BOEING 737-823

Full Narrative

HISTORY OF FLIGHTOn February 10, 2024, about 19:42 central standard time, American Airlines flight 1632, a Boeing 737-823, N991AN, experienced a brake system anomaly after landing on runway 17L at the Dallas-Fort Worth International Airport (DFW), Dallas-Fort Worth, Texas. The airplane came to a stop in the paved overrun area beyond the south end of the runway threshold (see figure 2). 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 in Dallas-Fort Worth, TX.


Figure 2. Photograph showing the final position of the airplane after the incident. (Source: American Airlines)

The incident occurred on the final day of a scheduled four-day trip, during a single-leg return flight to DFW. The first officer was the pilot flying (PF), and the captain was the pilot monitoring (PM). FDR data showed that the airplane departed DCA at 16:21 CST (17:21 eastern standard time) with no anomalies during the enroute phase of the flight.

According to data from the CVR, at 18:56:20, the first officer briefed the approach to DFW. He stated they would conduct a visual approach to runway 17L, with the instrument landing system (ILS) as a backup. The planned landing configuration included flaps set to 30°. At 19:15:23, the captain told the first officer that he would be landing with a tail wind. At 19:29:03, the flight crew conducted the before landing checklist. The captain said, “approach briefing was done, seatbelts have been on the entire flight, passenger announcement (PA) was completed, and the autobrakes are three”. In their post-incident statements, the flight crew explained that autobrakes were set to level 3 because the Automatic Terminal Information Service (ATIS) had reported winds from 070 degrees at 5 knots, resulting in a slight tailwind of one knot.

The crew reported that both the approach and touchdown were normal, with no issues noted regarding directional control.

According to the CVR transcript, at 19:41:33, a sound resembling touchdown was captured by the cockpit area microphone (CAM). At this moment, the FDR data indicated that the airplane’s main landing gear (MLG) had transitioned from air to ground, the airplane’s airspeed was 141 knots, ground speed was 150 knots, and all four ground spoilers were fully deployed. Approximately one second after touchdown, the autobrake parameter changed from “No Auto Brake” to “Auto Brake applied” then immediately reverted to “No Auto Brake” indicating that the autobrakes had deactivated. In their post-incident statements, the flight crew reported seeing the AUTOBRAKE DISARM light illuminate and the brakes did not engage requiring manual braking by applying pressure to the pedals. According to the CVR transcript, at 19:41:38, the first officer said “and, manual brakes“ followed by the captain stating “autobrakes are off” at 19:41:39.

At 19:41:40, approximately seven seconds after touchdown, the recorded right brake pressure peaked at around 3,000 pounds per square inch (psi), consistent with maximum manual braking. Approximately six seconds later, the left brake pressure also peaked at approximately 3,000 psi.

At 19:41:43, the thrust reverser parameters transitioned from “stowed” to “deployed.” They remained deployed for about 12 seconds, were stowed for six seconds, and then subsequently re-deployed for the remainder of the rollout. In a post-incident statement, the flight crew reported that the airplane decelerated with thrust reversers, but at a slower rate than expected.

According to the CVR transcript, at 19:41:52, the captain called out “80 knots” and then stated, “My aircraft.” The first officer responded, “Your aircraft”. At 19:41:56, the first officer said, “The brakes will not…you got it?” The captain replied “I got it. Brakes aren’t working.”

At 19:42:07, the first officer contacted the DFW tower and reported that the airplane had no braking capability. In response, the tower instructed an arriving airplane to go around.

According to FDR data, at 19:42:28, the airplane experienced several abrupt changes in acceleration, consistent with an overrun beyond the runway threshold. The airplane came to a complete stop five seconds later, at 19:42:33.

During the rollout phase, the airplane reached a maximum longitudinal deceleration of -0.27 g.

