The Flight
On 26 July 2001, a student pilot flying a Robin HR200/120B, registration G-VECA, departed on a solo cross-country navigation training exercise from Wellesbourne Mountford. While cruising at 2,400 feet on a leg from Wolverhampton to Tatenhill, the pilot detected an unusual engine vibration. He made a PAN call to Wolverhampton, indicating an intention to return and land there. Shortly after, he heard several bangs and the vibration intensified. The pilot reported the worsening condition to Wolverhampton ATC. Further bangs, loud clattering, a drop in engine RPM, and zero oil pressure led him to search for a suitable field for a forced landing, as he could no longer maintain altitude.
Receiving radar vectors, the pilot quickly located RAF Cosford visually. He observed that the active runway was Runway 24 but assessed that the approach would take him near a village. Unsure whether he could clear the buildings with the engine now at about 1,000 RPM and zero oil pressure, he elected to land on Runway 06, where fields on the approach offered a potential landing site if needed.
The aircraft touched down safely at Cosford. However, the pilot had difficulty exiting because the forward-sliding canopy became jammed by the engine cowling, which had bulged outward due to the engine failure. He praised the assistance provided by the ATC controllers during the emergency.
Engine Examination
Upon opening the cowling, it was evident that the No. 4 cylinder had become detached from the engine near the attachment flange. The connecting rod and piston were both broken. The engine bay and underside of the fuselage were covered in oil. The failed cylinder was sent to the AAIB for metallurgical examination.
Examination of both halves of the fractured cylinder indicated that a primary crack had developed on the front side of the barrel, leading to full 360° separation. Fatigue was present over the first approximately 5.9 inches of the crack, with the remainder being fast fracture. The origin, located between the attachment flange and the first cooling fin adjacent to the front hold-down bolt, contained a deep corrosion pit from which fatigue had propagated. Severe corrosion was also noted in other areas of the barrel and on the three other cylinders from the same engine. Material specifications and component dimensions were within drawing limits. In non-corroded areas, the black oxide coating had good adhesion.
Background on Cylinder Failures
On January 26, 2001, Textron Lycoming issued Service Instruction (SI) No.1504 for O-235 engines under the French DGAC. It cited a significant number of cylinder barrel failures in France, initiating from corrosion pits at the base of the cylinder. The French DGAC had issued Airworthiness Directive AD 1998-225(a), requiring periodic inspections for cracks. Since the depth of pitting or crack initiation could not be determined, Textron Lycoming recommended replacement of affected cylinders with new assemblies offering improved corrosion resistance. Engines shipped after September 1, 2000, incorporated these new assemblies.
The engine fitted to G-VECA was shipped from the factory on December 16, 1999, and installed on March 10, 2000. At the time of the accident, it had accrued about 299 hours from new. The improved corrosion resistance reportedly included zinc chromate treatment and shot-peening, the latter aiding fatigue resistance.
AD 1998-225(a) noted that the significant number of cylinder failures (22 in France up to June 29, 2000) included engines of various ages and flying hours, generally failing in areas similar to G-VECA. The AD required that new or overhauled engines be subjected to a white developer spray test to highlight oil leakage from cracks, repeated every 50 hours. Overhauled engines required additional visual inspections at 20-hour intervals.
Textron Lycoming stated that similar problems were unknown or extremely infrequent in the United States. In the United Kingdom, a search of the CAA's Mandatory Occurrence Report database revealed only one apparently similar case of cylinder cracking since 1976, with another 22 cases occurring remotely from the attachment flange area.
Conclusions
The reason why corrosion-induced fatigue failures appeared endemic only in France was puzzling. Textron Lycoming suggested climatic differences but such conditions would apply more to the UK. The failure of G-VECA's engine was isolated but concerning due to the low hours and calendar time. UK experience did not justify adoption of an AD similar to the DGAC's. The AAIB and CAA stated they would continue to monitor the situation for any future trends.