The NTSB issued an urgent recommendation to the FAA last week concerning unsafe wiring on Piper PA-31T Cheyennes that might lead to arcing and fires. The Safety Board also called for an emergency AD.

Its recommendation is based on preliminary findings in the ongoing investigation of the July 29, 2016 in-flight breakup of a Piper PA-31T on a medical transport flight. All four persons aboard the turboprop twin perished.

Investigators found evidence of thermal damage near the airplane’s main electrical bus circuit breaker panel. This enclosed space also includes hydraulic lines that run directly below the panel. The area showed evidence of electrical arcing, and sections of the adjacent hydraulic lines were consumed by fire in an area where these wires and hydraulic lines may have been in contact.

Current maintenance procedures, which specify only a general visual inspection, are not adequate guidance, according to the NTSB. A borescope examination of this area in six other Cheyennes noted electrical lines in direct contact with hydraulic lines in all six cases. The FAA issued a special airworthiness information bulletin (SAIB) on this issue last month, but the NTSB said an emergency AD would require mandatory action and a shorter timeline for addressing the problem than the SAIB.