The captain made the “Remain Seated” passenger announcement and instructed the FO to shutdown the right engine and start the auxiliary power unit (APU). Once the APU was operational, the left engine was shut down. AIRCRAFT INFORMATIONFDR 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 illumination of the AUTOBRAKE DISARM light on the flight deck. AIRPORT INFORMATIONFDR 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 illumination of the AUTOBRAKE DISARM light on the flight deck. WRECKAGE AND IMPACT INFORMATIONA post-incident inspection was conducted on the two NLG tires and the four MLG tires.
The MLG configuration is as follows:
o Left MLG: Wheel/tire number 1 (outboard), Wheel/tire number 2 (inboard)
o Right MLG: Wheel/tire number 3 (inboard), Wheel/tire number 4 (outboard)

The following tires exhibited normal wear with no damage: Left MLG number 2 (inboard),
Right MLG number 3 (inboard), and both NLG tires. These tires had intact tread and sidewalls, with no flat spots or structural anomalies observed.

The left and right MLG outboard tires (Goodyear) showed significant damage :
o Tire number 1 (Left MLG, outboard), shown in figure 4:
o Exhibited a flat spot worn through all plies.
o The rupture extended across both sidewalls near the outer rim.
o The tire was torn in an “X” pattern originating from the initial rupture.
o Tire number 4 (Right MLG, outboard), shown in figure 5:
o Also exhibited a flat spot worn through all plies.
o Torn in an “X” pattern from the rupture point.
o Several long strands of fabric cord were exposed between the ply layers.
Both failures are consistent with high-energy lateral loading, likely due to a skid-through event.


Figure 4. An overall photo of the number 1 tire, with noted tire failure circled in red.


Figure 5. Failure of number 4 tire, with flat spot worn through each tire ply.

Following the incident, American Airlines (AA) conducted troubleshooting and inspection of the brake control system. The investigation revealed that the flexible hydraulic hoses connected to the right MLG brakes had been improperly reconnected during the carbon brake and flow limiter installation. Specifically, the hydraulic hoses supplying the number 3 and number 4 MLG brakes were inadvertently swapped at their connections to the flow limiters (see figure 1).

The effect of the flexible hydraulic hoses being swapped when a skid occurs, results in the non-skidding wheel getting 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. This resulted in the skidding wheel continuing to skid and reduced pressure on the braking wheel further diminishing the braking action on the right main landing gear.

System troubleshooting also found a discrepancy with the wiring to the left MLG wheel speed transducers. During a wheel speed transducer operational test, maintenance found the wiring harness, located in the left MLG axle, had been installed incorrectly. The electrical connector for the number 1 (left outboard) and the No. 2 (left inboard) wheel speed transducer were swapped.

The effect of the wheel speed transducer connectors being swapped was the electrical equivalent of the flexible hydraulic hoses being swapped, causing the same result on the left main landing gear with the number1 tire continuing to skid and reduced pressure on the number 2 wheel brake, which was not skidding, further diminishing the braking action on the left main landing gear.

Antiskid Autobrake Control Unit (AACU) - Examination
The systems group had American Airlines remove the AACU from the airplane and ship it to Crane Aerospace at their facilities in Burbank, California for examination and testing. According to Crane Aerospace, the AACU does not have a continuous monitor that would be able to detect crossed hydraulic hoses or wheel speed transducer wiring that is swapped. The AACU had been installed onto the airplane on 02/04/2024 during the carbon brake modification.

The AACU was examined by the airplane systems group to confirm its operation and to examine the contents of the unit’s non-volatile memory. Although one (Transducer) fault was recorded in the AACU’s memory, the fault information did not include a time for the fault. Although the fault was recorded in the most recently used memory block of data, the time of the fault could not be determined; the fault could have been written during the flight or landing rollout.

The AACU also passed its acceptance test protocol (ATP) on the second attempt after the external cables were re-secured. The initial faults were attributed to external communication issues, cleared by the cable adjustment.

There was no evidence that the AACU did not operate normally during the accident flight and landing.

Hydraulic Brake Valve - Examination
The systems group met at the Woodward facility in Santa Clarita, California to test the autobrake assembly valve module removed from the airplane:

The valve module was transferred from the Woodward secured storage area to their Servo Assembly and Test Area's electrical test station, where a standard Woodward electrical evaluation was conducted. To prevent potential damage, the dielectric portion of the test was intentionally omitted. The completed tests included the Servovalve Insulation Resistance Test, Solenoid Insulation Resistance Test, and Coil Resistance Test—all of which the valve module successfully passed.

