View Full Version : B737 classic Bleed Trip Off
If one asked you very simply if one bleed is certified to supply one Pack together with the wing antiice or not, what would you answer?
Yes?
So why in the Bleed Trip Off NNC we absolutely have to avoid icing conditions?
No?
So why in the Engine Failure NNC that is allowed?
IFixPlanes
9th Jul 2005, 08:10
Sorry, but i do not understand the Question. :uhoh:
Can you rewrite (describe) it with other words.
And BTW what is "NNC". ( i am a greesemonkey not a pilot) ;)
Ingo
Bumblebee
9th Jul 2005, 08:32
NNC - Non Normal Checklist
LEM..
my thoughts...
Firstly - when you're one engine INOP your are already on the way to landing at the nearest suitable - and if you need wing anti-icing.....you need it...you are probably not so bothered about back pressuring the duct, pack and associated stuff on the failed side.
If you have a bleed trip off the Isolation valve is in what position when the packs are in Auto? Closed? Hence Assymetric Wing de/anti-icing = not exactly desirable.
Why can't you open the Isolation valve?...might not be desirable depending on what's caused a bleed trip off to start with..?
Any of that make sense?
:8
Hi Bumblebee,
Thanks for your reply.
I don't see the problem in "backpressuring the duct on the failed side"...
:confused:
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! :ok:
LEM
alexban
10th Jul 2005, 16:44
Hi LEM:
it's pilot judgement,as Boeing likes to say so often.
The bleed trip off NNC says avoid icing conditions. (indeed you arer right in assuming that in this case you can be at cruise alt,low chance of encoutering ice and also a high demand on the pack.)
But at the end of this particular NNC ,it says:
CAUTION : use of wing anti-ice above aprox FL350 may cause bleed trip off and possible loss of cabin pressure.
So you have your answer.You can use wing anti-ice at lower level,as your judgement dictates.(at lower levels...)
Otherwise it would've said : caution: use of wing anti-ice may cause bleed trip off.....(no level specified)
Brgds....
Alex
Hi Alex!
The problem is many people will strictly follow the checklist.
Disregarding a NNC is normally left in situations that you may qualify as the emergency of the emergency, or abnormal non normal, or multiple failures....
I still think it is badly designed, as it should have stated :" If wing antiice is needed below level 350 (or whatever), one pack switch OFF, wing antiice ON."