United Airlines Captain Felt Boeing 737 MAX Brakes Shaking Violently Before Plane Skidded Off Runway

The National Transportation Safety Board (NTSB) has released its preliminary report on an incident when a United Airlines Boeing 737 MAX 8 slid off the runway while landing at its destination airport on March 8, 2024. No injuries were reported following the incident.

Reduced braking deceleration

The Boeing 737 MAX 8, registered as N27290, was operating United Airlines flight UA2477 between Memphis International Airport (MEM) and Houston George Bush Intercontinental Airport (IAH) when the incident happened.

According to the report, there were 166 people onboard the aircraft, namely six crew members and 160 passengers. The captain of the flight, who was the pilot flying (PF), has been working at United Airlines since April 1987, having 15,191 flight hours (FH) on the Boeing 737, with 9,607 FH as the captain. Meanwhile, the first officer, who was the pilot monitoring (PM), has been at the airline since October 2019, accumulating 1,252 FH on the Boeing 737.

The report stated that the flight crew recalled that the PF saw the runway codes via the automatic terminal information system (ATIS) being 3/3/3 for runway 26L and runway 26R, while runway 27 had codes 5/5/5. However, the NTSB pointed out that the actual runway 27 surface conditions were rated as 3/3/3, indicating a slippery when wet runway, resulting in “braking deceleration is noticeably reduced for the wheel braking effort applied or directional control is noticeably reduced.”

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Changing landing runways

When the flight crew checked in with the air traffic control (ATC) at Houston Approach Control, they were told to expect a landing on runway 26L. Subsequently, the PF asked the PM to request a landing on runway 27, which the ATC approved with an instrument landing system (ILS) approach.

After contacting Houston Tower, at around 12,000 feet (3,657 meters) mean sea level (MSL), the captain asked the first officer to request to roll to the end of the runway, which the controller approved, requesting the flight crew to “keep [their] speed up.”

The captain changed the auto brake setting from 2 to 1, commanding a reduced deceleration rate and following an instrument meteorological conditions (IMC) approach, the Boeing 737 MAX broke out of the cloud cover at between 800 ft (243.8 m) and 1,000 ft (304.8 m) MSL. The flight crew told the NTSB that the visibility below the clouds was good, with the PF recalling that the runway appeared dry, while the PM said that the runway seemed to be wet.

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Initially uneventful touchdown

In interviews with the NTSB, the flight crew told the investigators that the initial touchdown was uneventful and at an appropriate speed. The speed brakes and thrust reversers deployed as intended, while the PF retracted the speed brakes shortly after the aircraft touched down on runway 27, disabling the auto brakes.

The captain recalled that he did not slow down the aircraft too much as the runway appeared dry, he wanted to expedite time on the runway, and he preferred to decelerate gradually to ensure passenger comfort.

At about 6,000 ft (1,828 m) from the end of the runway, the captain applied the brakes manually, noting that the deceleration was subpar. NTSB stated that Digital Flight Data Recorder (DFDR) and automatic dependent surveillance-broadcast (ADS-B) data showed that after the flight crew disabled the auto brakes, manual braking did not happen until around 4,000 ft (1,219 m) from the end of the runway.

Photo: United Airlines

The captain told the NTSB that he remembered that the runway awareness and advisory system (RAAS) indicated that there was 1,000 ft (304.8 m) of runway left, and the PF responded with additional pressure to the brakes. As the United Airlines aircraft approached the runway’s end, the captain attempted to turn onto the adjacent taxiway with the steering tiller and rudder pedals while still aggressively applying pressure to the brakes.

Substantial damage

As the aircraft was turning, the PF felt that the fuselage and the rudder and brake pedals began shaking violently, resulting in the captain letting off the brakes and reapplying pressure to them again, with the aircraft starting to shake again. Subsequently, the Boeing 737 MAX slid off the runway, and the left main landing gear (MLG) tires and nose wheel tires entered the grass before the aircraft stopped.

In a post-accident examination of the accident site, the NTSB determined that the left MLG tires had impacted a manhole, which is designed as an electrical junction box for lights and utilities at the airport. Because of the impact, the left MLG separated from the aircraft at its fuse pins near the rear spar, as it was designed to do to prevent more damage.

The investigators said that the United Airlines stopped with on its left engine nacelle, left winglet, and aft fuselage, receiving substantial damage to the left wing and aft fuselage. It remains to be seen whether the carrier will attempt to repair the aircraft, considering that Boeing is already struggling to deliver new 737 MAX aircraft to the airline on time.

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Airline’s auto brake recommended settings

According to the NTSB, photos and surveillance video during and after the incident showed wet runway and taxiway conditions. To note, the captain of the flight told the investigators that he assumed the runway was dry, with the PF setting the auto brake to 1.

Meanwhile, the airline’s procedures mandate the use maximum reverse thrust as soon as possible after the main wheel touches down, without waiting for the nose wheel touchdown. Furthermore, auto brake stopping procedures mandate that pilots determine the required distance to the desired runway exit and select an auto brake setting of 3 or MAX. United Airlines mandates that pilots be conservative and use the MAX setting if there is doubt regarding the runway conditions or available braking. The 1 setting is the lowest auto brake setting available to Boeing 737 pilots.

Photo: NTSB

Preliminary data review showed at touchdown, the speed brake lever position and auto brake parameters were full up and on, respectively, for about five seconds before going down and off for the remainder of the landing rollout. Furthermore, the thrust reversers were deployed for both engines for about 27 and 26 seconds, respectively, at idle reverse, with the left and right brake pressure increasing to their maximum pressure consistent with aggressive manual braking.

Lastly, ADS-B and DFDR data showed that the aircraft touched down about 1,000 ft (304.8 m) from the runway threshold with a ground speed of around 158 knots. When the Boeing 737 MAX had around 500 ft (152.4 m) of runway remaining, its ground speed was 57 kts, with the PF executing a right turn onto the taxiway at around 39 kts. The aircraft left the paved surface at about 22 kts.

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