While French investigators believe an intense oxygen-fuelled cockpit fire brought down an EgyptAir Airbus A320 eight years ago, they have yet to determine fully the reasons for the oxygen leak and pinpoint the source of ignition.
Investigation authority BEA has dismissed the Egyptian air accident commission’s official explanation – detonation of explosive material behind the cockpit – for the fatal 19 May 2016 accident.
But it acknowledges that, despite extensive tests, establishing the precise mechanism for the fire’s initiation has “not been possible”. It is unable to state whether the oxygen leak or the fire came first.
BEA says, however, that the speed at which the blaze developed “favours” a hypothesis of internal damage to the first officer’s oxygen system.
It has theorised that a component failure, either in the first officer’s oxygen mask storage box or the upstream distribution system, or perhaps the pressing of the ‘emergency’ knob on the mask itself, triggered a flow of oxygen.
While conducting studies into the ignition source, BEA learned about three recent in-flight oxygen-leak incidents on A320-family aircraft during 2022-23.
Initial analysis of these incidents, it says, suggests faulty installation or adjustment of the regulator which controls the cockpit-oxygen cylinder could cause overpressure of the oxygen system.
Overpressure could lead oxygen-mask regulators to fail and a high-pressure leak could occur in one, or all, of the masks.
BEA points out that the first sign of oxygen flow during the accident came via the first officer’s mask regulator.
“An initial hypothesis was that the mask knob had been pressed, but overpressure in the system could be another explanation,” it states.
BEA points out that the first officer’s oxygen-mask storage box was replaced 10 flights before the accident. It says a faulty regulator could be a “common explanation” for two “unusual” events – replacement of the storage box and the cockpit oxygen leak.
It states that further analysis would be needed to examine whether the incidents could generate new hypotheses for investigators.
BEA’s analysis also involved looking at several potential ignition sources, both internal and external to the mask storage box, to examine how the blaze began.
It considered the effect of a lit cigarette, as well the thermal runaway of a lithium battery-powered device such as a phone, placed in proximity to the storage box.
But the only external experiment which appeared to spark a rapidly-spreading oxygen-fed fire was the introduction of a lit cigarette into the storage box – a scenario BEA describes as “unlikely but possible”.
If the ignition source was internal to the box, BEA says, the oxygen hose could have been ruptured within a few seconds, causing a pressurised oxygen leak which then fed the fire.
BEA examined whether such internal ignition could have resulted from electrostatic discharge, heat from oxidation of grease, or a metal particle contaminant – introduced during maintenance – colliding under pressure with the oxygen distribution system.
Testing did not enable the investigators to reach conclusions on the source of the fire, and BEA says additional work would be needed to establish conditions for spontaneous grease combustion or particle impact ignition.
The Egyptian commission leading the inquiry concluded that detonation of a device – containing DNT- and TNT-based explosives – in the galley behind the cockpit started the fire, based on the apparent detection of explosives traces on casualties.
But BEA has described the factual accuracy of the commission’s report as “questionable”, and argues that noises captured by the cockpit-voice recorder are not consistent with such an explosion.
The cockpit-area microphone, it says, would normally have picked up the noise of an explosion in the galley, through structural transmission of the shock wave.
Yet the commission’s assumption of a TNT-based explosion is “never questioned”, BEA states, leading to an “unrealistic scenario” for the accident, one which is “incompatible” with warnings, failures and crew announcements.
“The report repeatedly mentions the presence of French experts concerning the discovery of TNT,” it adds.
“[We] would like to emphasise that the French experts acted as advisors and observers and had neither the role nor the possibility of validating any findings concerning the presence of explosives on human remains.”
