DCA24LA095

Improper maintenance due to human error during a braking system modification diminished braking performance. Contributing to the diminished braking performance was the lack of a functional check to verify the flexible hydraulic hoses and transducer wiring were connected correctly after the braking system modification.

American Airlines flight 1632 experienced a brake system anomaly shortly after landing on runway 17L at Dallas-Fort Worth International Airport (DFW), Dallas-Fort Worth, Texas. The anomaly resulted in a loss of braking effectiveness that increased the airplane stopping distance. The airplane came to a stop in the paved overrun area beyond the south end of the runway threshold. The crew executed emergency procedures, including engine shutdown and auxiliary power unit (APU) activation, ensuring a safe post-incident response. All 104 passengers and crew members safely evacuated the airplane via airstairs, with no injuries reported. The flight was operated as a scheduled domestic passenger service under the provisions of Title 14 Code of Federal Regulations (CFR) Part 121, traveling from Ronald Reagan Washington National Airport (DCA), Arlington, VA, to DFW. Each main landing gear (MLG) has a normal braking system powered by hydraulic system B and manually controlled by the flight crew via brake pedals in the flight deck. Pedal movement is transmitted through cables to left and right brake metering valves located in the wheel well, which regulate hydraulic pressure to the respective brake assemblies. Each wheel is equipped with a rotor-stator brake assembly that uses hydraulic pressure to generate braking force. On February 6, 2024—four days before the incident—American Airlines completed a scheduled modification on the airplane, replacing the original steel brakes with carbon brakes and associated wheel assemblies. This work was performed in accordance with an Engineering Order (EO) and associated cards dated November 2, 2022, based on Boeing Service Bulletin SB 737-32-1429, Revision 4. The modification applied to Boeing 737-800 series airplane equipped with Goodrich or Honeywell steel brakes and wheels. The EO and associated cards included detailed maintenance instructions with sign-off blocks and incorporated the technical content of the Boeing Service Bulletin, supplemented with additional information to ensure compliance with American Airlines’ continuous airworthiness maintenance program. As part of the conversion to carbon brakes, four flow limiters were installed, replacing the existing bulkhead unions between the rigid hydraulic tubes and flexible hydraulic hoses. Due to the increased length of the flow limiters, the original rigid tubes (four total) located inboard of each MLG within the wing were replaced with shorter ones. Installation required temporary disconnection of each flexible hose, removal of the bulkhead union, installation of the flow limiter, and reconnection of the flexible hose. Flight data recorder (FDR) data recorded brake pressure from two transducers—one for each side—located upstream of the antiskid valves. These transducers reflect hydraulic pressure supplied by the autobrake system or the pilot-controlled metering valves during manual braking. The recorded values during the incident appeared consistent with expected inputs, indicating no upstream braking system issues. However, the FDR does not record data downstream of the transducers, limiting the ability to evaluate antiskid valve functionality or overall brake system performance. Additional parameters such as wheel speed and brake pressure at the assemblies would enhance investigative capabilities. The flight crew indicated that they selected an autobrake setting of 3 for the landing at DFW. The autobrake system supplies metered brake pressure to help decelerate and stop the airplane after landing or if a rejected takeoff (RTO) occurs. It monitors airplane deceleration and controls metered pressure to maintain the target deceleration rate selected by the pilot on the AUTO BRAKE select switch until the airplane comes to a full stop, provided there is no flight crew input. Available settings include RTO, OFF, 1, 2, 3, and MAX. FDR data indicated that both the autobrake system and ground spoilers were functioning at the onset of the landing roll. Upon landing, the increase in the hydraulic brake pressures were consistent with the autobrake setting 3. However, manual brake application by the flight crew likely occurred within seconds of the autobrake application, overriding the autobrake system. The autobrake application discrete parameter confirmed the autobrake was applied for one sample, approximately one second. Once the flight crew’s manual brake application overrode the autobrake, the flight crew controlled the brakes for the remainder of the landing rollout. According to Boeing, the autobrake system disengages when the metered brake pressure reaches or exceeds 750 pounds per square inch (PSI). During this landing, the right brake pressure increased steadily from 0 to 3000 PSI within four seconds; the left brake pressure increased to 3000 PSI within ten seconds. At the time of autobrake disengagement, the right brake pressure was increasing past 750 psi. This data suggests that a crew member manually applied brakes resulting in the autobrake system disengaging. Post-incident troubleshooting