Boeing 737-823 N991AN

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 transducer operational test and the main landing gear installation job card has been updated with enhanced steps to ensure the transducer wires are not crossed. Following the incident, Boeing added Information Notice 02 to service bulletin 737-32-1429 Revision 4. This notice was distributed to alert operators to the risk of crossing hydraulic hoses when reconnecting the flexible hydraulic hoses to the new flow limiters. The notice emphasizes that each flexible hydraulic hose must be reconnected to its corresponding flow limiter and that hoses must not be crossed at any point along their length. Improper reconnection – such as attaching a flexible hydraulic hose to the incorrect flow limiter – will result in incorrect antiskid system operation, potentially leading to a loss of braking action and/or tire failure. Additionally, Boeing has issued two Fleet Team Digests - 737MAX-FTD-32-24002 and 737NG-FTD-32-24001 - to provide both interim actions as well as final actions to B737NG and B737MAX operators. The interim actions include a list of best practices such as: Temporarily labeling flexible hydraulic hoses to the corresponding brake positions, Labeling wiring harnesses when disconnecting wheel speed transducers, and Functional checks to verify flexible hydraulic hoses and transducer wiring is correctly connected. The final actions include revising specific AMM tasks and service bulletins where improper maintenance may occur. Boeing released Service Bulletin 737-32-1429 Revision 5 on August 14, 2025. Furthermore, Boeing issued SB 737-32A1599 on October 14, 2025, to ensure proper hydraulic routing following the implementation of service bulletins that alter brake configurations. The NTSB has investigated several aircraft incidents and accidents that have been the result of cross wiring of wheel speed transducers, all were the result of maintenance human error.

Quoted verbatim from the NTSB record.