B737 Wing anti-ice valve open NNC

[framer]

Hi there,
the 737-800 QRH NNC Wing anti-ice valve open has a step in it that I struggle with a bit and I would appreciate some other opinions.
Here is the scenario:
During the climb wing anti ice is used. It is turned off as the aircraft climbs through about FL280 and exits cloud and the L VALVE OPEN light remains illuminated bright blue. In step one the checklist confirms for you that the valve is now failed open.
Step two sates " If TAT is above ten degrees C or there is no visible moisture; isolation valve switch to close, pack switch off, bleed air switch off.
In the scenario the TAT is about minus twenty and it is clear skies so by following the QRH you go down to one bleed.
Is the primary reason for doing this so that you avoid a bleed trip off due to having 9th stage air required, or is there another reason why it is detrimental to have one wing heated in clear air?
Your thoughts?

[Lead]

It's asking, are you in icing? No? Then turn the bleed off (and thus the wing anti ice). No point wasting fuel.

Single pack op possible up to fl250 if I recall correctly.


Edit: you are right too, it's possible to get the bleed trip off at high alts with wing anti ice on.

[Lord Spandex Masher]

Hmmm, so turning off a bleed to avoid wing anti ice uses more fuel than flying at fl250 instead of 30something?

[arcdu]

Nothing stops you continueing the climb in this case, FL250 is an MEL restriction so only applies prior to dispatch. Just monitor the pressurisation to check it behaves ok.

[framer]

I don't think the reason is anything to do with fuel consumption.
I know that I can continue the climb to any level I think is suitable because the FL 250 restriction is for MEL dispatch. So...what I'm hoping someone can answer is Is the primary reason for doing this so that you avoid a bleed trip off due to having 9th stage air required, or is there another reason why it is detrimental to have one wing heated in clear air?
Ps, thanks for your replies :)

[slr737]

If this happens in flight, there is no restriction in FL. FL250 is only MEL and so applies in dispatch.

the main reason to close the bleed is to avoid having one wing anti iced and the other one not. Just in case you would not see that you are picking ice on the wing, with one wing being antiice and the other one not. that could result in an unexplained roll .... ;-)

Of course that scenario is not a problem when above FL300 where picking ice remotely is rare.

The other reason is to avoid having a bleed trip off on the side where the wing anti ice is on at high altitude.

[Capt Quentin McHale]

framer,


Methinks the PRIMARY reason for closing the wing isolation valve, shutting down the pack and closing the engine bleed is to avoid wing leading edge structural damage due to overheat. QRH stated tat above 10c and no visible moisture, thereby implying warmish dry air so no need for wing anti-ice.

[RAT 5]

It's never a good to have WAI applied to only 1 wing. KISS.

[framer]

QRH stated tat above 10c and no visible moisture,
No it says TAT above ten OR no visible moisture. So it can be -20 TAT and clear air and you still have to turn the bleed off.
This is getting to the crux of it now. If the thinking is to avoid wing leading edge structural damage due to overheat. then that's a very good reason but I've never seen it written anywhere that you can damage the lead leading edge with wing anti-ice when it's nice a cold outside ( ie cruise temperatures).
So faced with the prospect of having anti-ice on only one wing in clear air ( that is nice and cold ) v's having both wings with no anti-ice but only one bleed source, ....the QRH wins.....but why? Is it because Boeing is mitigating the risk of bleed trip off? Structural damage to leading edge? Increased fuel consumption? Just n case you don't notice you are picking up ice asymmetrically? We've had four different answers so far.
Cheers

[Yancey Slide]

TAT above 10C, OK... -20 is below 10C isn't it? Or am I (not unusually) being obtuse...

[Gas Bags]

Leading edge heat damage is right on the money.....

[framer]

Why is leading edge damage likely if there is no visible moisture? I'm keeping an open mind to the possibility that you are correct but I can't see how the visible moisture prevents leading edge damage.
Scenario 1: Minus twenty degrees TAT and no visible moisture.......QRH instructs you to turn the bleed off.
Scenario 2: Minus twenty degrees TAT in stratus and the bleed stays on. How does the stratus prevent leading edge damage?
Thanks again for your thoughts.

[PPRuNeUser0190]

Just as some extra info. What is written in the FCOM about a caution.

CAUTION: An operating procedure, technique, etc., that may result in
damage to equipment if not carefully followed.

Somewhere else in the FCOM:

"CAUTION: Do not use engine or wing anti–ice when OAT (on the
ground) or TAT (in flight) is above 10°C."


If Boeing is consistent in its terminology, damage might occur above the 10°C :)

[RVF750]

Possibly the moisture would assist in evaporating some heat from the surface?

[framer]

If Boeing is consistent in its terminology, damage might occur above the 10°C
That is understood but the question relates to when the temperature is cooler than ten degrees TAT but the checklist suggests you turn the bleed off , regardless of temperature, if there is no visible moisture. So the temperature is not what. I'm getting at.

[c100driver]

The answer is that with TAI wing and a pack operating it may exceed the bleed capability of the engine at high altitude. Also at altitude the 9th stage will have to open. This can cause very high temperatures in the ducting.