Here is a copy / paste repeat of what I posted in the past on this subject.
The first 2 drawings come from the Boeing web site and can be found in the
AIRPLANE RESCUE AND FIRE FIGHTING INFORMATION
767 SERIES FLAMMABLE MATERIAL LOCATIONS
This drawing gives the general location of the "CREW OXYGEN BOTTLE IN RIGHT SIDE, E & E COMPARTMENT"
E & E stands for electrical and equipment
The second drawing shows an X-ray view of the E&E compartment. Although it does not show the O2 tank,
it does give us a better idea of the size and location of the compartment in relation to the rest of the plane.
Note that the aft bulkhead of the nose wheel bay is also the fwd wall of the E&E compartment.
This is a photo of the E & E compartment. The O2 tank would be under the floor on the left hand side of the picture.
You can also see the external access door used to service the tank and battery.
An emergency O2 tank like the one on UA 175. It is pressurised to 1,800 psi with pure oxygen, not compressed air like scuba tanks.
The nose wheel undercarriage was located directly in front of of the E&E room and would have been the first major part of the plane to impact. When
GPers say hollow aluminum can, they are neglecting things like this. On impact this undercarriage would decelerate wile the rest of the plane
continued forward. So the under carriage smashed through the E&E room as well as knocking a large hole in the building. This is why the flash happens
outside of the wall and gives the illusion of happening before impact.
A photo of N612 UA in its original colors. The O2 tank would be within the red circle.
An O2 tank explosion on a Qantas Boeing 747.
This composite photo was made by Achimspok it's a boeing 767 photo resized and over laid on a frame from the Spiegel TV video to give a clear view of
the location of the flash.
This is the highest quality, best view video. The flash happens after the nose impacts the building.
The oxygen tank is the most likely explanation for the flash.