NTSB Factual Report

HISTORY OF FLIGHT

On January 30, 2008, at 2022 eastern standard time, American Airlines flight 1738, a Boeing 757-200, N624AA, declared an emergency due to smoke in the cockpit and diverted to Palm Beach International Airport (KPBI), West Palm Beach, Florida. The flight was being piloted by an airline transport pilot certificated captain and airline transport pilot certificated first officer. The airplane landed without incident at KPBI. Night visual meteorological conditions prevailed at the time of the incident. The scheduled domestic passenger flight was being conducted under the provisions of Title 14 CFR Part 121, and an instrument flight rules flight plan had been filed. The captain, first officer, three of the six flight attendants, and one passenger were transported to the hospital with minor injuries. The flight departed San Juan, Puerto Rico, at 1851 atlantic standard time and was enroute to Philadelphia, Pennsylvania.

According to the flight crew, the airplane was at flight level (FL) 340, approximately 35 miles south of the DRIBL intersection on radial R763, when they noticed a burning odor and large amounts of smoke in the cockpit, none of it coming from an obvious place. The crew donned their oxygen masks and smoke goggles, declared an emergency, and turned the airplane westbound towards the Florida coast. They accomplished the Quick Reference Handbook procedures for "Smoke/Fumes Removal". While trying to determine the source of the smoke, the first officer noticed that his windshield began to crack and small shards of glass began to separate and strike him. They began a descent and about 10,000 feet the first officer's windshield made a "very loud bang" and shattered, blocking all outside visibility. The airplane landed on Runway 27R and taxied to the gate under its own power.

PERSONNEL INFORMATION

Captain

The 46-year-old captain held an airline transport pilot certificate with a type rating for B757/767 airplanes and a first class medical certificate. His most recent medical certificate was issued on October 23, 2007, and his most recent flight review was performed on November 14, 2007. He had accrued 182, 49, and 7 flight hours in the last 90 days, 30 days, and 24 hours, respectively.

First Officer

The 41-year-old first officer held an airline transport pilot certificate with a type rating for B757/767 airplanes and a first class medical certificate. His most recent medical certificate was issued on August 14, 2007, and his most recent flight review was performed on October 5, 2007. He had accrued 122, 46, and 7 flight hours in the last 90 days, 30 days, and 24 hours, respectively.

AIRPLANE INFORMATION

The Boeing 757-200 airplane, N624AA, was last inspected during a scheduled A-check on January 25, 2008. According to American Airlines maintenance records, the windshield was installed on the airplane on October 20, 2002. Boeing Service Bulletin 757-30-0019 calling for inspection of the lower, aft terminal block for a loose connection had not been performed on the windshield at the time of the incident. The windshield was reported to have accrued 16,334 hours and 5,771 cycles since installation.

The windshield is manufactured of the following materials, from the outboard surface to the inboard surface; a 0.120" thick Herculite II chemically strengthened glass layer, a PPG Nesatron conductive heating film layer for deice capability, a 0.030" PPG 112 urethane interlayer, a 0.140" vinyl layer, a 0.020 PPG 112 urethane interlayer, a 0.380" Herculite II chemically strengthened glass layer, a 0.080" vinyl layer, and a 0.312" Herculite II chemically strengthened glass layer. The center and inner panes of glass and the vinyl and urethane layers are secured in a Phenolic edge and the windshield is secured in the fuselage with an aluminum retainer on top of the Phenolic edge. The anti-ice function is accomplished using PPG's Nesatraon heating film on the inner surface of the outboard glass ply. A polysulfide moisture (hump) seal around the window perimeter protects the interlayer materials from moisture ingression. A silicon seal on the inner surface of the Phenolic edge provides sealing between the windshield and the fuselage mounting structure. The middle and inboard panes of glass are considered structural panes and certification testing is performed to demonstrate that the windshield is capable of carrying a differential pressure load of 19.2 psi with one structural pane fractured. Boeing also requires that the windshield must withstand a differential pressure load of 11.8 psi for at least 15 minutes with all panes of glass fractured.

