Six halon fire extinguishers were used to combat an overheating aft galley oven on a KLM Boeing 777-200ER, but investigators state that this was ineffective – because there was no fire – and actually exacerbated the situation by generating smoke and toxins.
Investigators also found that electrical power to the oven remained connected during, and after, the incident. Cabin crew incorrectly assumed power had been disconnected, and this led the cockpit crew to believe there was no need to switch off the utility bus.
The oven overheated about 1h 20min into an Amsterdam-Johannesburg flight on 9 February last year, and the crew of the 777 (PH-BQO) opted to return to the departure airport.
Investigators from the Dutch Safety Board found the oven had been loaded with a meal-tray carrier with a rear face that was perforated but largely closed. This significantly extended the oven’s heating time and disturbed the airflow within it.
The resulting higher temperature behind the oven affected its control circuits in such a way that the heating elements were commanded to remain on, causing the temperature to rise further until the oven began emitting smoke.
According to the inquiry a safety device had been damaged sometime prior to the incident, but had not been replaced, and it was unable to disconnect power to the oven.
Although no flames were observed, the amount of smoke led the cabin crew to believe a fire was present, and six halon extinguishers were discharged into the oven. A seventh had also been prepared for use. The aircraft was equipped with 13.
But while halon is effective against flames, it does not provide a cooling effect. The halon gas was instead exposed to oven temperatures far above its decomposition threshold of 482°C, whereupon halon breaks down into such toxic by-products as hydrogen bromide, hydrogen chloride, hydrogen fluoride, free halogens, and small amounts of carbonyl halides. Halon can also deplete oxygen levels.
Twelve passengers and three cabin crew members experienced breathing difficulties or dizziness.
Prolonged overheating of the oven ultimately caused the heating elements to fail, and the temperature dropped significantly, enough for the control circuits to start function correctly and command the heating elements ‘off’ – thereby stopping the overheat event.
“All heat damage remained limited to the inside of the oven and did not cause damage to the surrounding area of the oven,” the inquiry states. The damage included a melted aluminium fan.
But the safety board points out that electrical power to the failed oven remained connected during the incident and the rest of the flight.
“At no point was the correct position of the galley area main power button verified by checking the powering of other galley electrical equipment,” it states. “The circuit-breaker of the oven was not pulled by the cabin crew nor were they directed to do so by the [oven smoke or fire] procedure.”
Investigators found that the cabin crews’ training stressed the importance of not manipulating circuit-breakers, which led to a general understanding that pulling a circuit breaker was not permitted, even in the presence of fire or smoke from electrical equipment.
