Learjet anti-icing

[sexygirl]

Anyone out there know why the AFM for the Learjet 40/45 has the wing and stab on prior to entering icing? Why does this aircraft use this system as anti-icing rather than allowing some ice build up prior to de-icing? Is this the norm or actually unusual for jet aircraft?

Many thanks...

[His dudeness]

Very much the norm on a jet. Its called anti ice opposed to de ice. One would not want ice building on the wing, break loose and enter the engines as solid pieces. Hence even the older staright wing Citations that have de ice boots, have anti ice on the inner wing (electrically or bleedair)

[roljoe]

Sexygirl, the scenario of giving ice a little bit time to build up before activating a de-icing system is restricted to boots system...otherwise what should be the efficiency of such a system..(simply mechanic)...

the lj45 like others jets using bleed air to warm up the leading edge and other sensitive parts is another way to avoid ice accumulation .

why waiting to see ice acretion to born ..if you have the possibility to get only water blowing on the wings..???

rgds

[Mad (Flt) Scientist]

Sexygirl, the scenario of giving ice a little bit time to build up before activating a de-icing system is restricted to boots system...otherwise what should be the efficiency of such a system..(simply mechanic)...

Not even all boots recommend this, afaik. There was a concern with what's called "ice bridging" with older, lower-pressure boots systems, but I believe some modern systems recommend use without waiting for ice buildup. As always, the definitive answer is type specific and so the manual for the a/c is the final word.

Note that per the current NASA Inflight icing Advice (http://aircrafticing.grc.nasa.gov/courses_inflight.html#) - slide 02 of the Aircraft Deicing Systems section - it states that the trend is away from "wait for ice to form" and towards "preventative use", even of booted systems.

Here's the slide in question, to save you going through the course (emphasis (red box) added):
http://img261.imageshack.us/img261/2507/nasabootsoz1.th.jpg (http://img261.imageshack.us/my.php?image=nasabootsoz1.jpg)

[BelArgUSA]

As mentioned above it is a norm on REAR engine tail mounted jet aircraft...
xxx
I used to fly 727s... we used to operate engine and wing anti-ice prior entering/reaching icing conditions. Our VHF antenna on top of the fuselage, was heated full time, to prevent ice ingestion into nbr. 2 engine.
xxx
In other airplanes I flew, the only "anti-ice" was engine... On the 707, the DC-8 or the 747, we would operate engine anti-ice prior to entering/reaching icing conditions... BUT ONE DIFFERENCE... wing anti-ice was, in pratice, more of a DE-ICE system, as we hardly ever operated wing anti-ice. The only time you would consider using wing anti-ice was i.e. holding (low speed) in icing conditions. I think I used wing anti-ice maybe only a dozen times in my entire career in 707-747-DC8... possibly 20,000 hrs total on these types.
xxx
:ok:
Merry Xmas to all - and happy contrails

[Bruce Waddington]

sexygirl,

I have asked the same question myself.

My background is large jet aircraft where, depending on the types certification procedures, you were allowed you to use the heated wing/stab systems for anti-icing or de-icing.

During my conversion to the LR45 it was made clear that the wing/stab system was for anti-icing only. And in fact the FAA Approved Flight Manual (AFM) say just that. The system is described as an anti-ice system and the procedure for its use states "Anti-ice systems should be turned on prior to operating in icing conditions. Icing conditions exist when the SAT is 10c to -40c and visible moisture in any form is present."

There are at least two reasons why the LR 45 has an 'anti-ice' system. They come from the FAA AFM.

1."Even small accumulations of ice on the wing leading edge can cause an increase in stall speed and possibly a degredation of stall characteristics."

2. "Accumulation of ice on the horizontal stabilizer may degrade pitch characteristics and result in elevator buffet or stick pumping."

The FAA AFM makes no reference to the possible ingestion of ice by the rear mounted engines as a reason for anti-ice only.

best regards,

Bruce Waddington

[SNS3Guppy]

Why does this aircraft use this system as anti-icing rather than allowing some ice build up prior to de-icing?