Power to the windshield heat system is supplied through three terminal blocks on the edge of the windshield; J1 at the upper, aft corner, J4 at the upper forward corner, and J5 at the lower aft corner. Two terminal blocks along the upper edge (J2 and J3) provide sensing functions for the system. At each of the upper terminal blocks a braid wire is soldered to one side of the terminal lug and is routed around the glass plies to a bus bar along the upper edge of the window heating film. At the lower terminal block two braid wires are soldered to the side of the terminal lug and routed around the glass plies to each end of the bus bar along the lower edge of the heating film. The terminal blocks are potted to the edge of the window with a polysufide sealant after soldering. The window heat controller supplies the power to the system. The Boeing electrical requirements for the system are a maximum voltage of 186 Volts root mean square (RMS) from the controller and a resistance between the upper and lower bus bars of 9.12 to 11.15 Ohms. At the end of each of the power wires from the airplane a terminal connector is installed. The connector is attached to the block with a screw and washer.

METEOROLOGICAL INFORMATION

The company reported the following weather at KPBI at 2053: wind from 150 degrees at 5 knots, visibility 10 miles, sky clear, temperature 22 degrees Celsius, dew point 17 degrees Celsius, and an altimeter of 30.12 inches of Mercury.

COMMUNICATIONS

The first officer reported that communication with air traffic control was difficult due to the distance between the airplane to the air traffic control facility and utilizing the oxygen masks.

MEDICAL AND PATHOLOGICAL INFORMATION

The captain, first officer, three flight attendants, and one passenger were transported to local hospitals for medical evaluation and released.

FIRE

The crew reported a burning smell and smoke in the cockpit but did not observe any visible flames. They were not able to determine the source of the smoke during the event. None of the fire extinguishers were expended during the incident.

TEST AND RESEARCH

The damaged right main windshield (P/N 141T4801-50, S/N 02280R7363) and attached wiring harness were removed from the airplane by American Airlines maintenance personnel and shipped to the manufacturer, PPG Aerospace in Huntsville, Alabama. The window was examined under National Transportation Safety Board (Safety Board) supervision on February 21, 2008. The windshield serial number indicates that the windshield was a remanufactured unit and was completed on the 280th day of 2002.

The first officer's windshield exhibited two distinct areas of fracture on the inner pane of glass. The upper 1/3 of the windshield exhibited long sparse cracks that curved from the upper, forward corner of the windshield towards the upper, aft corner. The lower 2/3 exhibited a dense network of small cracks that obscured the visibility through the windshield. The lower, aft (J5), terminal block was intact with no obvious damage. The upper, forward (J3) and aft (J2) terminal blocks were intact with no obvious damage. There was no evidence of moisture ingression on the windshield. There was evidence of a hump seal repair to the aft and upper edges. The upper bus bar exhibited some discoloration.

There was no evidence of chafing, rubbing, or repairs to the wires in the harness. The J4 screw and separated terminal block lug were attached to the J4 terminal connector and were removed for examination. There was no washer present between the screw and terminal connector. The J1 terminal connector lug and orange o-ring exhibited slight discoloration as compared to the J5 connector. The J4 terminal connector lug and orange o-ring exhibited moderate charring and discoloration as compared to both the J1 and J5 terminal connectors. The fastener side of the J4 terminal connector lug exhibited orange deposits and pitting consistent with corrosion. The cap shoulder areas on the J1, J4, and J5 terminal connectors were fractured from the fastener side of the connectors. The J2 and J3 terminal connectors were intact with no obvious damage.

The upper, aft (J1) terminal block was received separated from the windshield and exhibited minor charring on the upper and outboard sides. The area underneath the J1 terminal block had some small areas of charred polysulfide sealant along with some areas of soft pliable sealant. There was no evidence of the braid wire under the J1 terminal block. The surface of the glass exhibited a diagonal crevice traversing down and left about the normal location of the braid wire. There were no glass fractures initiating at the damaged glass area. The depth of the damaged area was measured with an optical micrometer as 0.054". A small stub of the braid wire protruded from the glass-Phenolic interface. The resistance between this braid wire and the J5 connector was 10.2 ohms. The bottom side of the J1 terminal block exhibited charring damage in a diagonal line coincident with the area of pitting on the glass surface. The Phenolic edge was removed to reveal the braid wire was intact from the edge of the Phenolic to the edge of the glass. The J1 terminal block lug exhibited moderate charred deposits on the connection face. The J1 connector was sectioned through the lug and revealed no evidence of cross-threading.

