B737 Engine anti ice below -40ºC SAT

[sudden Winds]

Hi there,

Today during recurrent training this old discussion came up.
Boeing instructs to not use engine anti ice during climb and cruise with OAT or SAT below -40ºC. I heard a good reason once that said a warm cowl lip would melt and turn into water ice pellets that would otherwise bounce harmlessly out of the way. What do you guys have to say about that? Ever heard or read different?

Thanks.

[Mad (Flt) Scientist]

The conventional wisdom is that supercooled liquid water - the kind that freezes on impact with the airframe (including the cowl) - cannot physically exist below -40C. Water at such temperatures can exist only as ice, not liquid.

Anti-ice systems don't help at all with ice pellets/crystals - best case scenario, the pellet still bounces off the surface, and goes where it goes (unless there is monstrous amounts of water, also physically impossible at such temperatures, the engine is going to easily cope with the volume of water, since it can cope with thunderstorm levels of rain) - worse case, the heat of the AI system actually causes the ice particle to NOT bounce but instead stick.

The only wrinkle I can think of is that the temperature of the water droplets could be above the "magic" -40C when the air temp is below -40C. (Maybe the supercooled liquid water is rising from a lower, warmer, altitude) In such a case it's conceivable that you could be getting ice accumulation even though the OAt is below -40C. But its a pretty extreme/unusual scenario. (And outside of the, admittedly imperfect, Appendix c icing envelope definition)

[tdracer]

Basically, what Mad Scientist wrote.
Below -40C, it's believed liquid water cannot exist, so what you're dealing with is Ice Crystal Icing (ICI). ICI has caused numerous headaches for engines, where they form ice internal to the engine flow path - forming ice where the temperature in the flow path is well above freezing. In simple terms, what happens is the ice crystals hit a warm surface and melt, additional ice crystals then hit the liquid water and cool it sufficiently that it re-freezes. It builds up then releases all at once - causing compressor damage or quenching the burner (or both).
In the case of inlet anti-ice, the design intent is that the inlet heat will not only melt any ice, it'll evaporate the water so you don't get runback. However below -40C, there may not be enough heat to evaporate the liquid - the danger is the melted crystals flow back and refreeze on the unheated portion of the inlet, then release in big slabs that can cause fan damage.

[ManaAdaSystem]

Boeing instructs to not use engine anti ice during climb and cruise with OAT or SAT below -40ºC.

Not sure if I agree with you. Boeing does not say not to use EAI below -40, they way I read it, it's not required to use it below -40.
I've been at FL 290 in clouds with OAT below -40 and slush building up under the wipers. EAI ON, please.

Engine anti–ice must be ON during all flight operations when icing conditions exist or are anticipated, except during climb and cruise when the temperature is below –40°C SAT. Engine anti–ice must be ON prior to, and during descent in all icing conditions, including temperatures below –40°C SAT.

[RAT 5]

Slush, not ice, can build up on, especially under, the wipers due to melting from window heating. It then builds up under the blade with airflow. That is why, perhaps, Boeing using the bolt as an indication; I know that is mostly for airframe icing. The -40c also depends on the thrust setting.
I'd always been told not to use EAI at very cold temps due to heating in the intake. This can cause very small ice crystals to warm and join together into bigger, not so friendly, ice pellets. I have to assume that the engine manufacturers have spent millions U$ on testing & researching this basic phenomenon. It's been around sine JT-8's. It's not new.
There are many new things they are learning about jet engines & cold temps; ask BA 777 crews. I think they might have got this one sorted, though.

[Centaurus]

the engine is going to easily cope with the volume of water, since it can cope with thunderstorm levels of rain) -

Not always. A Garuda 737 Classic had a double flameout due water ingestion when inadvertently penetrating a 70,000 monster CB over Indonesia and unable to relight. Did a dead stick ditching in a river. Analysis from the CVR revealed the noise from the rain hitting the aircraft was something like ten times heavier than engine certification tests. Figures from my memory of course so could be inaccurate

https://en.wikipedia.org/wiki/Garuda_Indonesia_Flight_421

[lomapaseo]

I'm having trouble understanding the answers posted above.

I interpreted the OP as a discussion about inlets which are part of the airframe systems.

The engine itself is a separate system. The linkage is the form of water as it enters the engine blading. and into the compressors. at -40C the only engine problem that I am aware in the service experience is that described by tdtracer as ice crystals melting and sticking to internal engine probes only to be shed into the small blading of high speed compressors or the burner itself as a slug of water.

Again I am not aware of any signifcant damage events at -40C having to do with inlet effects on such ice crystals.

If we are talking about icing at conditions above -40C that is another type of discussion as is water ingestion in massive rain storms.

[Mansfield]

It is important to distinguish between airframe/cowl icing and ice crystal icing. They are not the same thing, yet the use of engine anti-ice in some cases of ICI is recommended. From the 2011 Boeing Flight Ops Technical Bulletin regarding ice crystal icing:

Q9. In these conditions, should engine anti-ice be turned on for all aircraft?
Not for all aircraft. Engine anti-ice supplies heat to the cowl and, in these conditions, that is not where ice is forming. Engine anti-ice also has other effects: Accelerating the engine to approach idle, providing ignition, and promoting a higher fuel-to-air ratio in the combustor. We have recommended using engine anti-ice for only those engines where we believe these effects are beneficial – those engines that have a power loss during low power. If the power loss or damage problem occurs during cruise, none of these effects prevent icing-related power loss.

The entire Bulletin can be obtained at:

http://www.dispatcher.org/images/Library/All_Model_Tech_Bulletin_20110516_Ice_Crystal_Icing.pdf

[de facto]

NNC ICING------> Switch TAI ON

[Derfred]

Does a higher fuel/air ratio in the combustor help prevent ICI? I would have thought not, but stand by to be educated.

[tdracer]

Does a higher fuel/air ratio in the combustor help prevent ICI? I would have thought not, but stand by to be educated.
According to our engine operability types, a higher fuel/air ratio provides increased margin for flameout due to an ice shed.
At least until the GEnx came along, the CF6-80C2 had most of the issues with ICI. Typical scenario was an idle descent in ICI - ice would build up in the booster (LP compressor), when the engine was spooled up for level off the ice would shed and quench the combustor. Although the engines always relit (auto-relight is basic on the 747-400 and 767 CF6-80C2 FADEC), there were some cases of HP compressor damage from the ice shed (note that this fundamentally different than the GEnx ICI, where ice accumulated during steady state cruise). Since the operability people believed increased engine bleed would provide flameout margin, mitigating action was to maximize engine bleed by turning on Cowl Anti-Ice in suspected ICI conditions (also wing anti-ice at lower altitudes - there was a proposal to use WAI at cruise altitudes as well but trying to bleed that much air caused other issues). Long term the FADEC s/w was revised to keep the stability bleed (VBV) open more and longer to remove most of the ice shed from the core. That apparently helped - it's been several years since we've had a reported ICI flameout on the CF6-80C2.