Amateur-built THUNDER MUSTANG N244SW accident near Globe, Arizona

Probable cause

The separation of the main crank drive pulley from the engine’s accessory section due to three of its securing bolts backing out of their holes, which subsequently resulted in two accessory belts becoming unsecure, both crank sensor plugs becoming dislodged, the ignition source to the engine being interrupted, and the total loss of engine losing total power.

NTSB analysis

The commercial pilot, who was also the owner/builder of the experimental, amateur-built airplane, reported that the purpose of the flight was to practice instrument approaches. He added that, after completing a practice instrument approach, executing the published missed approach procedure, and climbing to 6,500 ft mean sea level, he felt a reduction in power, followed by an rpm gauge reading of 0. The pilot subsequently made an off-field emergency landing to a dry river bed, which resulted in substantial damage to the airplane.

A postaccident examination of the engine revealed that one of the three bolts that secures the main crank drive pulley to the associated damper was separated from the pulley and that the two bolts that remained in the pulley were loose. Additionally, both the left and right ignition leads were separated from their associated crank sensor plugs. Signatures on the main crank drive pulley's reluctor teeth were indicative of the main pulley having contacted the idler pulley and the idler arm subsequently separating from the pulley. This likely resulted in two accessory belts becoming unsecure, both crank sensor plugs becoming dislodged, and the subsequent total loss of engine power.

A review of the maintenance records revealed that the engine had accumulated about 190 hours since its most recent overhaul. The pilot/owner stated that, when he reassembled the main crank drive pulley to the overhauled engine, he followed the procedures outlined in the engine's repair manual by torqueing each bolt to 35 ft-lbs of pressure and applying a thread-locking compound. However, visual and spectrographic analysis of material in the threads of the two recovered bolts did not detect signatures of the required thread-locking compound. Although this finding suggests that no thread-locking compound was used, it is not conclusive evidence that the compound was not applied during its most recent reassembly. Thus, the reason for the bolts becoming unsecured could not be conclusively determined.

Text from the NTSB. Open the NTSB docket for WPR15LA020 for the full record.