Ice Protection - Large vs. Small Aircraft

[Capt. Amazing]

I've come across the following statement in a book on aircraft systems:

"On large transports, only the outer portion of the wing leading edges or slats and the engine cowl lips require protection but the air flow requirement can be similar to that for airconditioning."

The author then goes on to say in the next paragraph:

"Smaller aircraft, such as the BAe 146, also require protection of the tail surface leading edges, and when operating ice protection can take three times the flow for air-conditioning, for the short time that ice protection is required."

The author gave no additional reason, and cited no proof to validate his statement. So if he's in fact correct, my questions are:

1. With the second statement, the author seemed to imply that only small aircraft require icing protection for the tail. Is this true, and is so, why?

2. Why would the airflow requirement to protect "large transports" from icing be equivalent only to the flow required for air-conditioning, but for smaller aircraft, be three times the amount required for air conditioning.

Thanks in advance!

[Genghis the Engineer]

I can certainly confirm that tailplane leading edge icing was a significant issue on the 146, since I was involved in part of the testing of it (and I've spoken to people who did similar work on the PC12). However, that's as big an aeroplane as I've ever dealt with, so I've no idea about tailplane ice protection of bigger jets.


As to the wider question, I think that it probably comes down to scale laws. Let's take a hypothetical BAe 146 equivalent, with about 100 seats, and a wing area of somewhere around 100m².

Now let's assume that we're making a bigger version with 300 seats, so assuming that the payload fraction is about the same, the MTOM will roughly treble. Normally you'd expect to increase the stall speed with a bigger aeroplane, so let's assume it goes up from about 60kn to about 90kn. The result will be a wing area of around 245m². If it's the same shape, then the span will increase from the original 26m to about 40m, or around a 55% increase in span.

Now if we make the incredibly crude assumption that the amount of de-icing hot-air is proportional to the wingspan (after all - it's only really the leading edge that needs de-icing), you'll therefore need about 55% more hot air for the 300 seat jet than for the 100 seat jet. But since air conditioning flow is probably roughly proportional to the number of POB, you'll have 3 times as much available.

Putting that another way, if we take it as a given that a big jet will need about as much air for air-conditioning as de-icing, then the little jet will need 44% less air for de-icing, but 66% less air for air-conditioning. That comes out as the little jet needing about twice as much air for de-icing as for air-conditioning, which is pretty much in the ballpark of your textbook - and not too bad for some admittedly very crude back-of-envelope sums.

Moving slightly into the realms of conjecture now, the inboard section of a big-jet's wing is very very thick. As a consequence, you'd need quite terrifying amounts of ice to affect it's shape significantly, certainly the inch or so that created big problems on the BAe 146 tailplane would probably have no appreciable effect. I'd guess that's why there's no de-icing of the inboard section. Obviously if that's the case, then the big jet is again using less de-icing air, and so you start to move even more towards the statement that the 146 sized aircraft needs 3 times as much de-icing air per passenger as the 146x3 sized aircraft.

G

[safetypee]

I offer two additional explanations:
Ice accretion is proportional to the structure thickness; relatively thin projections such as wip aerials collect ice more quickly and to a greater extent than do thicker structures. Tailplanes usually have thinner sections than the main wing, thus require better ice protection; if you see ice on the wing then the tailplane will have a greater accumulation of ice! However these requirements are very much aircraft specific; some big Boeings do not have any tail de/anti icing, but then these structures are thick in comparison to a small turboprop wings and tailplanes.

The 146's relatively thick wing contributes to it’s good lift capability, especially when the near full span flaps are lowered. During flight with flaps retracted the ice protection system for both the main wing and the tailplane only uses the minimum hot air rneeded to meet the requirements of an ‘anti icing’ system.

With flaps down (or as required) the air flow to the wing de/anti icing system is increased (by crew selection). This gives the aircraft both a de icing and anti icing capability, which maintains the aircraft's high lift characteristics. It is essential to anti ice the tailplane in this type of aircraft as the tail is the control surface that ‘distributes’ the high lift generated by the wing. Note the difference between de icing and anti icing; crews need to understand these differences when considering use of the ice protection system, if in doubt select ice protection early to heat up the surfaces.

