>
> As in supply air to the HVAC system; or bleed air as in wing deicing?
They isolate the ducting system so that there is a left wing, right wing,
and cross body area. If a duct were leaking, closing the isolation valves
would mean that the two engines associated with it would continue to feed
the leak (until you turned off their bleeds), whilst the system on the
other side would go back to feeding packs rather than the great outdoors.
>
>>The total difference in time taken from the top to the bottom would
>>probably not differ by more than a minute, irrespective of just what
>>method you chose. Mind you, there probably isn't a right answer, though
>>there will always be a wrong one.
>
> OK, I was going to say someone has surely calculated every option from
> every altitude and the emergency checklist reflects the best SOP.
There is a graph in the manuals, but it sure isn't something you're going
to look at on the day. Let's see..find checklist, get page, read
options...'oh, you want the descent today!'.
>
>>The gear can be used, but you would need to lose around 50 kias before
>>being able to select it. Overall effect would have been even longer before
>>starting the descent, with a very similar total time. That's why this
>>method isn't normally used.
>
> I was thinking along the same lines; I don't know how fast that much
> aircraft slows down when you chop power at that altitude, but it's surely
> not instantaneous. [Might be an interesting simulator session: try mixes
> of power, speed brakes, nose up & gear down.... all looking at minimum
> time to reach 10,000.]
There is so little difference, and there are so many permutations that it
would be a waste of sim time.
>
>>The aim is to have an autopilot engaged, and that's how it is practiced.
>>The reason is that if the crew oxygen is compromised, assuming you've
>>taken all of the initial steps, then the aircraft will rapidly fly itself
>>down to the safety height. That also reduces the workload immensely, as
>>one pilot can then monitor what the aircraft is doing, whilst the other
>>attempts to contain the situation.
>
> That was fully understandable. Do newer ["all-glass"] birds have any
> pre-programmed descent plan built into the FMS? [I think there's one
> for aborted takeoffs.]
No. And no there isn't for aborted takeoffs either. There is automatic
braking (RTO), and that's just an offshoot of the normal auto braking
system for landing.
>
> In fact, I could see a automagic decent upon cabin pressure loss.
> Assuming the crew is still conscious, they could override it as required;
> but if they were not, the idea is the same as above, that they would
> regain consciousness at the lower altitude. [There are likely major
> downsides to such, but I've not thought of them yet..]
I think I'd rather trust the driver rather than some programmer who is
safely asleep 15,000 kms away. You would need to allow for the fact that
not all events are the same. Perhaps you've just lost control of the
outflow valves, or at the other extreme you've had a United Airlines style
door failure. You can't always go to 10,000 feet. Safety heights are often
up as high as 20,000 ft. We were at the bottom of the ATC stack, but it's
just as likely that there would be an aircraft right underneath you. And,
of course, the failure has to have occurred in such a way that the
automatics both stay engaged, and aren't fed incorrect data.
