Amateur-built LANCAIR IV P N541EM accident near Ooltewah, Tennessee

Probable cause

The inadequate torque of the propeller mounting bolts and inspection of the propeller, which resulted in the fatigue fracture of the bolts and a subsequent in-flight separation of the propeller assembly.

NTSB analysis

Shortly after departing on a personal cross-country flight and leveling off at the filed cruise altitude, the commercial pilot reported trouble maintaining altitude and descended to a lower altitude. He then reported both engine and instrument problems and requested to divert to a nearby airport. Subsequently, the pilot reported that the engine had lost power, oil was all over the windshield, and that there was no visibility due to the oil. Shortly thereafter, he stated that a forced landing was imminent; the last radar return for the flight was about 2 miles from a nearby airport and in the vicinity of the accident location.

On-scene examination of the wreckage revealed that the propeller hub and propeller blades were missing and that oil was noted covering the airplane and windshield fragments. The propeller blades and hub were later located about 8 miles from the accident location.

Five of the six propeller mounting bolts were found inside their bores. The sixth bolt was not located. Metallurgical examination determined that the remaining five mounting bolts failed due to reverse bending fatigue. The witness marks on the aft face of the propeller hub were consistent with marks from bolts or bolt fragments while the propeller hub was still partially attached. This would likely occur when the bolt or dowel was still intact before total separation of the propeller assembly. The reverse bending failure of the hub mounting bolts were likely indicative of a loose connection between the hub and the crankshaft.

Maintenance records revealed that the propeller was overhauled about 35 flight hours before the accident and was inspected about 15 flight hours before the accident; however, the records did not note, nor were they required to, the torque setting that was achieved. Considering the extensive damage to the propeller flange in conjunction with the limited number of flight hours, it is likely that at least one of the propeller mounting bolts was not torqued sufficiently at the time of installation and gradually loosened during the subsequent flights.

Text from the NTSB. Open the NTSB docket for ERA14FA421 for the full record.