Amateur-built ZENAIR 701 N538HB accident near Zephyrhills, Florida

Probable cause

The failure of the propeller speed reduction unit drive belts due to inadequate belt tension being applied during installation, which resulted in a complete loss of thrust.

NTSB analysis

The pilot reported that, while in the traffic pattern at his home airport practicing touch-and-go landings, the experimental, amateur-built airplane experienced a complete loss of thrust from the propeller. While turning from the crosswind leg of the traffic pattern onto the downwind leg of the traffic pattern, the engine suddenly "overreved." The pilot reduced power and chose to perform an emergency landing. He then tried adding power, but the propeller did not respond and was windmilling. He established best glide speed, but the airplane had developed a high sink rate and was losing altitude rapidly. He then slowed the airplane as it approached a set of powerlines. The airplane clipped one of the powerlines, which nearly severed the left wing. The pilot then "stalled" the airplane and touched down hard. The airplane was substantially damaged during the forced landing.

Examination of the engine revealed that a belt-driven propeller speed reduction unit (PSRU) was installed. Further examination revealed that both drive belts from the PSRU were broken into several pieces, and shredded material from the belts was present throughout the engine compartment. Small pieces of rubber were also present on the forward part of the engine, the propeller hub assembly, and the PSRU pulleys' grooves. Examination of the broken belts revealed jagged belt fractures and evidence of overheating, hardening, back cracks, land area wear, and "V" rib wear, all of which is indicative of inadequate belt tension, which can cause the belts to slip as they travel around the pulleys and generate heat buildup. The heat buildup can cause the rubber compounds in the belt to break down and crack and lead to a belt failure. According to maintenance records, the drive belts for the PSRU had been replaced about 20 hours of operation before the accident flight.

Text from the NTSB. Open the NTSB docket for ERA14LA004 for the full record.