The valve module was then installed onto the Woodward 60-12 hydraulic test stand. The Woodward standard evaluation testing was performed (excluding the initial proof testing). The valve module was tested to the Acceptance Test Procedure (ATP) as defined by the autobrake assembly valve module Component Maintenance Manual (CMM), 32-40-03 (revision dated October 24, 2019). The valve module passed all tested aspects of the ATP with no faults found, aside for a minor null shift of the steady state pressure gain. This deviation was deemed typical by the manufacturer and non-impactful. There was no evidence that the autobrake hydraulic valve module did not operate normally during the accident flight and landing.

Woodward indicated that they had no record that the module had been returned to Woodward after 2012. Paperwork shipped with the module indicated that the unit had been inspected and tested by Lufthansa Technik in January 2024. ADDITIONAL INFORMATIONOn February 6, 2024, American Airlines completed a modification on the incident airplane to replace the existing MLG steel brakes with carbon brakes and associated wheel assemblies.

The work performed on the incident airplane was done in accordance with Engineering Order (EO) 3222J004, and associated cards 3222J004-001 and 3222J004-002, all dated November 2, 2022, based on Boeing Service Bulletin SB 737-32-1429 Revision 4. The modification is applicable to 737-800 series airplanes equipped with Goodrich or Honeywell steel brakes and wheels.

The EO and associated cards provided detailed maintenance instructions with sign-off blocks and incorporated the technical content of the Boeing Service Bulletin, enhanced with additional information to ensure compliance with American Airlines’ continuous airworthiness maintenance program. EO 3222J004 applied to all 303 B737-800 airplanes in American Airline’s fleet. Prior to the modification of the incident airplane, a total of 53 airplanes had already been modified, leaving 249 airplanes still requiring modification as of April 23, 2025.

As part of the conversion to carbon brakes, four flow limiters were installed, replacing the existing bulkhead unions located 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 (see figure 8). Installation required temporary disconnection of each flexible hydraulic hose, removal of the bulkhead union, installation of the flow limiter, and reconnection of the flexible hydraulic hose.


Figure 8. Diagram showing the flow limiter installation (Source: Boeing. Image Copyright © Boeing. Reproduced with permission.)

The tests and operational checks called out by the SB 737-32-1429 revision 4 and EO revision date Nov 2, 2022, did not verify that each brake hydraulic line was correctly routed.

The EO was complied with on the incident airplane during a special maintenance visit in Tulsa, Oklahoma between February 4 and February 6, 2024. This work was completed at an American Airlines maintenance facility certificate number AALR025A. This facility is operated under 14 CFR 145.

Based upon the signoffs on the completed EO for the incident airplane, a total of six mechanics signed off work steps and all six provided written statements. In each written statement a synopsis of the work performed by the individual mechanic was provided. The mechanics all reported having at least 30 years’ experience with American Airlines. The mechanic who reconnected the flexible hydraulic hoses, detailed in his written statement “I disconnected the hoses and capped them and separated them to the individual sides so not to cross them, during the installation. Today I was notified that hoses were crossed, at the time I believed I installed them correctly.”

A review of the airplane’s maintenance logs for the 30-day period preceding the incident was conducted. No anomalies or discrepancies related to the anti-skid system or transducer wiring associated with tire flat spotting were noted in the records.

The NTSB investigated three incidents that involved cross-wiring of antiskid system components in transport-category airplanes. The first incident occurred on January 19, 1995, when an Air South Boeing 737-200, N4515W, overran the runway at Hartsfield Atlanta International Airport, Atlanta, Georgia. None of the airplane occupants were injured, and the airplane sustained minor damage. The NTSB’s investigation found that, during the landing rollout, the first officer felt the antiskid system releasing, so the captain took control of the airplane. The captain felt initial braking action, but all braking action was then lost. Postaccident examination of the airplane found that, among other brake system anomalies, the wiring to the left inboard and left outboard wheel speed transducers was crossed. This and the other brake system anomalies were not detected during antiskid system testing that was performed about 2 weeks before the incident. The NTSB determined that the probable cause of this incident was maintenance personnel’s “inadequate inspection of the aircraft…in that they did not properly diagnose discrepancies in the antiskid braking system.”

