Tom Horne

Ice-bridging brouhaha

March 6, 2009 by Thomas A. Horne, Editor At Large

In my recent Wx Watch article “Ice Bridging Redux” (March 2009 issue of AOPA Pilot) I mentioned the NTSB’s recent safety alert on ice brdiging and the operation of de-ice boots. “Pop the boots at the first sign of icing” is the distilled message from NTSB. The Board also expressed the belief that ice-bridging doesn’t exist at all! This concurs with the ideas expressed at a NASA ice-bridging conference in 1997.

So is ice-bridging real? Do you believe in it? If member feedback is any gauge, there are still a lot of pilots who will always wait for some small amount of ice to accrete before inflating de-ice boots. No matter what the guidance from the NTSB or the Airplane Flight Manual says.

The “Redux” article had no sooner hit your mailboxes than my e-mail inbox began to go berserk. The feedback was so informed, voluminous, and expansive that we couldn’t run the mail in the magazine’s “letters to the editor” section. Instead, we’re providing some of them below, so that you can see for yourself that this issue is still very much alive.

I remember the “knee jerk” reaction to the ATR accident at Rose Lawn. The lesson learned from that accident is don’t hold in severe icing…go somewhere else and you should be out of it in 100 miles or so. I’ve carried a bunch of ice on my leading edges and I do prefer the hot wing as opposed to an inflatable device. Ice bridging does exist. I’ve seen it. Other pilots that fly with me have seen it. I’ve unbuckled, gone back to get a closer look out the cabin window, and have actually seen it. Do I think it happens often? No. I think that many factors have to be just right for bridging to exist, i.e. CG, AOA, rate of accumulation, and type of ice for example. I have a two-position switch, Auto and Manual. I’ve experimented with both. Letting the ice accumulate and using the manual mode does leave a cleaner wing but the Auto mode works fine most of the time. Auto mode is good when you get “task saturated”.

–Jeff W., ATP/CFI/MEI

I invite you to come along on a typical flight in the Cessna Caravan. A “fact” in my world is that ice will simply not shed on the Caravan until there is adequate buildup. Usually, if I cycle too early, the ice fractures, the boots deflate, and the ice remains. I am in my first real season of night freight, five days a week. I strongly disagree with theory, and not reality. I argued with fellow pilots who were senior to me. “NASA says,” I would tell them. They would just say, “you’ll see!” I believe the relatively low speed of the Caravan and the massive leading edge contribute to this condition. Survey many Caravan pilots and I think you may be quite surprised

–Terry B.

I am no stranger to aircraft in-flight icing and frost on wings in general aviation types of aircraft. I had seven years of experience flying charter out of the Great Lakes region, and then over 25 years of charter and schedule flying in Alaska. My first experiences with learning about icing were warming the right seat on flights with very experienced pilots. All these pilots stressed never to inflate the boots until first experiencing a substantial ice buildup and loss of airspeed due to icing. I have personally never experienced bridging of operating boots, but every time I cycled the boots before there was a good buildup of ice I regretted inflating the boots early. Every time. The bottom line is not all planes are alike nor is the type of ice or its formation, and a one-size-fits-all recommendation like the constant use of boots as you and the FAA advise can cause more of a problem than it is supposed to eliminate. I’ll take real-world knowledge gained through real-world experience any day.

–Bill R.

A few years back I had an opportunity to fly a winter trip in the Midwest with ceilings at 8,000 with temperatures just below freezing at that altitude with warmer air below. I was flying a turbo-210 certified for flight in known-icing. ATC let me climb up into the icing ceiling and I immediately started to accrue rime ice. I activated the boots at a point that I would consider too soon and the ice just pushed out and stayed, though some did shed. ATC let me back down into the warmer weather and I waited until all the ice had melted (sublimated?) off. I then got ATC permission to climb back into the ice. This time I waited until I had a nice layer of ice accrued. I could see the impact of the ice accretion in the the slight pitch up and airspeed impact and I activated the boots. I got a much cleaner wing. ATC then let me down out of the ice. I flew home convinced that activating the boots too early could be a problem.

