Crew’s use of halon extinguishers on overheating oven ineffective: investigators

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.<br/>