I shall put forward my theory with my humble tech knowledge. Of
course each 'engineer' can have their theories, this is only a
speculation.
The fire may have started, based on previous cases,
electronics compartment below staircase or
the air conditioning unit.
The depot fire at Hong Kong in 2008 originated from an Enviro
500 electronics bay, there was a short circuit and fire spread to
other buses. Electronics fire can happen even after the engine has
cooled down, as capacitors wires are still carrying charge long
after the power supply is cut.
Air conditioning is also a place with electric charge, although
such charge may be insufficient to start a large fire on its own,
but its near to the engine, and with any little hot engine oil left
near the engine, it can pretty much ignite.
As a gauge and comparison, flash point for diesel is 51.6
degrees C, gasoline -42 degrees C, motor oil 215 to 251 degrees C.
This means that gasoline is the most inflammable, followed by
diesel and then motor oil. Different grades of oil may vary their
flash point.
This means that a same spark, landing on/near the engine with
only motor oil and little/no diesel around, would by theory not be
as dangerous as the same spark landing on/near the diesel tanks.
So, the electronics bay or air conditioner theory? Up to your own
judgement.
