History of the Flight
On 3 September 2018, a Robin DR400-120, registered F-GSBK, operated by Aéroclub de Quimper, departed from Quimper Pluguffan aerodrome for a local flight to the Glénan islands. The pilot and two passengers were on board. After descending to about 500 ft above the sea for photographs, the pilot attempted to climb but found the power lever ineffective. The engine began to vibrate, oil splashed onto the canopy, and smoke emerged from the engine cowling. Unable to find a suitable landing area on the islands, the pilot turned back toward the mainland and issued an emergency call. Residual engine power allowed level flight at 500 ft. The pilot configured for an east-facing landing on a beach. The engine stopped and propeller locked before the turn to align with the beach. A person on the beach caused the pilot to veer right toward the water. On landing, the aircraft turned onto its back and came to rest in the water. The passengers evacuated unaided.
Aircraft and Engine Information
The aircraft was equipped with a four-cylinder Lycoming O-235-L2A engine rated at 118 HP at 2,800 rpm, driving a Sensenich fixed-pitch two-blade metal propeller.
Examination of the Engine
The crankcase was perforated in line with cylinder No 4, located on the rear left side. Cylinder No 4 had failed completely around the circumference of its steel barrel near the base plate, which also had radial failure. The fracture surface showed peening damage over a large area, obscuring the crack initiation site. Indications of crack formation in at least two distinct areas, along with peened and oxidized appearance, suggested a progressive cracking process over time. The cylinder rod was not found; the piston was heavily damaged. No unusual lubrication was found. The failure of cylinder No 4 appeared to be due to fatigue cracking in the steel barrel. The outer surface of the failed cylinder and three others showed superficial corrosion and absence of paint.
Maintenance Findings
The aircraft's continuing airworthiness was managed by Xénon Aviation (CAMO); maintenance was performed by the club's mechanic. The last maintenance (50-hour inspection) was on 27 August 2018, at 2,259 engine hours (manufacturer's suggested overhaul interval: 2,400 hours). Leak rate measurements for the three 100-hour inspections between July and October 2017 were mostly above 70 psi, except one at 60 psi. For inspections on 5 February 2018 and 30 May 2018, leak rates were at or below 38 psi, attributed by the CAMO to incorrect tooling. Leak rates were not recorded for the 100-hour inspections on 11 July 2018 and 9 August 2018. The maintenance program specified an annual search for corrosion in salt-laden environments, but no such search was documented for F-GSBK.
Similar Occurrences
Thirty-four similar occurrences with the same engine type and failure mode were identified between 1995 and 2003. In six cases, cracks initiated near the base plate, with corrosion as a contributing factor. Two of those had normal leak rates before the event. An airworthiness directive issued by the French DGAC in 1998 mandated cylinder inspections, but was not adopted by EASA. Lycoming issued Service Bulletin No 1504 recommending replacement with corrosion-resistant cylinders. F-GSBK's cylinders were of the newer type. Only one cracking event in a new-type cylinder was recorded (2005), attributed to unknown high stresses. EASA published a Safety Information Bulletin in 2009 recommending barrel inspections for older cylinders.
Conclusions
The engine cylinder failure resulted from fatigue cracking in the steel barrel of cylinder No 4. Corrosion was observed along the fracture. The pilot successfully returned to the coast using residual power and performed a forced landing on a beach, where the aircraft overturned. The investigation could not determine the contributing factors leading to the cylinder failure.