N510UA - 1990 Aircraft Aircraft Registration

The ignition of the J5 power terminal located on the captain’s No. 1 windshield due to a loose electrical connection between the terminal connector lug and its respective terminal block. The loose connection resulted from a missing lock washer that allowed the resistance in the electrical path to increase sufficiently to generate high enough temperatures to ignite the terminal block. Contributing to the probable cause was the lack of instructions to ensure the lock washer was installed in the J5 power terminal block in the Boeing 757 aircraft maintenance manual (AMM). Additionally, contributing to the incident was the deferral of the related maintenance write-up before the incident, which resulted from information in the United Airlines AMM that stated, “When bus bar(s) show signs of blackening or burning, the condition is acceptable for continued service, although the window must be replaced within 50 flight-hours.