mandatory Service Bulletin MSB94-8C regarding preferred magneto timing methods into its approved Cessna 207 maintenance schedule. The continued use of the external engine timing mark method increased the risk of a magneto timing error. Additionally, a maintenance evaluation sheet addressing the Mandatory Service Bulletin was not prepared in accordance with the company's Maintenance Control Manual.
Aftermath
The aircraft was towed back to the company's hangar for examination. Cessna indicated it would incorporate information from Mandatory Service Bulletin MSB94-8C into the next revision of the Cessna 207 maintenance manual. The operator stated it would change its policy regarding service bulletins.