Overview
At approximately 1300 eastern daylight time, a Cessna 172N (registration C-GZLJ, serial number 17269614) operated by Toronto Airways Limited departed from Toronto/Buttonville Municipal Airport for a sightseeing flight over Toronto, Ontario. The aircraft carried one pilot and three passengers. A pre-takeoff engine ground run revealed no anomalies.
Engine Power Loss
After takeoff, the pilot applied full power, climbed to 2000 feet above sea level (1300 to 1400 feet above ground), levelled off, and selected the Toronto/City Centre Airport tower radio frequency. Shortly thereafter, the Lycoming O-320-H2AD engine began to lose power. The pilot informed the tower of the power loss and the intention to return to Buttonville.
Attempting to regain power, the pilot ensured full throttle was selected, checked the positions of the primer and magnetos, and switched fuel tanks. When these efforts failed, the pilot selected carburettor heat to the hot position, observed a further decrease in engine power, and reset the heat to cold. The engine could not produce enough power to maintain level flight and return to the airport.
Forced Landing
The pilot searched for a suitable forced landing location. The aircraft was over a densely populated area; the only suitable clearing was surrounded by trees and nearby buildings. On final approach, the engine lost power. The pilot selected flaps to full down, overflew the clearing, and stalled the aircraft into the trees. The aircraft sustained substantial damage, and one passenger received minor injuries.
Investigation Findings
Post-accident examination found no mechanical issues that would explain the loss of engine power. The aircraft was operating in conditions where carburettor ice was a real possibility, and the engine symptoms resembled those associated with carburettor icing. The investigation concluded that the loss of power was related to carburettor ice.
The ambient temperature and dew point values were in the range where moderate carburettor icing could occur. Although the pilot applied carburettor heat after the power loss, the heat was not applied long enough to remove any ice that had formed. The initial degradation of engine performance following carburettor heat application should have been anticipated.
Additionally, a straw-coloured fuel found in the fuel line to the gascolator did not meet the specification of grade 100 LL aviation fuel, but the concentration of foreign components was insufficient to affect engine operation. Clean 100 LL fuel was found between this fuel and the engine and would have been supplying the engine during the flight.
Causes and Contributing Factors
The investigation identified that ambient temperature and dew point conditions most likely resulted in carburettor icing, which caused the engine to lose power. The pilot applied carburettor heat but did not allow enough time for it to be effective, turning it off after noting a further power decrease.
Conclusion
This report concludes the Transportation Safety Board's investigation into this occurrence. The Board authorized the release of this report on 08 July 2004.