revealed that the flexible hydraulic hoses connected to the number 3 (right inboard) and number 4 (right outboard) MLG brakes had been improperly reconnected following the carbon brake and flow limiter installation. Specifically, the flexible hydraulic hoses supplying pressure to the number 3 and 4 brakes had been swapped at their connections with the flow limiters (see figure 1). Figure 1. Photograph showing the swapped flexible hydraulic hoses on the right MLG. (Source: American Airlines) Further system troubleshooting by American airlines revealed a discrepancy with the wiring to the left MLG wheel speed transducers. During an operational test, maintenance personnel discovered that the wiring harness within the MLG axle had been installed incorrectly. Specifically, the electrical connectors for the number 1 (left outboard) and number 2 (left inboard) wheel speed transducers were swapped. As a result of these two discrepancies, when a skid occurs, the non-skidding wheel would receive the brake pressure release intended for the wheel on the same MLG that is skidding, and the skidding wheel would receive the (potentially full) metered brake pressure due to its brake not being released. This is because the skidding wheel would receive the antiskid commands intended for the non-skidding wheel, and vice versa. Given the combination of the switched hydraulic brake hoses and wheel speed transducers, the use of manual braking would not have released the subsequently locked and skidding outboard tires. Eventually, the skidding tires were worn flat until the tires failed. The failed tire would lead to lost braking effectiveness that increased the airplane’s stopping distance beyond the end of the runway. The investigation confirmed that the only mechanical discrepancies were the reversed hydraulic hoses to the number 3 and number 4 MLG brakes and the reversed wheel speed transducer wires for the number 1 and number 2 MLG wheels. Safety Actions American Airlines subsequently revised engineering order card 3222J004-001 adding a step to label the flexible hydraulic hoses prior to removal and requiring an inspector to verify the flexible hydraulic hoses are properly re-installed. In addition, the transducer operational test was incorporated into engineering order card 3222J004-001. This operational test is capable of detecting either swapped flexible hydraulic hoses or swapped transducer connectors. Additionally, American Airlines issued engineering authorizations (EA) and engineering orders to check the entire B737 fleet for swapped transducer wiring. A total of 50 airplanes were inspected by EA and 253 airplanes were inspected by EO. Of all 303 airplanes inspected, zero were found with transducer wiring swapped. American Airlines also revised their B737NG and MAX aircraft maintenance manuals (AMMs) to enhance the tr

FDR data showed that the active brake system during the incident was the normal braking system. In addition, the FDR data revealed that the autobrake engaged for approximately one second after touchdown before disengaging. At that time, the pressure from the left normal (manual) brake metering valve increased to more than 750 psi, and by design the autobrakes disengaged. The brakes were manually controlled by the crew for the remainder of the landing. Therefore, the following systems descriptions will focus on manual braking with the normal braking system. The MLG brakes and wheel assemblies are identified from left to right as 1, 2, 3, and 4, with 1 referring to the left outboard and 4 referring to the right outboard. The Boeing 737-800 airplane has a retractable tricycle-type landing gear, composed of the nose landing gear (NLG) and the left and right MLG. The NLG contains two wheels, a left wheel and a right wheel. For the MLG system, the left MLG has the number 1 wheel (outboard) and the number 2 wheel (inboard ), while the right MLG has the number 3 wheel (inboard) and the number 4 wheel (outboard). Normal Braking System The normal braking system uses hydraulic system B as its hydraulic source (figure 3). The brakes are controlled by the flight crew using the brake pedals in the flight deck. Brake pedal movement is transmitted by cables to the left and right brake metering valves located in the main landing gear wheel well. The left brake metering valve supplies metered hydraulic pressure to the left main gear wheel brake assemblies in response to the input from the control cables. The right brake metering valve supplies metered hydraulic pressure to the right main gear wheel brake assemblies in response to the input from the control cables. The metered hydraulic pressure passes through a shuttle valve and then to the respective inboard and outboard antiskid valves. Between the shuttle valve and the antiskid valves is a brake pressure transducer (one for the left brake system and one for the right brake system). This is the location where the brake pressures recorded on the FDR originate. Between each antiskid valve and brake assembly there is a hydraulic fuse to prevent hydraulic fluid loss if there is an external leak downstream of the fuse, and there is an alternate brake shuttle valve to allow brake pressure to come from the alternate brake system if required. Each wheel has one brake assembly. The brake assemblies are rotor-stator units that use hydraulic pressure to push the