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Black Stain
20th Aug 2012, 00:31
FCOM PRO-SUP-30:

WING ANTI-ICE may either be used to prevent ice formation or to remove ice accumulation from the wing leading edges. WING ANT-ICE should be selected ON whenever there is an indication that airframe icing exists.

It seems a simple procedure, turn it on and leave it on in airframe icing conditions. Apply the Vapp additive and LDR factor.

However, it has been suggested that leaving it on may result in significant clear ice build up aft of the leading edges.

A technique has been suggested to counter this possibility: Use the Wing Anti-Ice as a de-icing system only, by cycling it on and off at regular intervals.

The problem is, that without data on how long the leading edges take to cool down after Ant-Ice use the technique is at best hit and miss, probably pointless and possibly dangerous. If the Anti-Ice is cycled too quickly it's pointless, excessively long interval potentially dangerous.

Yes the A320 wingtips can be assessed visually from the flight-deck, but not the A321. Not sure about other types.

The variables:

Bleed Temperature
TAT
Size of the supercooled water droplets

Questions:

What is the normal operating temperature range of Slats 3, 4 & 5 with the Wing Anti-Ice on? Not bleed temp, how hot are the slats.
How quickly do they cool down, a few seconds, a few minutes?
Has anyone noticed after landing the build up of clear ice behind Slats 3, 4 & 5?
Has anyone experienced controllability difficulty in severe airframe icing conditions using the published Airbus procedure?

Cheers

Slasher
21st Aug 2012, 20:37
1. Wouldn't have a clue, but obviously well above 0*C
2. 1-2 minutes depending on OAT and water drop temp (so the engineers tell
me) after you punch the WAI switch off.
3. Nope.
4. Nope.

Disclaimer - my only regular icing encounters in the 320 is China and once in Moskva.

hawk37
21st Aug 2012, 22:15
it has been suggested that leaving it on may result in significant clear ice build up aft of the leading edges.

Just wondering, has this been "suggested" by Airbus, general airmanship, or physical observation?

I'd have to say this is the first I've heard of it, except for booted aircraft.

Mad (Flt) Scientist
21st Aug 2012, 22:34
It would seem to me that the only way operation of a heated AI surface can lead to the formation of ice aft of the protected area is if the protected area is "running wet" as opposed to "fully evaporative" i.e. there is (only) sufficient energy to raise the temperature of the supercooled droplets such that they are simply droplets, and run back along the protected surface, only to freeze in contact with the (colder) unprotected surface further aft.

In such a scenario, turning the system on and off would seem only to further guarantee that in some circumstances the running wet condition occurs - as the protected surface heats up or cools down, its almost certain to pass through the running wet case, especially if the full power case is also running wet. So I fail to see any practical gain to be had - the same risk is still present.

If the protection actually is fully evaporative then cycling is worse, since it exposes you to the heatup-coooldown cycle and a possible running wet phase that is not present in full power operations.

Additionally, Airbus will (I am 100% sure) have demonstrated the performance of their system under some quite extreme conditions and developed procedures accordingly. If they did get runback ice, it will have been accounted for. Compromising the protected zone (as any cycling must do) to protect against a threat that's either theoretical or, if real, already addressed, seems like a poor option.

Disclaimer - "our" systems are fully evaporative and and anyone suggesting such a procedure on our types is being, at best, a bloody idiot.

cav-not-ok
22nd Aug 2012, 03:31
the idea of cycling wing anti ice is used on turboprops wing de icing boots, which are on the leading edge of the wing

the boots cycle (expand and decompress) at a predetermined rate to allow build up of ice, then shed it by breaking it. no actual heat element is applied other than the heat of the bleed air blowing in the boots to expand them.

but as far as jets and airbi go, i'd listen to the mad scientist, he is mad... :p
"our" systems are fully evaporative - thanks, i learn something new everyday

hawk37
22nd Aug 2012, 10:32
"our" systems are fully evaporative

MFS,

are there manufacturers that have systems to heat leading edges that are not fully evaporative? Dassault? Gulfstream?

Will that mean in some cases "running wet" will occur, but they will have had to demonstrate this is still safe under some pretty extreme conditions?

Always great to read your inputs

Hawk

safetypee
22nd Aug 2012, 19:55
“are there … systems … heat leading edges that are not fully evaporative?”
Although recent certification requirements have significantly improved aspects of flight in icing conditions, the performance of any system depends on the environment encountered, and possibly the era of certification.

E.g. an older aircraft with a leading-edge hot-air system may be fully evaporative in light to moderate icing (note these are the operational definitions – severe icing is that which the system cannot cope with). The system is identified as ‘anti-icing’, but it has a boost setting of ‘de-ice’ for use when exiting ‘severe’ conditions or with landing flap when presumably the ice accretion can change.
There may be cases where the aircraft is technically in severe icing – the evaporative system is overcome, but the leading edge remains clear, thus there could be runback icing. The certification of this type may have shown (within the confines of the tests) that runback icing in cruise conditions was not a problem, but it may be a hazard during the approach.
Icing operations require a good understanding of the aircraft’s capability and great respect for the environment.

There is good ice, bad ice, and hazardous ice; … good ice is found in the galley.

http://www.ead.eurocontrol.int/eadbasic/pamslight-4D61C9AAC1C2415BF4AAC434F815ED3A/7FE5QZZF3FXUS/EN/AIP/GEN/EG_GEN_3_5_en_2012-07-26.pdf para 6.3.3.

http://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo/all_safos/media/2008/SAFO08006.pdf

Black Stain
27th Aug 2012, 00:59
Thanks for the information gentlemen, my question answered. Diverge from FCOM at your own peril.

I also found from Airbus, "Getting to Grips with Cold Weather Operations" and an article on runback icing from Edward Whalen. Both interesting.

Cheers