NTSB: melted thermal plugs caused tyre failure on American 737 Max takeoff at Denver

US investigators have tied a tyre and wheel failure on an American Airlines Boeing 737 Max 8 to thermal relief plugs that melted during a long ground delay in hot weather.

The incident happened on 26 July last year as the aircraft prepared to leave Denver for Miami. According to the National Transportation Safety Board, the jet sat through a long taxi and wait in 35°C heat as winds kept shifting. It first lined up for runway 08, but a tailwind ruled that out. The crew then taxied to 17L, found a tailwind there too, and moved on to 34L on the far side of the airport.

By the time it reached 34L, the aircraft had taxied roughly six nautical miles and waited a further 25 minutes. Its weight was reported to be near performance limits given the heat and Denver's elevation. The crew had told American Airlines they were approaching minimum fuel and duty-time limits.

During the eventual takeoff roll, at about 90-100kt, the crew heard a loud pop and the aircraft tilted abruptly to the left. Investigators said the crew sensed a significant loss of controllability, possibly from a landing-gear or multiple tyre failure, and rejected the takeoff. The jet stopped near the runway centreline. Passengers evacuated after signs of smoke and fire were observed.

None of the 169 passengers or six crew members were hurt. Examination afterward found that several thermal relief plugs on the left main wheels had melted, according to the inquiry, consistent with prolonged heat exposure during the taxi delays.