Use of wing anti ice (Airbus)
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Consider that there are two ways to construct a thermal ice protection system: one is known as fully evaporative, the other is called running wet. Some designs are intended as a running wet system...my old Convair 240 comes to mind...but most jet designs begin with the intention of a fully evaporative system. That said, you can probably guess which one requires the most energy (read: fuel consumption), so it would seem that more contemporary designs are limiting the energy output rather more than they used to.
A running wet system, or a fully evaporative system being operated in a running wet state (perhaps because of reduced power during a descent) will likely generate some runback. This is due to the supercooled liquid water droplets being temporarily un-supercooled, so-to-speak, and then quickly freezing once the water mass has "run back" off the heated surface. The fully evaporative system is supposed to sizzle the little buggers into steam, so that by the time they get around to freezing again, the wing has left the building.
It takes anywhere up to five or six minutes to thoroughly heat the wing protected surface after you activate the thermal ice protection system, depending on the SAT, mass of water impinging, size of the wing and actual bleed output. During that time, the system will necessarily run wet at least briefly.
Boeing's current spin on this is to operate the system in a de-ice mode in order to preclude a runback ridge. Their boilerplate language looks something like this:
However, a number of us have raised the question over the years regarding how you are supposed to know when the cycle the thing if using it in a de-ice mode? Can't see the wings, wouldn't know what we were looking at even if we could (can you estimate in the fractions of an inch at a range of seventy or eighty feet...at night?)...
At the end of the day, the thing was supposed to be a fully evaporative anti-ice system. Fuel conservation pressures, coupled with a rather barren accident record due to inflight icing, have allowed procedures to be "relaxed" as it were.
Regarding the slat issue, I'd be cautious about asserting that the TAI is less "efficient" in one configuration or another. FAR/JAR 25.1419 has a lot of detail, particularly in the advisory material, about how to get your ice protection system certificated. No where does it say anything about "the system can be less efficient" at any particular point in time. It either meets the criteria or it doesn't.
That said, the manufacturer's procedures are written in a way that will respect the certification results. If they say turn it on, then turn it on. If they say it can be turned off here or turned off there, then doing so will respect the certification. But that doesn't mean you can't be more conservative, should you be so inclined!
A running wet system, or a fully evaporative system being operated in a running wet state (perhaps because of reduced power during a descent) will likely generate some runback. This is due to the supercooled liquid water droplets being temporarily un-supercooled, so-to-speak, and then quickly freezing once the water mass has "run back" off the heated surface. The fully evaporative system is supposed to sizzle the little buggers into steam, so that by the time they get around to freezing again, the wing has left the building.
It takes anywhere up to five or six minutes to thoroughly heat the wing protected surface after you activate the thermal ice protection system, depending on the SAT, mass of water impinging, size of the wing and actual bleed output. During that time, the system will necessarily run wet at least briefly.
Boeing's current spin on this is to operate the system in a de-ice mode in order to preclude a runback ridge. Their boilerplate language looks something like this:
The wing anti–ice system may be used as a de–icer or anti–icer in flight
only. The primary method is to use it as a de–icer by allowing ice to
accumulate before turning wing anti–ice on. This procedure provides the
cleanest airfoil surface, the least possible runback ice formation, and the
least thrust and fuel penalty. Normally it is not necessary to shed ice
periodically unless extended flight through icing conditions is necessary
(holding).
The secondary method is to select the WING ANTI–ICE switch ON
when wing icing is possible and use the system as an anti–icer.
In my personal opinion, this is driven by Boeing's general attitude that their airplanes really don't need ice protection. I don't agree with that sense, but there is no doubt that larger scale airplanes have less of an issue with ice than smaller scale machines. only. The primary method is to use it as a de–icer by allowing ice to
accumulate before turning wing anti–ice on. This procedure provides the
cleanest airfoil surface, the least possible runback ice formation, and the
least thrust and fuel penalty. Normally it is not necessary to shed ice
periodically unless extended flight through icing conditions is necessary
(holding).
The secondary method is to select the WING ANTI–ICE switch ON
when wing icing is possible and use the system as an anti–icer.
However, a number of us have raised the question over the years regarding how you are supposed to know when the cycle the thing if using it in a de-ice mode? Can't see the wings, wouldn't know what we were looking at even if we could (can you estimate in the fractions of an inch at a range of seventy or eighty feet...at night?)...
At the end of the day, the thing was supposed to be a fully evaporative anti-ice system. Fuel conservation pressures, coupled with a rather barren accident record due to inflight icing, have allowed procedures to be "relaxed" as it were.
