ENG10IA029

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.

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.

HISTORY OF FLIGHT On May 16, 2010, about 2117 eastern daylight time, the flight crew of United Airlines flight 27, a Boeing 757-200, N510UA, declared an emergency because of a fire in the cockpit and diverted to Washington Dulles International Airport (IAD), Chantilly, Virginia. The flight was en route from John F. Kennedy International Airport (JFK), Queens, New York, to Los Angeles International Airport (LAX), Los Angeles, California. The airplane landed at IAD without incident. No evacuation was conducted, and none of the 7 crewmembers or the 105 passengers sustained injuries. The scheduled, domestic passenger flight was operating under the provisions of 14 Code of Federal Regulations Part 121 on an instrument flight rules flight plan. Night visual meteorological conditions prevailed at the time of the incident. The captain was the flying pilot. During post incident interviews, the captain stated that the flight departed JFK for LAX about 2030. The flight crew reported that the taxi, takeoff, and climbing phases of the flight were normal. According to the captain, the first indication of a problem occurred shortly after the airplane had leveled off at its cruise altitude of 36,000 feet. Subsequently, both the captain and first officer heard a hissing sound and saw smoke emanating from below the glare shield. The captain stated that, “within a couple of seconds,” a fire started and that he saw flames emanating from the top of the glare shield. He stated that the smoke was not thick but that it was acrid. The captain got a burning sensation in his eyes and nose before he and the first officer were able to don their oxygen masks and smoke goggles. The captain stated that he transferred control of the airplane to the first officer and told him to declare an emergency with air traffic control (ATC), which the first officer did at about 2136. According to the captain, he got out of his seat because flames were in front of him and he needed to immediately reach the Halon bottle (fire extinguisher). The extinguisher was not reachable from his seat because it was located on the back wall of the cockpit behind his seat. As he reached for the Halon bottle, his oxygen mask and goggles were “torn off” because he moved beyond the reach of the oxygen mask hose. He retrieved the Halon bottle, re-donned his mask and goggles, and discharged the bottle. After he emptied the Halon bottle, the fire went out momentarily, but it re-ignited after a few seconds. His oxygen mask came off again when he opened the cockpit door to receive another Halon bottle from a flight attendant. He discharged the second Halon bottle at the fire, and it went out. According to the first officer, he turned all four window heat switches off shortly after the fire re-ignited. After the fire was extinguished, the captain returned to his seat and re-gained command of the aircraft. The first officer informed the captain that ATC was asking where they wanted to go, and the captain replied, “go to Washington Dulles Airport,” because it was about 50 miles away. The captain indicated that he thought IAD was the best choice because the airport had better maintenance and fire support. The autopilot remained engaged until the airplane descended through about 10,000 feet, at which time, the flight crew began to slow the airplane to configure it for landing. As the airplane descended through about 500 feet, the flight crew heard a “loud explosive bang,” and the inner pane of the captain’s windshield cracked. The captain then transferred control of the airplane to the first officer because his vision was impaired because of the shattered windshield. The captain stated that he could see through the window but it was “spider webbed”. The first officer landed the airplane and reduced its speed. The captain then took control of the airplane and taxied it off the runway. DAMAGE TO AIRPLANE On May 16, 2010, a National Transportation Safety Board (NTSB) investigator visually examined the captain’s No. 1 windshield and the area near the windshield. The windshield had fractured and its lower aft portion exhibited signs of high thermal damage and soot staining. Remnants of the J5 power terminal block, which had been mostly consumed by fire, were in this area. Visual examination of the J5 power terminal block and its attachment hardware revealed that a lock washer was not present beneath the screw that connects the wiring harness terminal lug to the terminal block. Examination of the remaining terminal blocks on the captain’s windshield, as well as the three power terminal blocks and the two terminal sensing blocks on the first officer’s windshield, revealed that no lock washers were present beneath any of the screws that attach the wiring harness terminal lugs to their respective terminal blocks. PERSONNEL INFORMATION The captain, age 59, held a multi-engine land airline transport pilot (ATP) certificate with type ratings in Boeing 737, 747-4, 757, and 767; Citation CE-500; Lockheed L-300; and Airbus A320 airplanes. The captain held a first-class Federal Aviation Administration (FAA) medical certificate, dated May 3, 2010. His most recent Boeing 757 flight proficiency check occurred in May 2009. He had flown 170, 60, and 7 hours in the 90 days, 30 days, and 24 hours, respectively, before the incident. The first officer, age 45, held a multi-engine land ATP certificate with type ratings in Boeing 757 and 767 and A320 airplanes. The first officer held a first-class FAA medical certificate, dated August 20, 2009. His most recent Boeing 757 flight proficiency check occurred in November 2009 He had flown 191, 79, and 7 hours in the 90 days, 30 days, and 24 hours, respectively, before the incident. AIRCRAFT INFORMATION All six cockpit windows have an electrically heated conductive layer for anti-icing and defogging. Two thin electrical conductors, called bus bars, are installed on the upper and lower edges of the window to transmit electrical current from the power wires to the conductive heating film. The windshield heat system for the captain and first officer’s No. 1 windshield comprises three power terminal blocks that are located on the edge of the windshield; J1 is located at the upper aft corner, J4 at the upper forward corner, and J5 at the lower aft corner. Two sensing terminal blocks (J2 and J3) are located along the upper edge of the windshield to provide sensing functions for the system. Power is supplied to the windshield heat system by a window heat controller through a wiring harness with molded terminal connector lugs on the end. One side of the wire harness connects to the airplane wiring (to the heat controller), and the other side connects to each of the windshield terminal blocks via the terminal lugs. Each terminal lug is connected to its respective terminal block on the windshield with a screw and a lock washer. The windshield is a bolted-edge design that uses internal and external aluminum retainers to secure the windshield in the fuselage opening. The anti-ice function is accomplished using PPG’s Nesatron heating film to provide clear vision during inclement conditions. A polysulfide moisture seal around the windshield perimeter protects the interlayer materials from moisture ingression. According to United’s maintenance records, at the time of the incident, the windshield had accumulated 12,065 hours and 2,325 cycles since installation. A review of United Airlines maintenance records revealed that TIMCO Aviation Services, Inc. (GSO) performed a scheduled maintenance check on the airplane in January 2007. During the maintenance check, GSO replaced the Number 1 left-side window because its lower, forward, inboard corner exhibited signs of delamination. The newly manufactured replacement window (part number 141T4801-49) was installed on January 29, 2007, in accordance wit