What happened
On May 10, 2006, a Cessna 172N, registration CS-AZJ, was performing a recreational flight from Chaves to Vila Real. The aircraft, operated by Aeroclube de Bragança, was on approach to runway 20 when the engine suddenly stopped.
During the final stages of the descent, the pilot had reduced engine power and deployed 20 degrees of flaps. Approximately half a mile from the runway, as the pilot attempted to increase power, the engine failed. In response to the failure, the pilot instinctively pulled back on the control column, causing the aircraft to climb momentarily. This maneuver caused the airspeed to drop near the stall speed, which the pilot initially misinterpreted as a sink rate caused by local terrain.
Realizing the runway could not be reached, the pilot diverted the aircraft away from trees and performed an emergency landing in an area covered with low vegetation and shrubs near the northern boundary of the aerodrome. The aircraft struck bushes and the aerodrome fence, coming to a halt within ten meters. The pilot sustained minor injuries, including abrasions and a small sternal fracture, and was able to evacuate the aircraft independently.
The investigation
The GPIAAF examined the wreckage and the engine, a Lycoming O-320-H2AD. Technical inspections of the engine by specialized technicians found no mechanical anomalies, malfunctions, or issues within the fuel, ignition, or lubrication systems.
Investigators also reviewed the pilot's procedures and the environmental conditions. At the time of the accident, the temperature was 23°C with a dew point of 8.5°C. The investigation also noted that the operator was using an incorrect abbreviated checklist, which did not include the use of carburetor heat during in-flight engine restart attempts, unlike the manufacturer's recommended procedures.
Findings
- The engine failure occurred at a moment when the pilot was attempting to increase power.
- The aircraft was flying at a high angle of attack and near stall speed during the emergency landing, which contributed to the impact with the vegetation.
- Atmospheric conditions were susceptible to carburetor icing, as the dew point depression and humidity levels could allow ice to form in the carburetor, especially since the pilot had been operating with the engine at low power and the carburetor heat was set to the 'cold' position.
- The pilot did not select carburetor heat during the in-flight restart attempts.
- The pilot failed to close the fuel valve as prescribed in the emergency checklist for an engine failure.
- No mechanical failure or fuel contamination could be definitively identified as the cause of the engine stoppage.