Bits & bobs
Pictures of the wreck seem to imply the following:
- one engine was fully shut down (no blade damage after ground impact, so can't have been spinning, or was just windmilling).
- other engine wasn't operating under power, and may have been spinning down (blades damaged/tips shed, but no catastrophic failure so can't have been doing significant rpm).
- no bird strikes as no visible damage to wings, nose or fans.
- APU exhaust vent open, so APU was starting or started, maybe under manual control though startup is automatic after dual engine fail.
- RAT appeared to be deployed, though this might possibly have been due to ground impact. RAT deploys automatically after electrical or hydraulic failures, or on manual control.
- apparently there were still 'significant' amounts of fuel on board as there was leakage post-impact, though the tanks never actually run fully dry even if there is no usable fuel left. 'Empty' wing tanks don't necessarily much as the centre tank reserve is the last bit to go. Also being an 'ER' variant there is extra fuel storage within the fuselage.
Fuel contamination is a possibility, water for example could freeze out of the fuel, and thaw during descent when it would be much more of a problem due to the lower tank levels. This could certainly explain a dual engine failure.
Another possibility is a catastrophic electrical failure, as per previous 777 electrical fires. Though there was no sign of fire on the ground. And each engine is independently controlled. And has failover control so any likely failure certainly shouldn't take out both engines at the same time.
Another possibility is a software glitch. These aren't exactly unknown, and in the past have been due to all kinds of unusual interactions. Sometimes they're just a mystery!
In any case they did well to get the plane onto the airport. Once the aircraft is configured for landing and the gear is down you can't glide particularly well due to the drag, so you have to use power to maintain the descent on the glideslope, and maintain the correct approach speed.
At the stage of descent they were at they didn't have enough airspeed to make the distance, nor much height to trade for airspeed. As energy management goes it was pretty marginal, and as a problem to manage it wasn't exactly easy given it must have been all of 40 seconds from beginning to end.
At the end they seem to have hit aerodynamic stall speed i.e. the plane was too slow to fly, and fell out of the sky. Luckily it had gone far enough, and was low enough that this didn't matter too much, though it didn't do the landing gear any favours.
I would guess a few people will be hitting the sims & trying this scenario out for themselves, just to see how they get on. I would also guess some of them won't make it far enough.
Re: F104 & other fast jets. The F104 didn't exactly have a great reputation when it was working properly, I doubt many people tried to test the glide characteristics in un-powered flight. At least not many who survived to talk about it, or if they did it was because they left the aircraft at the first opportunity.
Most current fast jets don't exactly have great flight characteristics if the engine(s) fail, for one thing they don't tend to have the same level of fallback in the electrics & hydraulics so once the engines go down, taking the generators and pumps with them, it's just you and the battery warning horn riding a plane which suddenly flies like a brick and which you can't control. So you do the sensible thing & pull the handle to get out.
On a civil aircraft this isn't an available option so you just deal with the situation, luckily the plane is designed to help you do this so the outcome is generally positive.