What happened
On November 24, 2013, a Cessna 172 H Reims, registration EC-CXP, departed from Madrid Cuatro Vientos for a local flight accompanied by two passengers. The flight was planned to pass through El Escorial and Toledo. During the flight, the pilot encountered significant turbulence, leading to a change in the planned route directly toward Toledo.
While approaching Toledo, the pilot noted an unusual engine noise. Shortly after, a loud bang occurred, and engine power dropped to 2,000 rpm. As the aircraft struggled to maintain altitude, the pilot identified a suitable plowed field for an emergency landing. The pilot maintained a speed of 65 kt, deployed full flaps, and secured the cabin. The aircraft landed in the field, and the occupants remained uninjured.
The investigation
The investigation focused on the engine failure and the condition of the cylinder components. Technical analysis of the engine revealed that cylinder number 6 had fractured and completely separated from the cylinder head. Laboratory examination of the fracture site showed that the failure originated from a pit caused by localized corrosion on the exterior wall of the cylinder between the cooling fins. This corrosion led to a fatigue crack, which eventually resulted in a final failure due to static overload.
Investigators also examined maintenance records and found that the recent 100-hour inspection, conducted in November 2013, as well as a previous inspection in May 2013, had not implemented the manufacturer's Service Bulletin SB96-12. This bulletin provides specific instructions for leak checks and visual inspections of cylinders to detect defects that might be hidden by cooling fins.
Findings
- The loss of engine power was caused by the fracture of cylinder number 6 due to static overload following a fatigue crack.
- The fatigue crack originated from a corrosion pit on the cylinder wall.
- A contributing factor was the failure to apply Service Bulletin SB96-12 during the two most recent maintenance inspections, which would have provided more thorough inspection procedures for the cylinders.
Safety action
- It is recommended that Cessna incorporate the contents of SB96-12 into the engine maintenance manual for all cylinders, regardless of the manufacturer, to facilitate the detection of leaks and other defects that are not easily visible during standard inspections.