"At one stage the captain turns and says "Good job the cabin pressurisation is working, we dont want the oxygen masks to drop and panic the passengers"-why is the pressurisation working? from my (admittedly limited) knowledge, the pressurisation comes from the engines. The engines stop, so does the pressurisation. Does it run off the little drop-down turbine they use? "
The drop-down turbine (RAT) is only for providing hydraulics. It is activated above 80kts in the air with both engine N2's below 50%. However, the pressurisation system may have been able to maintain some sort of pressure in the cabin simply by slamming shut the outflow valve (in the absence of sufficient air coming from the aircon packs). It would be stale air, but I wouldn't be complaining. I couldn't say for sure if ram air, windmilling the engines would be able to provide sufficient pneumatic pressure to power the aircon "packs" to provide cabin air pressure. The pack valve may close at a specific level of (low) bleed air pressure like they do on certain other aircraft. Actually, I'm not even sure that the packs would operate at all without proper elec power(???).
"When the engines fail, so do the electrics (and therefore the transponder) , untill the drop-down generator thingie comes into action, and ATC naturally get a little upset that the aircraft drops off their screen."
No electrics from the RAT, unlike some aircraft. The power to operate a transponder may have come from a Hydraulic Motor Generator (some 767's have three of them, one on each hydraulic system). I'm guessing that the windmilling engines might provide sufficient power to operate 2 of the HMG's. There may have been a delay in the HMG's powering up, however: The hydraulic pressure coming from the EDP's would be less than normal and some HMG's are a little reluctant to start up, if at all, if not exercised regularly. Also, there is a sequencing of HMG's. An HMG may wait until others have failed before coming to life.
More speculation, I admit, but this scenario is certainly is great food for thought.
Regards.
Q.