Incident Overview
On May 27, 2016, at 0915 central daylight time, a Cessna 402B airplane, registered as N76MD, encountered an in-flight left engine fire while maneuvering near the Joe Foss Field Airport (FSD) in Sioux Falls, South Dakota. The airplane was operated by Encore Air Cargo of Sioux Falls and was conducting an instructional flight under 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and a company flight plan was filed. The local flight had departed FSD at 0840.
Sequence of Events
According to the airline transport pilot, the flight crew noticed a rough running left engine while maneuvering for a practice approach. The commercial pilot activated the auxiliary fuel pump, after which the engine operated without issue. Shortly thereafter, the left engine tachometer dropped to zero, and the commercial pilot, seated in the left seat, observed flames emitting from the left engine cowling. The flight crew declared an emergency and secured the left engine. The airplane landed uneventfully and taxied to the company ramp.
Postaccident Examination
A postaccident examination by a Federal Aviation Administration inspector and company mechanics revealed substantial damage to the left wing structure and firewall. The number 4 cylinder was burned through at the exhaust port, and the number 6 cylinder valve cover displayed thermal damage consistent with the damage to the number 4 cylinder. The number 4 cylinder was removed and sent to Continental Motors Group in Mobile, Alabama, for further examination.
Cylinder Examination
On October 13, 2016, Continental Motors Group examined the number 4 cylinder under the supervision of a National Transportation Safety Board investigator. The examination showed a burned through area at the cylinder head exhaust port, consistent with a preexisting crack that allowed exhaust gas to erode the cylinder head. However, the preexisting crack could not be determined due to the erosion of the burned through area. Additionally, a crack was noted on the opposite side of the exhaust port.
Maintenance History
According to the maintenance records, the number 4 cylinder had been overhauled 178.3 hours prior to the accident.