What happened
On September 3, 2006, at 16:35 UTC, a Cessna 152, registration CS-AZB, departed from the Torres Vedras Municipal Aerodrome (Santa Cruz) for a recreational flight along the coast toward Ericeira. The aircraft was carrying a pilot and one passenger.
While flying at approximately 1,000 feet AMSL, the pilot noticed that the engine was failing to produce the selected power, with RPMs dropping to between 1,200 and 1,700, significantly below the expected 2,000 to 2,750 RPM range. The pilot attempted to follow the manufacturer's recommended checklists to restore normal operation but was unable to do so. After performing a 180-degree turn to return to the departure aerodrome, the pilot realized the aircraft could no longer maintain altitude and was unable to reach Santa Cruz.
Seeking an alternative landing site, the pilot identified an unprepared agricultural field near Barril. The pilot executed an emergency landing on the sloping terrain. During the landing roll, the nose wheel struck a depression in the ground, causing the nose and propeller to impact the soil. Additionally, the left wingtip struck the ground, causing structural deformation to the wing root and fuselage.
The investigation
The GPIAAF investigation focused on the engine's performance and the environmental conditions at the time of the accident. Investigators performed a thorough inspection of the Lycoming O-235-L2C engine, specifically examining the fuel, ignition, and lubrication systems. Subsequent tests conducted by the Aero Clube de Torres Vedras confirmed that all engine systems were functioning normally and that the engine was capable of producing its rated power.
Meteorological data indicated that while the weather was generally good, there was high relative humidity (near 100%) and temperatures around 20°C along the coast. The investigation examined the possibility of atmospheric conditions contributing to the power loss.
Findings
- The pilot and passenger sustained no injuries and evacuated the aircraft unaided.
- The aircraft sustained significant damage to the propeller, engine mount, engine lower shielding, nose gear, and the left wing structure.
- Carburetor icing is the most likely cause of the partial power loss. The high humidity and temperatures around 20°C provided sufficient conditions for ice to form in the carburetor venturi, restricting airflow and altering the air-fuel mixture.
- The formation of ice was likely exacerbated by the pilot not having the carburetor heat in the "hot" position.