rotors and stators together, causing the wheel to slow. Figure 3. Hydraulic Brake System. (Source: Boeing. Image Copyright © Boeing. Reproduced with permission.) Antiskid/Autobrake System The airplane’s antiskid and autobrake systems are controlled and monitored by the antiskid/autobrake control unit (AACU). The AACU is the central component of the two systems and also monitors the two systems for faults. The AACU is located on the E1-3 shelf of the forward electronic equipment (EE) bay. The unit includes circuit cards that control the autobrake function, inboard/outboard antiskid, and a built in test equipment (BITE) function. The AACU receives input from a number of sources, including the four MLG wheel speed transducers, the autobrake pressure control module, and the air data inertial reference unit (ADIRU). Antiskid System The antiskid system monitors wheel deceleration and controls the metered brake pressure to prevent wheel skids during brake application. The antiskid system is operational whenever the associated electrical buses are powered and requires no flight crew action. When brake pressure is released to a wheel that is skidding, the wheel speed is permitted to increase which stops the skid condition. When the normal braking system is active there is an antiskid valve for each wheel brake. The antiskid valve releases pressure to its associated wheel brake when commanded by the AACU. The unwanted pressure is released through the parking brake valve. A transducer for each main landing gear wheel, installed in the axle, supplies wheel speed data to the AACU. The ANTISKID INOP amber light comes on if the built-in test card in the AACU detects a fault in the antiskid system. The AACU monitors faults related to system power, wheel speed transducers, parking brake lever and parking brake shutoff valve disagree, antiskid valves, and the AACU itself. When certain faults (including an open antiskid inboard or outboard circuit breaker) are detected in the antiskid system the autobrake system becomes inoperative. These are some of the antiskid functions: o Skid control operates at a ground speed of more than eight knots to control each wheel deceleration independently during normal braking system antiskid operation. Skid control compares the calculated wheel speed velocity with a velocity model to control wheel deceleration. If a wheel slows down too quickly, the skid control releases brake pressure until the wheel speed increases. o Locked wheel protection compares the wheel speed of the two outboard or the two inboard pair of wheels. If the slower wheel speed decreases to less than 30 percent of the faster wheel speed, the locked wheel protection releases brake pressure from the slower wheel. Locked wheel protection does not operate at a ground speed less than 25 knots. Autobrake System The autobrake system supplies metered brake pressure to stop the airplane after the airplane lands or if a rejected takeoff (RTO) occurs. The autobrake system monitors airplane deceleration and controls metered pressure to maintain the target deceleration rate selected by the pilot on the AUTO BRAKE select switch until the airplane comes to a full stop, provided there is not flight crew input. The pilot can select a setting of RTO, OFF, 1, 2, 3, or MAX depending on the desired deceleration rate. The autobrake system arms for landing when there are no associated faults in the autobrake system or the normal antiskid system, and all the following conditions occur: o The AUTO BRAKE select switch is moved to a landing deceleration position (1, 2, 3, or MAX) o Both air/ground systems in air mode, or both thrust levers at idle, or one or both air/ground systems in the ground mode for less than or equal to three seconds o Valid input from the left air data inertial reference unit (ADIRU) o Normal brake metered pressure is less than 750 psi The autobrake function applies the brakes when these conditions occur: o Landing autobrake is armed o Both thrust levers at idle o Either air/ground system continuously indicates ground for 0.2 seconds (if wheel spin-up occurs more than one second before ground is sensed) or 0.7 seconds (if wheel spin-up occurs less than one second before ground is sensed). o Wheel spin-up detection occurs or the spin-up latch sets. Wheel spin-up detection occurs when one wheel on each main landing gear increases to 60 kts or greater and the wheel speed stays above 30 kts. The spin-up latch sets 3 seconds after the air/ground system is in ground mode and the wheel spin-up detection occurs. During autobrake operation the AACU unit uses its inputs to determine brake applications commands to the autobrake pressure control module. In addition, the antiskid portion of the AACU sends brake release commands to the antiskid valves. The AACU also monitors the two systems for faults; the fault monitoring function includes non-volatile memory to record faults. The BITE system will display faults on the BITE portion of the AACU’s front panel. Brake pressure transducers provide brake pressure information to the flight data recorder (FDR) system independent of the AACU. Durning normal operations, the autobrake system function may disarm for various conditions. Manual brake application(s) by the pilot can override and disarm the autobrake system when metered brake pressure meets or exceeds 750psi. This triggers illumi