As others have indicated, very small changes in the lear wing leading edge make large differences in the aerodynamic properties of the wing. So much so, in fact, that if the leading edge is removed for maintenance, dive tests must be performed by factory pilots in order to recertify the aircraft before returning it to service...even though the removal involves putting everything back in exactly the same place, using the same screws, in the same screw holes.

The anti-ice capabilities on the lear wing are limited. I've done icing research in certain of the Learjets, and in significant icing conditions, one can see a point farther out on the wing where ice removal is less effective; the icing may remain as the cooler air near the wingtip doesn't do as effective job at removing or preventing ice.

The significance of heating the leading edge early is that the idea to prevent ice from forming, and it will form if the leading edge hasn't had enough time and exposure to bleed air before entry into icing conditions. Ice which forms, as others mentioned, poses a significant danger to aft mounted engines. This is also the reason that the Lears equipped with an alcohol de-ice system must have the radome anti-ice turned on prior to ice forming; the system isn't effective at removing ice, but is effective at preventing it. Ice which does form becomes a hazard if removed, because then solid ice moves aft with the slipstream and can cause power interruption in the aft mounted engines, or blade damage.

Some aircraft do use wing anti-ice only after ice has formed. In the 747 we do this based on Boeing recommendations, and don't generally turn it on at the first sign of ice. In this case, we're more concerned with "run-back" in which ice may melt or liquid may flow back from a heated leading edge, and then freeze. While runback can occur on the learjet, it's far less of a concern than the hazards to the engines, and it's the leading edge that's most critical. The lear uses a thin wing and the leading edge is much more important to it's function than a big, fat, thick-winged aircraft such as the 747.

[Von Klinkerhoffen]

Just to tidy up what Guppy has said about Lear wing leading edges . The 45 has a new and completely different wing design to all the Lears that have gone before . It's not quite so critical or sensitive and , as such , does not require a stall test to be carried out if a L/E section is removed and refitted during maintenace . Indeed , every 600 hrs the short L/E sections at the wing root and at about mid span are removed to inspect the anti-ice piccolo joints . Previous Lears though , up to and including the 60 , are a different kettle of fish ! You are not even allowed to remove two adjacent screws holding the L/E sections in place , without having a stall test carried out , before returning the aircraft to service ! One screw out , one screw back in before continuing on . The wing is very sensitive and just the minute difference in L/E position when refitting is enough to completly alter the stall characteristics of one wing.....with alarming results at the stall !! Many jets from this era share the same traits.....

[Mad (Flt) Scientist]

To continue the discussion on deicing vs anti-icing and the use of boots, I noticed this item from AIN today:

NTSB: Don’t Wait For Ice Buildup Before Inflating Boots

The NTSB released a new safety alert warning pilots not to wait for ice to build before inflating de-icer boots. “As little as one quarter inch of ice can be deadly,” the alert warns; “as little as a quarter inch of leading-edge ice can increase the stall speed 25 to 40 knots;” and “early activation of the de-ice boots limits the effects of leading-edge ice and improves the operating safety margin.”

The NTSB said some OEMs still advise pilots to wait for ice to build and that pilots should follow AFM or POH directions. Cessna’s Citation V AFM, for example, requires pilots to wait for ice to build to between a quarter and half an inch before boot deployment during normal operations. According to Cessna, “Our extensive research has shown activation of de-ice boots with a quarter- to half-inch buildup of ice on the wing is the most effective way to shed ice. It is significant to note that if our procedure was shown to be outdated and no longer acceptable, we would expect an AD [airworthiness directive].”

In fact, in the 1990s the FAA did attempt to issue an AD that would have forced OEMs to tell pilots not to wait for ice to build before deploying boots, but the FAA withdrew that proposed AD before it could become law.

In my opi nion, that first part of the Cessna spokesman's answer is fine: they believe what they are doing is the right way, and they can justify it.

But the second part is appalling - "in any case, we expect to be told what to do by AD" ??? What kind of abdication of your responsibility as an engineer is that? If something's wrong you do something about it, not wait for the FAA to tell you to fix it! That answer would be a great deal more reassuring without the second sentence.