In addition the 146 ice protection system is designed to be fully evaporative (within the certification criteria), thus should not suffer the problems seen on some turboprop aircraft. I also suspect that the high quality of ice protection relates to the use of manual controls.

[bookworm]

I agree with both the scaling argument and the effect of greater radius of curvature.

One more factor is that the higher speeds for larger aircraft give a higher skin temperature through compressive heating.

[steamchicken]

What's the definition of de-ice vs anti ice?

[safetypee]

steamchicken

De icing, to the purist, is like the use of mechanical de-icing boots; first the ice has to form, or is allowed form on the airframe before it is removed. The boots are activated to remove the ice, and then cycled to off to let a new layer of ice form. Hot air systems may be used in this way; this is where the system is only selected on when ice has formed on the airframe.

Anti icing is usually a continuous process, like the use of hot air to evaporate water or ice particles from the airframe before they can form into ice accumulations. The leading edge ‘deicing fluid' system is another example of anti icing; where icing condition are suspected then the system should always be on

Recent changes to regulations suggest that mechanical boot systems be used continuously, like an anti icing system. Whatever the arguments for or against a particular method of operation, always follow the manufacturers instructions, these should explain how and when to use the system. Although the AFM may not always state whether it is a de icing or anti icing system, the required method of operation should indicate which definition applies.

[steamchicken]

So (roughly) anti-ice=prevention, de-ice=cure. Thanks.

[FlyingForFun]

Safetypee,Recent changes to regulations suggest that mechanical boot systems be used continuously, like an anti icing systemCould you elaborate on this a little please?

My understanding - purely from reading up on the theory - is that if boots are operated before a sufficiently thick layer of ice has formed, the ice will crack, but not be removed. Subsequent ice will then build up on top of the initial layer, but because the first layer of ice is already cracked, further use of the boots will not be able to remove it.

If whoever is writing these "regulations" is suggesting that boots be used all the time, would that not prevent the boots from working? Or is my understanding of how they work flawed? (And what regulations are you refering to?)

Thanks,

FFF
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[bookworm]

AC 23.1419B (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/1ab39b4ed563b08985256a35006d56af/591e1fdabbeef9e786256c670066e649/$FILE/ac23.1419-2b.pdf) recommends:

Pneumatic Deicer Boots. Many AFMs specify a minimum ice accumulation thickness prior to activation of the deicer boot system. This practice has been in existence due to the belief that a bridge of ice could form if the boots are operated prematurely. Flight testing and icing tunnel testing of several “modern” boot designs have not shown evidence of “ice bridging”, and no degradation in ice shedding performance, when the boots were activated at the first sign of ice accretion. Although the ice may not shed completely with one cycle of the boots, this residual ice will be removed during subsequent boot cycles. Tunnel testing is documented in FAA Technical Report DOT/FAA/AR-02/68, "Effect of Residual and Intercycle Ice Accretions on Airfoil Performance" (May 2002), and recommends that activating the deicing boots “early and often” be given more consideration as a means of limiting the size of intercycle and residual ice accretions. “Modern” boots are defined as high operating pressure (nominal greater than 15 pounds per square inch gauge (psig)) and fast inflation and deflation times. Both one-inch diameter tube designs operating at a nominal 18 psig, and 1.75-inch diameter tube designs, operating at a nominal 15 psig, have been evaluated. The recommended AFM procedure for boot operation should be to operate the boots in an appropriate continuous mode at the first sign of ice and not to wait for a specific amount of ice to accumulate. The boots should be operated until icing conditions are exited and ice no longer adheres to the airframe.

See also:

EFFECT OF RESIDUAL AND INTERCYCLE ICE ACCRETIONS ON
AIRFOIL PERFORMANCE (May 2002) (http://www.tc.faa.gov/its/worldpac/techrpt/ar02-68.pdf)

[spannersatcx]

de-ice - to remove ice that is already formed

anti-ice - to prevent the formation of ice

[safetypee]

FlyingForFun, you must be prepared for a possible illogical debate with the FAA, who following two major accidents (Roselawn and Detroit) decided to mandate the use of de icing boots at all times in icing conditions (about 4 years ago). This was against the advice of some sections of the industry; there is at least one European manufacturer that has successfully defended the change based on some very sensible and in depth flight testing. Thus my caution for each aircraft to be flown i.a.w. the AFM.

