NTSB Identification: ENG10IA029
Scheduled 14 CFR Part 121: Air Carrier operation of United Airlines (D.B.A. operation of United Airlines)
Incident occurred Sunday, May 16, 2010 in Chantilly, VA
Probable Cause Approval Date: 03/14/2011
Aircraft: BOEING 757, registration: N510UA
Injuries: 112 Uninjured.
NTSB investigators traveled in support of this investigation and used data obtained from various sources to prepare this aircraft incident report.
The Boeing 757-200 airplane experienced a fire in the cockpit shortly after the airplane had leveled off at its cruise altitude of 36,000 feet. As a result of the immediate actions of the flight crew, the fire was extinguished. As the airplane descended through about 500 feet, the inner pane of the captain’s windshield cracked. Because he could not see clearly out of his shattered windshield, the captain transferred control of the airplane to the first officer, who was able to land the airplane without incident.
Post incident examination of the airplane revealed the J5 terminal block on the captain’s side windshield had ignited and was mostly consumed by fire. The J5 terminal connector lug was found secured to the terminal block with only a screw; no lock washer was present. Examinations revealed that none of the other terminal block installations on the captain’s and first officer’s windshields contained a lock washer.
Further examination of the J5 terminal block did not find any evidence of a condition that would result in the terminal block overheating (such as a misthreaded screw or damaged wiring). It is likely that the connection between the connector lug and the terminal block was loose because of the missing lock washer. A loose connection can create a point of high resistance in the electrical path between the terminal lug and terminal block, which can generate temperatures high enough to cause the terminal block to ignite.
A review of United Airlines’ maintenance records found that the captain’s No. 1 windshield was installed on January 29, 2007, in accordance with the United Airlines 757 Aircraft Maintenance Manual (AMM). A review of the AMM found that it was consistent with Boeing’s AMM and that both AMMs lacked information to alert the maintenance technician that a lock washer must be installed at all power and sensor terminal block installations. After the May 16, 2010 incident, Boeing released a temporary revision (TR56-1004) to AMM 56-11-01/401 which adds specific instructions for the installation of the lock washer and graphics depicting its installation. United has revised their AMM per the changes identified in the temporary revision.
A review of the airplane’s service history revealed that evidence indicating that an electrical anomaly had recently occurred in the cockpit were clearly available to United Airlines maintenance personnel a day before the incident flight. Specifically, there were reports of electrical odors in the cockpit on two of the three flights before the incident flight. On the flight immediately before the incident flight, the captain reported to maintenance that his No. 1 windshield lower outboard power connector appeared burnt and was hot to the touch. At the time of the maintenance inspection, the lead mechanic thought that the power terminal block was a part of the bus bar. He referred to the United Airlines AMM, which indicated that the window should be replaced due to the discoloration of the bus bar; however, the limitations section of the manual stated that the item could be deferred for 50 flight hours. After discussing this with another mechanic, they decided to defer the maintenance write-up for 50 flight hours. This deferral option was not found in the Boeing AMM. This lack of clarity may have caused inadequate troubleshooting to be performed as well as the deferral of a failing component that subsequently resulted in a fire. United Airlines has modified its AMM to enhance the troubleshooting procedures and remove the 50 hour deferral.
United Airlines’ maintenance organization was aware of Boeing Service Bulletins (SB) 757-30-0019, revision 2, dated April 19, 2010, which provides instructions for initial and repeat inspections of the terminal blocks. However, at the time of the incident, United Airlines had not yet performed the actions specified in the SB on the incident airplane, nor were they required to do so by the FAA. If the actions specified in SB 757-30-0019 had been performed, it is likely that an anomaly would have been discovered at the J5 terminal block and the fire prevented.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: 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. Full narrative available
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