N991AN

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 whee...