One of the counter arguments was that continuous operation of boots may lead to a hazardous condition of ice bridging – as you state cracking the ice, but not fully shedding it. However modern boot systems have their own improved characteristics, ribbed, ridges, longitudinal, chordwise, etc, and in general do not suffer bridging to the same extent if at all. In these cases the airframe manufacturers have fallen in line with the FAA; it’s cheaper than arguing or conducting more flight tests.

The illogical crux of the decision was that in the US accidents and similar accidents and incidents world wide, crew error was a predominant factor. The crew failed to follow AFM procedures or abide by limitations; too slow, incorrect flap, incorrect autopilot mode. In addition there is an industry wide ambivalence to the hazards of ice; “we always see it, the aircraft continues to fly, we’ll be alright” etc. The problem is that some of the modern aircraft (and systems / engines) are more susceptible to ice than were the older aircraft (new supercritical wing sections, reflex control surfaces – cleaver aerodynamics that do not tolerate being mucked about by ice or any other contaminants, and this includes de-icing fluid). Thus some of the industry saw the focus of action as the need to re-educate the operators on the hazards of ice and the need to adhere to procedures, but then there was commercial pressure.

Forgive me for the long post, IMHO all of this is important.
The more successful experience a pilot gains in icing, the more he may believe in his ability to evaluate ice accretions for handling and performance degradations. Unfortunately, the subtle warnings that many perceive will be seen or felt prior to a major upset … simply may not exist. The airplane’s behavior may reveal nothing about the effects of the ice accretion until a non-linearity is encountered. A small increase in angle of attack may result in a) a gradual drag rise suddenly increasing so rapidly as to exceed the available excess power; b) the wing’s ability to produce lift diminishing quite rapidly and a dangerous and asymmetric stall developing, and c) leading to enormous control loadings driving rapid roll or pitch changes. Small changes in angle of attack can cross the line.
What the line pilot lacks is the careful measurement of flight conditions and parameters, the accounting for variables and unknowns that the flight test program can take advantage of. The line pilot has no way of measuring the ice thickness, roughness, chordwise extent or location, horn height or horn angle, or even or recognizing how uncertain he actually is. So while he feels that the ice he encounters on any given day is within the range of his experience, he can base this only on the crudest of qualitative assessments. The fine details which actually influence the effects of an ice accretion are not even within the pilot’s sensory capabilities.

For the present time, the line pilot’ best approach to this problem is to adopt a cautious skepticism. He should be wary of what his experience has taught him. Although it is somewhat of an icon within our profession to believe that with suitable experience, we become seasoned, wise, confident and charmingly crusty, we often forget that by default, those of us who have survived to become experienced have not acquired the experience of those who did not survive. Thus we are not afforded a clear perspective on whether our survival is due to wisdom or just to the good humor of the gods. The true wisdom of the seasoned pilot is to not overestimate his experience. “The Question of Experience in Icing Operations”
S.D. Green, Air Line Pilots Association, International
There is a very useful document on icing that also covers the FAA position here:New Zealand CAA Icing Handbook (http://www.caa.govt.nz/fulltext/safety_booklets/aircraft_icing_handbook.pdf)

A smaller guide here: New Zealand CAA Winter Flying (http://www.caa.govt.nz/fulltext/safety_booklets/winter%20flying.pdf)

In aviation there are three types of ice.
Good Ice, Bad Ice and Hazardous Ice.
Good Ice is found in the galley.

[Miles Magister]

Please forgive me if I am repeating anyones gen, but the Campaign Against Aviation are distributing a NASA DVD all about icing and are about to publish a new AIC. Don't have the details with me at the moment but they are both very good. Unusually good gen from big brother.

[Old Aero Guy]

Without getting into too much detail, two of the major differences between large and small airplanes and the need for h-tail ice protection are:

Trimmable vs fixed h-tails

Powered vs manual elevators

[FlyingForFun]

Safetypee and Bookworm,

Thanks for the quotes and links - very enlightening! :ok:

FFF
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