The upper, forward (J4) terminal block remained attached to the windshield and exhibited extensive charring on the inboard, outboard, and upper sides. The lug was not present in the J4 terminal block and there was a large extensively charred hole through the middle of the block where the lug normally is installed. There was a crack in the block material along the length of the lug on the lower side. The J4 terminal block was removed from the windshield and revealed charred polysufide sealant underneath and no soft pliable sealant. There was an area of damaged glass coincident with the normal location of the braid wire. The Phenolic edge was removed above and back to the edge of the glass and no braid wire was found. There was extensive heat damage and small melted metal balls along with carbon residue present in the damaged glass area. The outboard edge of Phenolic was then removed by cutting through it. During the cutting of the Phenolic edge the outer ply of glass fractured. The vinyl at the edge of the glass where the braid wire should have been exhibited a melted appearance. The braid wire was evident along the edge of the glass plies up to the point where it wraps around to the inner pane of glass. The resistance between the J4 and J5 connector was 11.7 ohms with the outer pane fractured. The J1 to J4 resistance was measured at 0.7 ohms. The J4 terminal block lug exhibited extensive charred deposits on the connection face. There was also charred material deposited on the upper surface where the braid wire attaches. The J4 terminal block lug was sectioned and revealed no evidence of cross-threading.

The J4 screw measured 0.75" in length and the head was 0.12". There was no evidence of cross threading on the J4 screw. The J4 screw exhibited orange deposits similar to corrosion on the head and the threads that were not engaged with the lug. There was discoloration and charred/melted deposits on the neck area of the screw. The clockwise faces of the Phillips point receptacles on the screw head exhibited mechanical damage consistent with tightening of the screw. The terminal block lug depth measured 0.520". About 0.285" of the screw protruded out of the terminal connector when installed.

American Airlines removed a windshield with the screw-type terminal blocks from one of their airplanes in order to perform additional testing on the windshield heat system. The removed windshield (P/N 141T4801-50RPR, S/N 99124H6722) was sent to Boeing with a test plan calling for the removal of the J1, J4, and J5 terminal blocks leaving a portion of braid wire attached. The Safety Board was not present for this testing. The solder connection between the terminal block and braid wire was examined under magnification for each terminal block. The J1 terminal block exhibited one broken strand of the braid wire and the J5 terminal block exhibited two broken strands and some exposed copper on the braid strands. The J4 terminal block did not exhibit any anomalies. The J1 and J5 terminal blocks were attached to actual shipside terminal connectors with the screw torqued to 25 in-lbs. The assemblies were then spray painted flat black so that a thermal imaging camera could be used to measure the temperature at the solder joint when power was applied. The J1 connector had 191 Volts AC at 400 Hz with a current of 10.5 Amps applied and the J5 connector had 191 Volts AC at 400 Hz with a current of 21.3 Amps applied for 480 seconds and 720 seconds, respectively. The solder joint reached a maximum temperature of 83.6 degrees Fahrenheit (F) for the J1 connector and 90.4 degrees F for the J5 connector. The screws were then loosened and the test was repeated at varying current levels until the solder joint failed. Both the J1 and J5 solder joints failed when a current of 21.4 Amps was applied with a loose connection and the solder joint temperature was more than 400 degrees F.

ADDITIONAL INFORMATION

The Safety Board has investigated several similar events of smoke/fire at the windshield heat terminal connection since 2004 and is aware of many more events. In some of the events the cause of the event was traced to a cross-threaded screw at the J5 terminal block, while in others the initiating cause was undetermined. Boeing has redesigned the 757 windshield to incorporate a pin-socket connection from the shipside wire to the windshield and a screw-crimped ring connection from the braid wire to the terminal block. The new windshield was released after production on the 757 stopped but is available as a spare replacement. Boeing Service Bulletins 757-30-0019 and 757-30-0020 provide for inspection of the terminal blocks on all old windshields and replacement of the windshields on all 757 airplanes with a new windshield that incorporates a pin-socket type terminal block. The Safety Board issued recommendation A-07-50 in September 2007 asking the FAA to issue airworthiness directives to replace the windshield heat terminal block on all Boeing 747, 757, 767, and 777 airplanes in accordance with the Boeing service bulletins. The FAA issued a Notice of Proposed Rulemaking in April 2008 to make the inspection of windshields and eventual replacement mandatory on 757, 767, and 777 airplanes.

NTSB Probable Cause

the arcing of the windshield heat system at the upper, aft (J1) and upper, forward (J4) terminal locations on the first officer's windshield, which caused the inner pane of glass to fracture.