The Accident
On a visual flight rules flight from Lac du Bonnet, Manitoba, to George Lake, a float-equipped de Havilland DHC-3 Otter, registration C-GGON, serial number 225, departed at approximately 1100 central daylight time. The aircraft, operated by Whiteshell Air Service Ltd., carried one pilot and three passengers. After a normal take-off from the Winnipeg River in an easterly direction, the aircraft began a shallow climb. At about 400 feet above ground level, a loud backfire occurred, followed by a complete loss of engine power. The pilot performed a forced landing straight ahead. The aircraft struck several large trees and came to rest in a swampy area, striking the ground on its left side. Both wings detached, and the engine became buried in the swamp. There was no fire.
The pilot and one passenger in the cockpit sustained minor injuries. One cabin passenger was thrown clear of the aircraft while still strapped in the seat and sustained minor injuries. The other cabin passenger was thrown forward, still strapped in the seat, struck interior structure, and sustained serious injuries.
Aircraft and Maintenance History
The day before the accident, the aircraft had returned to Lac du Bonnet with an unusual engine noise. The operator's approved maintenance organization (AMO) inspected the engine and found the No. 1 cylinder showed excessive blow-by. The cylinder head was separating from the barrel. A new cylinder was installed. On the accident day, before the first flight after replacement, a lengthy engine run-up showed no anomalies. At take-off, the pilot confirmed engine performance by observing 2250 rpm at a manifold pressure of 36.5 inches. However, the approved aircraft flight manual specifies that when the manifold pressure equals the field barometric pressure, engine rpm should range from 2000 to 2200 rpm. The required field barometric power reference check was not completed after the cylinder replacement or prior to take-off.
Engine Failure Investigation
Examination determined that a spacer in the No. 3 cylinder exhaust push rod tube was incorrectly installed end-wise under the ball end during assembly. This error, likely set and partially bent during installation, shortened the push rod length over time due to repetitive lifting action. The increased valve clearance led to pounding of the cam roller against the cam ring lobes. Despite likely adjustments, the wear progressed until the exhaust valve failed to open. Hot exhaust gases then accumulated in the combustion chamber and migrated back through the intake valve into the induction system, igniting the fuel-air mixture and causing the backfire and power loss.
Additionally, the breakage of the top portion of the cam roller slot tappet guide likely resulted from wear and was probably a consequence of the backfire, not a cause of power loss. The investigation could not determine when the No. 3 exhaust valve was last adjusted or when the valve adjustment screw protrusion was set beyond limits. The company did not check valve clearances on the required 400-hour schedule. The ELT had been removed during an annual inspection on 08 May 2003 for re-certification, and a log book entry noted its removal, but the aircraft was not placarded as required by Canadian Aviation Regulations. The pilot believed a re-certified ELT had been installed.
Findings
Causes and Contributing Factors
- A spacer in the No. 3 cylinder push rod tube was installed incorrectly, initiating increased wear that eventually prevented the No. 3 exhaust valve from opening. This led to hot exhaust gases migrating into the induction system, causing the engine failure.
- Following replacement of the No. 1 cylinder, the operator did not complete a field barometric power reference check, which is used for early identification of engine problems.
- The valve adjustment screw protrusion was set beyond specified limits. Had this been noted during adjustment or a scheduled inspection, excessive wear in the valve train might have been detected and the failure prevented.
Risk Findings
- The aircraft maintenance engineer (AME) who signed the journey log entries held an expired licence (expired 05 October 2000, not renewed). The pilot/AME performed the last inspection after the expiry date.
Other Findings
- The removal of the ELT was not placarded as required. The pilot was unaware that the ELT was not installed.
Conclusion
The Transportation Safety Board released this report on 23 February 2004, concluding the investigation. High deceleration forces during the crash likely exceeded seat design strength, causing overload failure of the cabin passenger seat support structures.