Swedish investigators are urging regulators to take a closer look at engine propeller vibration incidents on ATR 72s that occurred in 2007-14 and remain unexplained based on their work on a November 2014 incident on a Braathens ATR 72.
The Swedish Accident Investigation Authority (SHK) urged the European Aviation Safety Agency (EASA) to restrict ATR 72 operating profiles until the incidents can be better understood. The problem is not new to EASA. In 2014, France’s BEA investigative board issued four recommendations based on six of the incidents. In response, EASA issued a safety bulletin urging operators to reduce descent speed (all the incidents happened during the descent phase) and take other precautions. Propeller manufacturer Hamilton Sundstrand believes the incidents stem from excessive friction in the propeller blade-retention bearings, which causes the blades to have different pitch angles. The company developed an improved ball bearing separator. SHK is not convinced, however, and says that “additional, extensive engineering initiatives” are needed in order to solve the mystery.