What happened
On December 16, 2002, a Cessna 152, registration CS-AYH, was performing a positioning flight from Cascais to Portimão following an annual inspection and engine overhaul. During the flight, the pilot noticed an unusual odor inside the cockpit. Upon inspecting the aircraft's exterior, the pilot observed smoke trailing toward the tail and identified an oil stain on the leading edge of the left horizontal stabilizer.
Further monitoring of the engine instruments revealed a gradual decrease in oil pressure accompanied by an increase in temperature. The pilot declared an emergency with Lisbon Military Control. Following a suggestion from the controller, the pilot diverted to the Sines airfield—which was inactive but available for emergency use—and landed safely on runway 14 at approximately 13:30 UTC. There were no injuries and no damage to the aircraft.
The investigation
The GPIAAF investigation focused on the mechanical failure and the maintenance oversight. Technical teams examined the engine and identified that the oil radiator was leaking under pressure. The investigation found that the radiator exhibited significant dents, twists, and torn cooling fins, which would have been apparent upon a simple visual inspection.
Investigators also reviewed maintenance records, noting that the aircraft had recently undergone a major overhaul by a subcontracted maintenance company. While the inspection protocol required checking the radiator for leaks and fractures, the component had been cleared for service despite its visible physical condition. The investigation also noted that the operator, an aero club, lacked its own maintenance oversight capabilities and relied heavily on the documentation provided by third-party contractors.
Findings
- The primary cause of the incident was an oil leak from the engine radiator body when subjected to operational pressure.
- The leak caused a significant loss of oil, leading to the drop in oil pressure.
- The contact of leaking oil with hot engine surfaces created the burning smell detected by the pilot.
- A contributing factor was a deficiency in the maintenance verification process, as the radiator's poor physical condition was not adequately addressed during the post-overhaul inspection.
- The lack of an exhaustive pre-flight inspection by the aircraft owner or representative prevented the detection of the radiator's degraded state before the flight.