Regarding the slat issue, I'd be cautious about asserting that the TAI is less "efficient" in one configuration or another. FAR/JAR 25.1419 has a lot of detail, particularly in the advisory material, about how to get your ice protection system certificated. No where does it say anything about "the system can be less efficient" at any particular point in time. It either meets the criteria or it doesn't.
That said, the manufacturer's procedures are written in a way that will respect the certification results. If they say turn it on, then turn it on. If they say it can be turned off here or turned off there, then doing so will respect the certification. But that doesn't mean you can't be more conservative, should you be so inclined!
Last edited by Mansfield; 3rd Nov 2010 at 20:54.
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I'm going to speculate that Airbus may be more "relaxed" about turning on WAI with slats deployed not because the system works less well - there is no reason why slat deployment should materially affect the performance of the system - but rather because the aerodynamic effects of ice with slats deployed was found more tolerable than for the slats retracted case.
It's not that it's less effective - but rather less required. As a guess.
It's not that it's less effective - but rather less required. As a guess.
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and the definitive use of wing anti ice from airbus is???? ..............there's no definitive and that's why I asked in the question in the OP. Maybe they may add "may your own arrangements re useage thereafter" because some folk insist in having it on all the time in icing condts cause airbus says so and others use it as required which imo would be the preferred option. The useage of continously on in severe icing is I think is where airbus is coming from not ever whisp of cloud.
Thank you for your contributions, I'm still none the wiser.....no fault of yours but the lack of definitive from airbus irks me. Rgds
..............there's no definitive and that's why I asked in the question in the OP. Maybe they may add "may your own arrangements re useage thereafter" because some folk insist in having it on all the time in icing condts cause airbus says so and others use it as required which imo would be the preferred option. The useage of continously on in severe icing is I think is where airbus is coming from not ever whisp of cloud. Thank you for your contributions, I'm still none the wiser.....no fault of yours but the lack of definitive from airbus irks me. Rgds
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Hi Meikleour
Refer to the reply to your posting of 25.02.10 - the slats are still heated due to ducting. The worry behind flight in severe icing conditions with slats extended is due to what can build up in the "slots" and the reason they reccomend turning it off on final approach (unless in severe icing conditions) is to ensure plenty of power on the go around.
Refer to the reply to your posting of 25.02.10 - the slats are still heated due to ducting. The worry behind flight in severe icing conditions with slats extended is due to what can build up in the "slots" and the reason they reccomend turning it off on final approach (unless in severe icing conditions) is to ensure plenty of power on the go around.
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Strongly recommend reading latest Business and Commercial Aviation magazine issue October 2010. Page 72 entitled "Fine-Grain Icing on Aircraft" by Richard N.Aarons. Four pages of the best informaion around on take off icing.
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Finally a definitive answer
The AFM recommends avoiding extended flight in icing conditions with extended slats
and flaps, as accreted ice may block the retraction of the high lift devices causing
mechanical damage to the slat / flap system
From Getting to Grips Icing
and flaps, as accreted ice may block the retraction of the high lift devices causing
mechanical damage to the slat / flap system
From Getting to Grips Icing
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Airbus also wants you to switch WING ANTI ICE to ON if VOLCANIC ASH HAS BEEN ENCOUNTERED. Hopefully an unlikely subject for airliners, but does anyone know or has an idea why Airbus recomends this procedure?
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Originally Posted by redfly
Airbus also wants you to switch WING ANTI ICE to ON if VOLCANIC ASH HAS BEEN ENCOUNTERED. Hopefully an unlikely subject for airliners, but does anyone know or has an idea why Airbus recomends this procedure?
Last edited by TopSwiss 737; 29th Nov 2013 at 21:28. Reason: typo
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Well, maybe a rather dumb question but doesn't do maximum bleed extraction calls for MORE thrust from the engines? And that's what you should try to avoid while finding yourself situated in volcanic ash clouds, no?
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Most, if not all, aircraft require the thrust levers to be retarded to idle as part of their volcanic ash procedure.
This reduces possible engine damage or flameout, or both, by decreasing EGT. The lower temperature in the engine at idle thrust reduces the glass build-up from molten ash in the combustor and on the turbine blades (this glass deposit blocks cooling holes in the turbine blades, which may overheat them by a considerable amount).
Extracting maximum bleed from the engines by also switching on wing and engine anti-ice reduces the pressure gradient in the compressor, thereby increasing engine stall margin.
This reduces possible engine damage or flameout, or both, by decreasing EGT. The lower temperature in the engine at idle thrust reduces the glass build-up from molten ash in the combustor and on the turbine blades (this glass deposit blocks cooling holes in the turbine blades, which may overheat them by a considerable amount).
Extracting maximum bleed from the engines by also switching on wing and engine anti-ice reduces the pressure gradient in the compressor, thereby increasing engine stall margin.
