WASHINGTON — The National Transportation Safety Board issued an urgent safety recommendation to the FAA Monday concerning unsafe wiring on Piper PA-31T-series aircraft that may lead to arcing and cause fires.
The urgent safety recommendation is based upon preliminary findings in the ongoing investigation of the July 29, 2016, in-flight 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 in-flight fire. Evidence thus far indicates that the in-flight fire occurred in the area where these electrical wires and adjacent hydraulic lines may have been in contact.
(In this photo taken in an exemplar Piper PA-31T, white electrical lines are in close proximity to metal hydraulic lines. The NTSB believes this wiring may chafe and then arc, causing a fire and subsequent damage to hydraulic lines. NTSB photo by Mike Bauer)
Current maintenance procedures, 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 boroscope 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.
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.
The text of the recommendation letter is here: https://www.ntsb.gov/safety/safety-recs/RecLetters/A-17-001.pdf
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