Simon, wouldn't such a fire in the MEC (Main Equipment Center) of a 777 take out the ACARS satcom link as well; thereby disabling the "handshake" feature?
That is not absolutely so. We do not know the nature, exact location, or cause of any fire nor exactly what was involved. Just because you can't imagine it, does not mean it isn't so. This is clearly an exceptional event with a complex combination of factors whichever explanation you subscribe to. No simple single cause can replicate all the known features of this disappearance and the cascading effect of electrical failure, electrical fire, explosive decompression comes closest to explaining all the imponderables.
For example, the fifth prototype Boeing 787 test aircraft suffered a generator fire on approach to land at Laredo on 10 November 10, 2010 which knocked out cockpit displays and disabled the auto-throttle. However the crew isolated the fault and managed to land.
En-route to Northern Sudan, on 24 August 2010, an Airbus A321-200 operated by British Midland suffered loss of cockpit displays and un-commanded turns which vanished with de-selection of No.1 generator.
A McDonnall Douglas DC-9, AirTran flight 913, on 8th August 2000, experienced inflight arcing of relay bus cables though the flight continued after the fault was isolated.
The point is electrical faults can be isolated and aircraft continue to fly.
What about the the Flight management System Computers (FMS), the Air Data Computers (ADC's), the Autoflight Director Computers (AFDC), the Inertial reference system (IRS) boxes and the FBW Flight Control computers? They are all in that area beneath the forward deck and they all supply required inputs to the autopilots for them to function.
No that is a false assumption. In reality a fire could breach the hull in just 20 seconds and rapid explosive decompression could knock out the fire before vital electrical components were destroyed. In particular a ruptured oxygen line could cause a highly concentrated blow torch effect.
I am surprised that someone like you who seems well informed about the multiple redundancy features of the Boeing 777, for example dual ADIRUs, that you of all people would advocate the fragility of this aircraft?
In principle pilots could quite effectively manage to isolate the electrical problem, turn the aircraft around and only then discover a blaze. Insulation has been known to catch fire in airliner avionics bays from electrical arcing and as such insulation can keep burning after isolation of the electrical fault has removed the ignition source. Once depressurised the fire would snuff out.
Your fire needs to avoid all these boxes for the AP to work.
Processor units in the underfloor avionics bay of a Boeing 777 are widely spaced to ensure that a fire in one processor box does not affect others.
It is implicit in the scenario which I suggested that pilots first isolated an electrical fault and turned the aircraft about before any fire broke out. This is the state of the avionics bay in Egyptair 667 after the cockpit fire was extinguished. Whilst much of the console interface was destroyed or charred, the actual processors were not:
In the oxygen fed fire of Egyptair 667, the hose down by the right leg of the co-pilot had an anti-kink spring which acted like an induction coil and heated to over 600 degrees F. This melted and ruptured the hose.
That fire took just 20 seconds to breach the hull from when the co-pilot noticed smoke beside his leg, jumped up and discharged a fire extinguisher.
Granted that these anti kink hoses were all due to be replaced within 18 months of a Service Bulletin issued in April 2012, but then Malaysian Airlines was also required to remove Black Boxes for servicing and battery replacement by the original manufacturer and that did not happen either.
Electrically conductive Oxygen hoses are not the only possibility... just one illustration of what might have happened.
That it did that and also took out the pilots and all the comms gear bar the ACARS Satcom is difficult to believe.
No, not that it took out all the comms gear... The pilots were following training, aviate, navigate and before they got to communicate were overcome by fire and explosive decompression.
There is no inference that communications equipment was disabled, rather the possibility that pilots died fighting an emergency before they had any chance to communicate.
The ADS-C system sent information via a Mode S transponder on MH370. You cannot say just because Kuala Lumpur lost contact with that transponder at 17:21 UTC that the transponder was disabled.
IGARI is just south of Ho Chi Minh FIR, therefore the pilots had to Log In by entering a new code. The effect of entering a new code is also to log off with Kuala Lumpur.
It is also worth noting that ADS-C information continued to be fed to SITA via ground relay stations in Vietnam after 17:21 UTC which suggests MH370 did not descend or turn west.