Chantilly, VA, USA
N510UA
BOEING 757
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 with the Boeing 757 Aircraft Maintenance Manual (AMM) 56-11-01/401, “No.1 Windshield – Removal/Installation.” A review of the airplane’s service history revealed that, on May 15, 2010, the day before the incident, the airplane diverted to McCarran International Airport (LAS), Las Vegas, Nevada, because of a report of an electrical odor in the cockpit. United Airlines maintenance personnel inspected the airplane at LAS, and no defects or odors were found. The airplane was then ferried to San Francisco International Airport (SFO), San Francisco, California, for additional maintenance. During the flight from LAS to SFO, no electrical odors were noted by the flight crew, and the only unusual odor was reported to have come from the forward galley ovens. Upon arrival in SFO, United Airlines maintenance personnel inspected the airplane and could not find the source of the smell; however, they replaced both forward galley ovens as a precaution. The airplane was returned to service on May 16, 2010. During the first revenue flight from SFO to JFK, after the maintenance was performed, the flight crew reported that the right No. 1 windshield lower outboard power connector appeared burnt and was hot to the touch. During post incident interviews, the captain of that flight stated that, based on the previous maintenance write-ups, he was fully aware of the reports of smoke and fumes in the cockpit. During the flight, he looked the cockpit over to see if he could determine where the smoke and fumes might have initiated. He stated that when he looked at the power connector, it looked unusual, and he noticed that the lower left terminal block on his windshield was blackened or charred. When he put his finger on it, he noticed that it was too hot to keep his finger on, which he thought was unusual. He got out of his seat, felt every other terminal block on every other window, and noted that none of them were hot. He then submitted an information item to the maintenance reporting manual (United’s maintenance write-up system) via the Aircraft Communications Addressing and Reporting System (ACARS). Upon arrival at JFK, a JFK maintenance technician met with the captain in the cockpit to discuss the maintenance write-up. According to the captain, he touched the connector at the captain’s No. 1 windshield to show the mechanic the connector that he was concerned about and told him that it was hot and looked charred. The window heat had been turned off before the technician came on board the airplane as part of the normal aircraft shutdown procedures. During a post incident interview, the JFK maintenance technician stated that he visually inspected the window, bus bar, power terminal, connector, and wires. He stated that, at the time of the inspection, he thought that the power terminal was part of the bus bar. He referred to the United Airlines AMM 56-11-00/601, “Flight Compartment Windows – Inspection / Check,” which indicated that the window should be replaced due to the discoloration of the bus bar. (When an AMM is referred to as “United Airlines AMM” as opposed to a “Boeing AMM”, it indicates that United Airlines has customized the original “Boeing AMM” by either including or clarifying maintenance procedures to help the maintenance technician perform the task.) However, the limitations section of the United Airlines AMM states, “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.” After the inspection, the technician spoke with the other lead technician at JFK, and they decided to defer the maintenance write-up for 50 flight hours. The mechanic signed off the maintenance write-up and returned the airplane back to service. A review of the Boeing AMM 56-11-00/601, “Flight Compartment Windows – Inspection / Check,” revealed that it does not contain a limitations section and that it states to replace the windshield if there are any indications of arcing. On May 20, 2010, United Airlines revised AMM 56-11-00/601 from a significantly customized United AMM to a pure Boeing AMM to adopt the latest damage limits and definitions for in-service damage. FLIGHT RECORDERS The airplane was equipped with a Honeywell Model 9804700 solid-state flight data recorder (FDR). The FDR was in good condition, and the data were extracted normally. The data did not contain any information regarding the windshield heating system. The airplane was equipped with a Honeywell Model 6020 solid-state cockpit voice recorder (CVR). The CVR was sent to the NTSB’s laboratory for readout evaluation. The CVR was in good condition, and the audio information was extracted without difficulty. None of the audio information was pertinent to the incident, and it was consistent with the recording being overwritten by subsequent events. FIRE On May 16, 2010, a NTSB investigator visually examined the captain’s No. 1 windshield (part number 141T4801-49, serial number 06039H6554) and its respective wiring harness before the components were removed from the airplane. (The Boeing 757 cockpit has six windows: three on the captain’s side [left] and three on the first officer’s side [right], and they are numbered center to outboard 1, 2, and 3.) Examination revealed that the airplane exhibited relatively little fire damage except for some soot staining and surface charring in the area of the captain’s windshield. Soot staining was evident moving up the aft portion of the window frame brace and below the instrument panel glareshield. In addition, some slight charring existed in about a 1-square-inch area on the top surface of the instrument panel glareshield. The lower aft portion of the windshield exhibited the most thermal damage and soot staining. Remnants of the J5 power terminal block, which had been mostly consumed by fire, were found in this area. The terminal connector lug associated with the J5 terminal block was charred and melted, and the electrical contacts inside appeared oxidized and corroded consistent with exposure to high temperature. The screw that had been embedded in the terminal block also exhibited a heavily oxidized appearance consistent with exposure to high temperature. The visual examination did not reveal any additional evidence of fire damage in the cockpit. TEST AND RESEARCH The windshield, wiring harness, and all of the attachment hardware were removed from the airplane and shipped to PPG Aerospace facility in Huntsville, Alabama for more detailed examinations. The examinations were conducted on June 16, 2010 under the supervision of the NTSB and witnessed by representatives from PPG Aerospace, FAA, United Airlines, Boeing, International Brotherhood of Teamsters, and Air Line Pilots Association. The windshield part and serial numbers indicated that the windshield was manufactured in 2006 under PPG’s Parts Manufacturing Approval from the FAA. Examinations revealed that the upper 75 percent of the windshield’s inner pane exhibited a dense network of cracks that emanated from the lower, aft corner of the windshield. The upper and lower bus bars were intact and exhibited very little discoloration. No evidence of moisture ingression was found on the windshield or and there were no seal repairs to the aft and upper edges. Soot was found on the inboard glass pane along the entire length of the aft windshield edge. A check of the electrical resistance between each terminal (power and sensor) and its respective braid wire found that all of the connections were within their specified limits. Other than the lower, aft J5 connector, all power terminal blocks and sensors remained intact and attached with no apparent damage. The J5 connector was found to be extensively burned with only the aft edge remaining. The area on the glass underneath the J5 terminal block had some small areas of charred polysulfide sealant and some areas of soft pliable sealant. Examination of the wiring harness revealed no evidence of chafing, rubbing, or repairs to any of the wires in the harness. The J5 terminal connector and lug exhibited charring damage, and the housing was cracked in multiple places. A small area of bluing was found on the surface of the lug
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.
Source: NTSB Aviation Accident Database
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