Two B787 pilots and a maintenance technician reported all 4 crew oxygen masks were leaking. During the maintenance process a flashover occurred and the aircraft was evacuated.
Synopsis
Two B787 pilots and a maintenance technician reported all 4 crew oxygen masks were leaking. During the maintenance process a flashover occurred and the aircraft was evacuated.
Narrative
After making the crew bunks during pre-flight I returned to the flight deck and sat in the Relief Pilot seat. The Captain advised me Maintenance had been called due to a leaking O2 mask. After Maintenance arrived it was determined that all four crew O2 masks were leaking oxygen and all four were then disconnected from their respective supply lines.It appeared the Maintenance personnel was trying to trouble shoot the problem and was attempting to reattach the Relief Pilot O2 mask which caused more O2 to leak onto the flight deck and was ignited by an unknown spark causing an explosive event containing a loud pop; flash of light and heat; and finally a cloud of smoke. The occupants on the flight deck being the Captain; First Officer; 2 Maintenance personnel; and myself quickly exited the flight deck and gathered around the L1 door. The Captain directed the Purser to initiate an urgent de-planing.All passengers safely exited the aircraft without incident.Mitigation suggestions would be to avoid working on O2 systems while pressurized if not needed and not in the presence of unnecessary personnel.
Second reporter narrative
I was the Relief Pilot on Flight ABC ZZZ-ZZZZ After I had finished the walk-around; I went up to the flight deck to inform the Captain; that the exterior preflight inspection was complete. The Captain mentioned his oxygen mask was continuously leaking oxygen and that our maintenance personnel was looking for a replacement mask. When Maintenance came back on board with a new mask; I left the flight deck so that they could have the space needed in order to make all the necessary repairs. As I stood by the L1 door; I heard multiple whoosh sounds coming from the masks; as they were being removed from the storage box inside the oxygen mask panel. Shortly after; I heard a loud pop; similar to an inflated plastic bag that had just burst. The mechanics and the rest of the pilots exited the flight deck. The Captain advised the purser to make a PA for all passengers to deplane. While the passengers were deplaning; I was told that my oxygen mask had been arced.Cause: Not sure what the corrective action would be since this was a maintenance function. I do not know what their procedures are for removal of multiple masks from the storage box at the same time.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.