I don't think the WAI on the 737 works like that. What you describe is how boots work on older airplanes, and the NGX doesn't have wing boots. The WAI can remove ice, yes, but i think it's best used to prevent ice buildup. Maybe someone with more technical knowledge can give us a better idea.

 

The wing anit-ice can be used a deicer or an anti-icer.  The preferred method at my company is to use it as a deicer by allowing ice to accumulate before turning the system on.  Boeing states this method provides the cleanest airfoil surface, least possible runback ice formation and the least thrust and fuel penalty.

 

You rarely see ice on the wing in this plane.  You aren't in the icing conditions long enough to build up a significant amount unless you are holding in moderate or greater icing conditions, which you shouldn't be doing anyway.

I should also point out that the entire wing surface isn't heated as well.  Only the three inboard slats have anti-ice, the outboard slat and the leading edge flap are not heated.

What distance of runway would you say is necessary for a bleeds off takeoff?

 

It depends on a lot of things.  how much payload did they put on the airplane?  What is the temprature?  The runway condition?  Our takeoff planning system will take all the performance factors into account and will plan the lowest thrust setting we can get away with for that takeoff.  It will start with 22K thrust and go up from there.  If 26K max thrust isn't enough for the planned payload it will try 26K max with the bleeds off.  If that doesn't work it will bump up to 27K if we are at an airport the company allows its use at.  If that still doesn't work the system will start to limit the amout of payload the station can load and you start bumping people.

Great stuff Matt, thanks for the information. I will consult the NG FCOM about the possible double trip (scary indeed).

 

What if you happen to hit ice crystals at FL350 or above in the NG? I know it's next to impossible for these to accumulate, but I've heard  a few nasty stories about it. Maybe descend to a lower level?

 

Thanks again!

cheers

-E 

Nothing happens when you hit Ice crystals. You experience icing if you hit supercooled water droplets.

 

Anyway, at FL350 ISA temperature is -55*C. At temperatures below -40*C only very small supercooled water droplets can exist. All others will have become ice. Freezing of very small supercooled droplets will occur without the aid of nuclei. Icing at temperatures below -40*C is usually negligible.

The WAI is just too much for the system at altitude.

 

 

What if you happen to hit ice crystals at FL350 or above in the NG?

 

 

Anyway, at FL350 ISA temperature is -55*C. At temperatures below -40*C only very small supercooled water droplets can exist.

A bit off topic:

interestingly that this is exactly what happend to AF447. They were at FL350, the OAT was just -41°C and they had EAI + WAI running! In addition they had just entered the cell top of a CB cluster inside the ITCZ. On entering this zone they were in an updraft of warmer air carrying a lot of moisture from heavy rain below. (This is why the crew decided to turn on even the WAI at this altitude). The page in the T7 FCOMv1, SP.16.23 "Avoid Ice Crystal Icing Weather" says it all (quote): "Exit the ice crystal icing conditions. Request a route change to minimize the time above red and amber radar returns." (quote end)

It was published on december 12th 2011, 6 month after the initial and 6 month before the final investigation report of AF447.

Sorry again for being off-topic, but it just came into my mind.

 

Greetings,

Claus

Claus, I remember the case of AF447 indeed. Also, there have been reports of ice crystals creating problems in the GENX engine in use by the 747-8 and the 787. I've heard stories of strange behaviour caused by ice crystals at altitudes above FL350 in a MD-11 and older 737... but not a formal report of accumulation of such (if such a thing is possible, which according to g_precentralis it is not) and no need for using WAI at all. Then again, it is not something I'd like to encounter.

 

cheers

-E

Ice crystal icing in the engine are "new" if you will. These aren't going to affect the wings.

Exactly, Matt. Reports of ice crystals building up inside the GEnx have nothing to do with the wing. The things I've heard involving the MD-11 and 737 in ice crystal icing conditions are strange behavior such as random stall warnings and severe turbulence which led to the crew fearing ice buildup in the wings... something that has never been reported and seems outright impossible.

 

Cheers

-E