What happened
At around 13:00 EDT, a Cessna 172N, registration C-GZLJ, departed from Toronto/Buttonville Municipal Airport for a sightseeing excursion. The aircraft was carrying a pilot and three passengers. Following a successful engine ground run, the aircraft climbed to approximately 2,000 feet above sea level. After establishing contact with the Toronto/City Centre Airport tower, the engine began to lose power.
The pilot attempted several troubleshooting steps, including verifying full throttle, checking the magnetos and primer, and switching fuel tanks. When these actions failed to restore power, the pilot applied carburetor heat. However, this resulted in a further drop in engine performance. Realizing the engine could no longer maintain level flight or return to the departure airport, the pilot searched for an emergency landing site. Due to the densely populated surroundings, the only available clearing was heavily obstructed by trees and nearby structures. During the final approach, the engine lost power entirely. The pilot deployed full flaps but overflew the clearing, resulting in a stall into the trees. The impact caused substantial damage to the aircraft, and one passenger sustained minor injuries.
The investigation
The investigation focused on the mechanical state of the engine and the environmental conditions during the flight. While the fuel in the fuel line contained foreign components that did not meet grade 100 LL specifications, the fuel reaching the engine was clean and sufficient for operation. The aircraft was found to be in serviceable condition, with no mechanical defects identified that could explain the power loss.
Investigators examined the effects of applying carburetor heat during an engine malfunction. It was noted that applying heat introduces warmer, less dense air, which can initially reduce power and cause engine roughness as ice melts and is ingested into the intake. The investigation also assessed the ambient weather conditions, specifically the relationship between temperature and dew point.
Findings
- The primary cause of the engine power loss was carburetor icing, driven by ambient temperature and dew point conditions conducive to ice formation.
- The pilot applied carburetor heat as part of standard procedure but did not leave the heat on long enough to allow the ice to melt.
- The further reduction in power observed after applying heat was an expected consequence of the change in air density and the ingestion of melting ice.
- The pilot's decision to stall the aircraft into trees was a result of the lack of suitable landing alternatives in the populated area.