What happened
On 22 March 2023, a CESSNA 172P, registration EC-GVD, was conducting a dual flight training mission departing from Madrid - Cuatro Vientos Airport. The flight, operated by AEROFAN, included an instructor and a student pilot. After climbing to 3,000 feet and establishing cruise at 90 knots, a loud explosion occurred in the engine approximately 4 km from the airport.
The explosion caused significant vibration and forced a hole in the right-side engine cowling, with debris being ejected from the engine compartment. The instructor performed a 18/0 turn to return to the airport. During the return, smoke entered the cockpit and a burst of flame was observed, leading the crew to declare a MAYDAY due to an engine fire. To maintain altitude for the return, the instructor briefly applied power, which caused further vibrations and increased smoke levels. The instructor eventually shut down the engine, set a glide speed of 75 knots, and successfully landed on runway 09. Both crew members were unharmed.
The investigation
Investigators examined the wreckage and the engine, which was a Lycoming O-360-A3A. The inspection revealed that cylinder no. 3 had completely detached from the engine block. While the cylinder remained within the cowling, the investigation found that all eight studs intended to secure the cylinder to the block had failed.
Detailed fractographic analysis of the fractured studs revealed clear evidence of fatigue, characterized by fatigue beaches and striations. Furthermore, when the other three cylinders (no. 1, 2, and 4) were disassembled, investigators found significant wear on the internal threads of the studs, suggesting that all cylinders were improperly secured. The investigation also reviewed maintenance records, noting that the engine had undergone a major overhaul by a previous maintenance organization, SINMA AVIACIÓN, which had since had its approvals revoked due to various shortcomings.
Findings
- The primary cause of the accident was the detachment of cylinder no. 3 due to the fatigue failure of the studs securing it to the engine block.
- The fatigue failure was driven by insufficient tightening torque applied to the nuts during the engine overhaul, which resulted in inadequate preload and excessive vibration/play.
- Evidence of thread wear on the studs of all four cylinders suggests that the improper assembly was a systemic issue affecting the entire engine, not just the failed cylinder.
- The in-flight fire was contained within the engine compartment and extinguished itself before the aircraft landed.
- The instructor's decision to return to the airport and the subsequent emergency landing were executed successfully, preventing injuries to the crew.