[Von Klinkerhoffen]

In Cessna's eyes , nothing is wrong , their aircraft are not falling out of the sky each winter due to icing ! I think the FAA's position is one from the result of the ATR crash some years ago but they look to bring out an all encompassing AD to cover all aircraft equipped with de-ice boots . Each aircraft is different , they respond to ice in different ways and their boots may well shed ice in different ways too . I think what Cessna are saying is that why should we go to the expense of an AD when we know our AFM/POH procedures work for our aircraft ...if the FAA tests each of our types and proves otherwise... then we'll change !

[SNS3Guppy]

Both the NTSB and NASA has been hawking this demand to blow the boots as soon as possible for a time now. One chief assertion in this counsel is that ice bridging doesn't occur. I've no doubt that ice bridging DOES occur as I've experienced it on many occasions.

There are times to initiate boot inflation early in the icing process, and there are times to wait.

[rick1128]

A couple of points. First the Citation 1SP used TKS for the inboard leading edges. The only model to do so. The only jet aircraft to use this is the Hawker.
The Westwind, Astra and Galaxy (or G100 and G200) use boots.

From what I have been told, the reason for the stall test on the Lears is due to the original razorblade leading edge. The fat leading edges were originally mods, so it was easier for Lear not to change the maintenance manual. All the screws in the leading edges are counter sunk and I have yet to find any reference in the maintenance manual about checking angles or placement. I have done stall tests on Lear 35's with the fat LE's and as long as the flight controls are in rig, it stalls reasonably well.

As for turning them on prior to entering icing conditions, it is a long trip for the engines to the leading edges and it take awhile to warm them up properly, especially during descent.

[His dudeness]

TKS`ed Citation was the 550S (the one "between" the 2 and the 5) If you switch on ENG Anti ice on that one, the inner part was TKS ed (IMSC that part was called "cuff") . All the other 500/501/550/551/Bravo had electrically heated inner wing leading edges. The 5 and Ultra were bleed air heated plus boots on the rest of the wing and tail.

[Von Klinkerhoffen]

As for turning them on prior to entering icing conditions, it is a long trip for the engines to the leading edges and it take awhile to warm them up properly, especially during descent.I think not , it is not all that far to the wings from the anti-ice valves just I/B of the engine pylons , besides the duct work is insulated to keep the heat in and protect the surrounding structure from heat damage. Even in decent with power levers back , bleed air (it will be HP air ) will still be somewhere in the region of 200 degrees C plus I would think . I know from experience on the ground that the L/E's get very hot very quickly , granted you have no cooling airflow . If you allow ice to form before turning on anti-ice , that ice will very quickly shed in the airflow as soon as the heat breaks the adhesion.....where are the engines on a LJ45 !!! Better not to let it form at all.

It's interesting to note that the 45 has no radome anti-ice at all . Once , after an icy decent and landing , watching ice form on the vortilons and winglets , I managed to remove a fruit bowl sized 'ice mould' of the radome in one piece , the rest of the airframe was clean though .

[con-pilot]

I've no doubt that ice bridging DOES occur as I've experienced it on many occasions.


I have had ice-bridging as well in a King Air 90 and a 200. This occurred in moderate to heavy icing. After landing we broke the ice off the leading edge of the wing and we could see where the boots had been operating under the ice. All I could say was, thank God for those thick Beech wings.

I read the new report by NASA with great interest. I must say I do not know if I agree with the report 100%. I think that more attention should be given to the condition of the de-ice boots. The boots must be clean and some type of surface lubricant/polish must be on the surface of the boots to assist the ice blowing off the leading edge of the wing. If nothing else spray silicone on the boots prior to flight when you know you are going to encounter icing conditions.

As Guppy posted.

There are times to initiate boot inflation early in the icing process, and there are times to wait.

I agree.

[julian_storey]

I have had ice-bridging as well in a King Air 90

Me too and that was WITH a reasonable amount of ice.

The book says wait for half an inch before inflating the boots.