An equally interesting comment a long ways back that got little discussion also seems relevant. A question about accessibility and security of the MEC (Mechanical Equipment Center) on the 777.
A little research turns up some very interesting info.
First lets look at
the MEC location - below and immediately behind the flight deck seems a perfect spot to have catastrophic consequences.
A search seems to show incidents which exhibited characteristics that well fit some of the scenarios discussed here at PPRuNe. Scenarios that address the seemingly unlikely event of loss of comms, loss of telemetry (ACARS), loss of transponder etc.
On 26 February 2007, a Boeing 777-222 operated by United Airlines, after pushback from the stand at London Heathrow Airport, experienced internal failure of an electrical component which subsequently led to under-floor fire. The aircraft returned to a stand where was attended by the Airfield Fire Service and the passengers were evacuated.
After engines start, about the time the engine driven Integrated Drive Generators (IDGs) would normally come on-line, the flight deck instrument displays flickered, the crew heard an abnormal noise and an EICAS message, amongst other related messages, indicated that the Right Main AC Bus had failed.
Some 40 seconds after the engines had stabilised at ground idle, the smoke detector in the Main Equipment Centre (MEC), located beneath the flight deck and forward vestibule, detected smoke. About two and a half minutes after the electrical failure, the crew became aware of a faint smell of electrical burning on the flight deck, following which the right engine was shut down. The crew were alerted by the ground handling crew that smoke was seen to be coming from the MEC vent and, a couple of minutes later, ATC also advised that smoke had been seen coming from the aircraft.
The
investigation found:
The accident occurred during engine start after pushback from the stand. After the right generator came online an electrical failure occurred in the right main bus. The failure resulted in severe internal arcing and short circuits inside the two main power cont actors of the right main bus. The heat generated during the failure resulted in the contactor casings becoming compromised, causing molten metal droplets to fall down onto the insulation blankets below. The insulation blankets ignited and a fire spread underneath a floor panel to the opposite electrical panel (P205) (see MEC compartment drawing above), causing heat and fire damage to structure, cooling ducts and wiring.
The Report identifies the following causal factors:
*An internal failure of the Right Generator Circuit Breaker or Right Bus Tie Breaker contactor on the P200 power panel inside the Main Equipment Centre resulted in severe internal arcing and short-circuits which melted the contactor casings. The root cause of contactor failure could not be determined.
*The open base of the P200 power panel allowed molten metal droplets from the failed contactors to drop down onto the insulation blankets and ignite them.
*The aircraft’s electrical protection system was not designed to detect and rapidly remove power from a contactor suffering from severe internal arcing and short-circuits.
*The contactors had internal design features that probably contributed to the uncontained failures.
It does not in my opinion take an engineer or accident investigator to see that had this incident occurred at cruise that the results could very well have matched what has been speculated here.
A failure (or even a breach) in the MEC causes fire. As the crew is unable to shut down and deplane, the incident escalates with smoke filling cabin/cockpit, followed quickly by electrical failure as we saw happen in this incident ...
As the fire continues uncontained, with the aircraft deaf and dumb - electrical; and as a result comms, transponder, ACARS etc all down, the fire breaches the aircraft hull - as it was well on its way to doing in this incident - followed by a rapid depressurization.
It seems entirely possible and plausible that in such instance, as the damage would be advanced before the crew even were aware - and with the likely rapid loss of electric bus and all related, that no Mayday would get out between discovery and loss of power.
I think it also at least plausible that the pilots MAY have been able to initiate an emergency procedure, but may have rapidly lost consciousness.
More experienced pilots please comment, but I believe the response to a rapid decompression at cruise is to push the nose over hard into a banked descent. I can imagine pilots initiating the emergency descent maneuver at same time they dial in a heading and lower altitude, then being overcome - by depressurization and as the descent continues, then by smoke.
The aircraft, even in distress, could likely fly a fairly long ways. With in the fairly long delay before anyone realized or were worried about the aircraft being missing, it could have traveled well out of the area.
If the MEC is accessible from the cabin it seems terrorism is at least possible - and seems something to look at. But regardless, this incident seems to show everything necessary to casue the scenario we have been discussing, is present in a previous incident with the aircraft.
References:
Short Version:
Air Accidents Investigation: S2/2007 Boeing 777-222, N786UA
Air Investigation Bulletin - Overview:
http://www.aaib.gov.uk/cms_resources...7%20N786UA.pdf
Air Investigation - Complete:
http://www.skybrary.aero/bookshelf/books/647.pdf