US investigators have determined that a Boeing 737 Max 9 aborted take-off with a damaging wheel fire, the result of its brakes overheating as the crew attempted to burn excess fuel while taxiing.
Denver is a high-altitude airport and the air temperature was high, which left the heavily-laden United Airlines aircraft with little weight margin for departure.
When the crew of the aircraft – bound for Boston on 30 September 2023 – arrived on board, they reviewed performance data for take-off on runway 16R, the longest available.
But they discovered that the initial flightplan, which resulted in a maximum take-off weight of 174,500lb (79.1t), had been based on an air temperature of 29°C, whereas the temperature had since risen to 31°C.
Although the crew tried various runway and engine bleed scenarios, the US National Transportation Safety Board says the aircraft, which had a weight of 172,800lb at the gate, needed to be below 171,700lb.
This resulted in three revisions to the flightplan. The revisions involved removing of 1,000lb of cargo and eight passengers, and also included an extended taxi route to burn off 1,000lb of excess fuel.
The fuel-burn would enable the aircraft to depart without having to offload more passengers.
According to the inquiry, the crew conducted a “long, slow taxi” to runway 16R using higher-than-normal power balanced by additional braking in order to burn off the fuel.
After the 737 reached the runway it held for a further 10-15min, with its parking-brake set, in order to reduce the fuel weight further.
“The pilots reported that they never had any indication that the brakes were getting hot,” says the inquiry, pointing out that the cockpit had no brake-temperature monitor.
Once the jet’s weight fell below the maximum limit, the flight was cleared for take-off. But the captain, who was flying, noticed quickly that the aircraft was not accelerating normally.
“The tower then observed and notified the flight crew that there was smoke and fire on the right side of the airplane,” says the inquiry.
There was no cockpit indication of fire to the pilots. The crew aborted the take-off and firefighting personnel responded, releasing fire retardant on both main landing-gear assemblies.
Passengers were not immediately evacuated but subsequently disembarked via stairs. None of those on board, comprising 167 passengers and six crew members, was injured.
Examination of the aircraft (N37560) found it had suffered substantial damage. Both left-hand main-gear tyres had deflated while both right-hand tyres had separated from the wheel, with fragments striking the airframe in several areas.
Loss of the tyres and the grinding-down of the right-hand wheels resulted in the right engine nacelle scraping the runway. Several fairing panels were heat-damaged, tyre debris was embedded in part of the engine structure, and the horizontal stabiliser was also damaged.
