Hi Bumblebee,
Thanks for your reply.
I don't see the problem in "backpressuring the duct on the failed side"...
On the 737, in cases like this, the isolation valve is always operated
indirectly, by turning one pack off.
Very simple, thus no asymmetric wing deice.
Regarding your third point, I still don't see the problem...
A bleed trips off because of excessive pressure or temperature.
Once it is closed, if you make the isolation valve open, the duct will be pressurized by the other bleed, as in the engine failed case.
What would be the harm?
Thinking about that today inflight, I concluded Boeing must have balanced the risk of losing pressurization against the risk of really needing wing antiice.
Technically speaking, one bleed
is capable of supplying one pack
and wing antiice.
But, as you said, in the engine failed scenario, you are probably already descending to lower altitudes, thus better to allow the use of wing antiice since the loss of the second bleed would not be so traumatic.
In the bleed trip off case, you have two engines, you're flying at high altitudes, and the risk of losing the second bleed (and pressurization) is much heavier than having to use wing antiice (quite unlikely).
Am I wrong?
However, the NNC is too simplistic and badly designed: imagine you lost one bleed while in the sequence for approach at Munich during a snowy winter.
How can you "avoid icing conditions"?????
Impossible!
In this case, I would go against the NNC and turn one pack off, and use wing antiicing if needed!
Keep your opinions coming!
LEM