Cargo of used lithium cells, and one in the middle is almost at failure. At take off it is fine, but as the hold depressurises to cabin cruise pressure it expends slightly, then this triggers the cell to short internally. It heats up from the stored energy, and heats up the other used cells in the same pack. The cell protection will not prevent this, as the cell is failing internally. The failing cell eventually bursts and the vapour released catches fire, fuelled by the heat and the ambient air and the hot cells around it. This then triggers the other cells, and they also start to vent and burn the plastic housings. The smoke released, along with the CO and other toxic chemicals in the smoke are circulated by the plane's AC system, and as the first vents are the cockpit the CO starts to build up. The pilots might then notice smoke in the cockpit, and start to initiate a RTO immediately, programming in the waypoints in reverse order to get there. By now the smoke probably has triggered a loud cargo hold smoke alarm, a fire warning alarm and a few caution warnings as the smouldering fire starts to burn cargo hold wiring. The pilots, thoroughly confused, wearing possibly masks, are trying to contain and triage this sudden unexpected pattern of events, not covered fully in the simulator, where an engine fire is the most common simulation. Thus one or the other mistypes a waypoint, and confirms without checking, setting the autopilot on a track to some far off waypoint via a great circle. Fire by now has burnt through control wiring for the cabin entertainment, and trips breakers, taking out at the same time one AC bus and one DC bus. Aircraft systems reconfigure to a failed state but keep running in degraded mode, and there are suddenly a whole load of new warnings on an already overloaded crew. fire burns through control wiring and half the cabin instrumentation goes dead, and kills the comms systems and radios. Fire burns through bottom of cargo hold, and vents cabin pressure at a rapid rate, dumping the smoke, but the crew have already lost consciousness from the sudden pressure loss popping the masks off them, and they fall down unconscious.
Plane flies to waypoint on autopilot, then turns and attempts to follow to the other waypoints programmed in by the pilots by error. In all this fuel management and trim keeps the aircraft in balance, transferring fuel as needed from tanks to keep the aircraft balanced at the optimum flight attitude for the altitude and speed. Plane flies till autopilot detects low fuel, and it gives strident warnings to the cabin, then eventually degrades to attitude hold as the engines run dry. Last engine runs out, and suddenly the AC busses disappear, and the DC battery is called on to supply all power. RAT is deployed automatically by the failed AC bus and DC bus monitor, and provides emergency power and hydraulics, and autopilot stops attitude hold and does minimum airspeed management in a long glide. No ground warning, no power for the radio altimeter, though the air data system would be triggering warnings about level.
Impact.
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