The second incident occurred on October 9, 2007, when a United Airlines Airbus A320, N431UA, departed the runway and impacted runway lighting during landing at O’Hare International Airport, Chicago, Illinois. All 127 airplane occupants were uninjured except for 1 flight attendant and 1 passenger, who sustained minor injuries. The airplane sustained minor damage. The NTSB’s investigation found that, during the landing, the left MLG inboard wheel went to a high braking level and that the left MLG outboard wheel did not apply braking. The NTSB also found that the wiring for the airplane’s left MLG inboard and outboard antiskid tachometer (Airbus’ nomenclature for a wheel speed transducer) was reversed during scheduled maintenance. The NTSB determined that the probable cause of the incident was “the misrouted and reversed antiskid wiring by vendor maintenance personnel leading to the runway excursion.” Contributing factors to the incident included the operator’s maintenance procedures for the dual tachometer replacement, which were unclear to the vendor’s maintenance personnel.

The third incident occurred on February 25, 2008, when a United Airlines Airbus A320, N442UA, departed the right side of the runway during landing at Jackson Hole Airport, Jackson, Wyoming. All 125 airplane occupants were uninjured except for 1 passenger, who received minor injuries during the evacuation. The airplane sustained minor damage. The NTSB’s investigation of this incident found that the inboard and outboard tachometer wires on the left MLG were cross-connected. As a result, when the inboard tire began to skid, there was a reduction in the hydraulic pressure to the outboard brake instead of the inboard brake, causing a loss of braking on the outboard wheel. Also, the hydraulic pressure to the inboard brake increased, causing it to fully skid and fail. As a result, when the captain applied full manual braking, the braking action on the left MLG was almost fully lost while the right MLG braking remained normal, resulting in the airplane veering to the right and exiting the runway. The NTSB determined that the probable cause of this incident was “the loss of braking action on the left main landing gear due to the cross connection of the wheel speed tachometer wires that was caused by inadequate maintenance performed on the airplane during the installation of the main landing gear.” FLIGHT RECORDERSThe airplane involved in the event was manufactured in 2009 and operated under regulations requiring it to be equipped with a FDR that records at least 91 parameters, as specified in 14 CFR 121.344(f). The National Transportation Safety Board (NTSB) Vehicle Recorder Division received an electronic file containing FDR data from N991AN. The recording included approximately 10 hours of data, with time measured in subframe reference numbers (SRNs), where each SRN represents one second.

The event flight was the final segment in the recording and lasted approximately 3 hours and 25 minutes. A review of the data was completed, and a plot of the braking parameters is presented in figure 6. The data showed that at touchdown, the “Auto BRK Applied” parameter briefly (for less than two seconds) changed from “No Auto Brk” to “Auto BrK” before reverting to “No Auto Brk” for the remainder of the landing rollout. During this time, both left and right brake pressures increased to their maximum levels.


Figure 6. Plot of the FDR data from the event landing.

The airplane was equipped with a Honeywell model 6022 CVR. The CVR recorded a minimum of 120 minutes of digital audio stored on solid state memory modules. Specifically, it contains a 2-channel recording of the last 120 minutes of operation and separately contains 4-channel recording of the last 30 minutes of operation. The 120-minute portion of the recording is comprised of one channel that combines three audio panel sources and a second channel that contains the cockpit area microphone (CAM) source. The 30-minute portion of the recording contains 4 channels of audio information: one channel for each flight crew, one channel for a cockpit observer, and one channel for the CAM. The CVR was removed from the airplane and sent to the NTSB’s Vehicle Recorder Laboratory in Washington, DC, for analysis. The CVR arrived at the laboratory with no visible damage, and the audio information was extracted from the recorder normally, without difficulty. The two-channel download had good to excellent quality audio, and a summary was created of the audio associated with approximately the last hour of operation of the accident flight.

Certified ADS-B data, which records more accurate latitude and longitude data than the FDR, was provided to the NTSB by the FAA (see figure 7).  Analysis of the ADS-B and FDR data shows that the left and right MLG touched down about 1,500 feet from the runway threshold and the nose came down just after 2,000 feet. Groundspeed on the initial touchdown was 150 knots and 140 knots for the nose.  The airplane groundspeed when it departed the end of the runway surface was 30 kts.

Figure 7. Plot of the ADS-B & FDR data from the event landing.