The NTSB issued a rare urgent safety recommendation on Tuesday, warning pilots that Piper PA-31T-series aircraft may have unsafe wiring that could lead to arcing and cause fires. The urgent safety recommendation is based upon preliminary findings in the ongoing investigation of the July 29, 2016, inflight breakup of a Piper PA-31T medical transport flight in California. 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. Further examination of the wiring in this area showed evidence of electrical arcing, and sections of the adjacent hydraulic lines were consumed by inflight fire. Evidence thus far indicates that the inflight fire occurred in the area where these electrical wires and adjacent hydraulic lines may have been in contact, the NTSB said. NTSB urgent safety recommendations are issued in advance of the completion of an investigation when the Board believes an imminent threat to life and safety exists, based upon findings of the ongoing investigation.
According to the NTSB, current maintenance procedures for the Piper series, which only specify a general visual inspection, do not provide adequate guidance for inspection in the area of the floor-mounted circuit breaker panel because of its location and the confined space in that area. Thus, contact between electrical wires and hydraulic fluid lines can persist undetected. NTSB and FAA investigators examined this area in six other Piper PA-31T-series airplanes using a borescope and camera and noted electrical lines in direct contact with hydraulic lines in all six cases.
The NTSB believes that owners and operators must identify and repair (or replace) damaged wires in the floor below the main circuit breaker panel and ensure proper clearance between wires and hydraulic lines. The FAA issued a special airworthiness information bulletin (SAIB) on this issue in December 2016.The NTSB believes the wiring condition on Piper PA-31T aircraft merits an FAA emergency airworthiness directive, which would require mandatory action and a shorter timeline for addressing the issue than the SAIB. The PA-31T is a twin turboprop commonly known as a Cheyenne.