–Gary R., CFIA

In my former position as the senior FAA icing specialist, I have worked with NASA and sponsored the tailplane stall research and was the FAA project engineer for tailplane stall, roll upset and SLD and taught icing accident investigation at the Transportation Safety Institute after I retired from the FAA.

I seem to recall one of the pilots at NASA telling me rime was no problem. To which I can cite a number of cases which even a little (trace) was too much.  One that stands out was a production flight test on the stall protection system of a Gulfstream. The pilot took the airplane toward the stall and the airplane rolled approximately 90 degrees. The aerodynamicist sitting in the back observed a thin narrow band of ice on the engine inlet and suggested that the pilot turn on engine and wing heat. The few seconds cleaned up the wing and the next stall was a clean nose down break.

I try never to use the terms “rime”, “glaze” or “mixed”. I have gotten so many disparate and meaningless descriptions that I cringe when I hear that. The wing is only interested in location, shape, texture and thickness—thickness being not the most critical attribute. If all goes well, the FAA will adopt a revision to the AIM soon that introduces the concept of icing severity effects in lieu of icing severity. I developed it about 11 years ago based on hundreds of icing events I researched and I am surprised it has progressed this quickly!

While we have had no modern accounts of bridging, there have been cases of what is best described as “distributed roughness elements”. These start life as a small accretion on the leading edge that need not be much larger than a few thousandths of an inch high. Other droplets will quickly form on these and build a very small but aerodynamically adverse shape, in some cases resembling the tooth of a rasp or perhaps a shark’s tooth. Since these do not touch they are unlikely to shed and will remain on the surface of the boot. The boot will inflate and deflate and that is what some accounts of bridging reflect. These can be very nasty too and in just a few minutes will increase stall angle and increase drag. These accretion sites can be formed in Appendix C conditions or supercooled drizzle drops. All icing clouds are not the same and change in time and space.

Most critically, what would have saved the ATR-72 in Roselawn, the ATR-42 in Crezzo, Italy, the EMB-120 in Monroe, Michigan and perhaps the DHC-8-Q400 in Buffalo is a change to the post stall recovery training?

There are a few people who got it right and kept the airplane flying and right side up even though it meant an off airport landing.

–John P. Dow, Sr.

I have 8,000 hours in a Jetstream, most of it PIC. We had 3 derivatives of the Model 31 and at least one had boots that could be operated either manually or automatically (periodically). Nobody used the “auto” mode because of bridging. I subscribed to the “bridging” concept for many years, but something the NTSB said and an important event in my life cause doubts.

NTSB: “No known cases where ice bridging has caused an incident or accident”. Hmmm, interesting.

One day as we lifted off, I noted that the oil pressure gauge was going berserk. I did not comment on it and there were no other indications that we had a problem. I was grooming the F.O. for Captain and wanted to see how long it would take her to notice the gauge. During the trip I had pulled out the Emergency Check List just to feel confident on how to handle this situation. Don’t do anything until there is a second indication. She commanded an engine shut down which I performed. She had everything perfect but the plane was icing up and performance going south. She asked for boots and I advised her I did not think there was enough ice yet and perhaps closer in would be better. She confirmed that the plane was not doing what it should and repeated the boot request. I popped the boots and although some ice was still visible on the leading edge of my wing, the plane became vigorous, speed increased and we went above glide slope. I took the plane and landed without incident. The change in single engine performance was incredible! Caused me to think hard about bridging.

–Marty

I am an 18,000 hour airline captain whose background is 100 percent GA trained over 43 years. There is most certainly a phenomenon known, years ago, as rubber ice. I have experienced it.  I do have a different explanation on the phenomenon. If ice is allowed to build up to say half an inch, the boots crack it into, for arguments sake, four-inch pieces. There is an edge created that is one-half by four inches, or two square inches of the ice’s edge exposed. The airflow has this area to force the ice to peel away from the boots. However if the ice is only one-eighth inch, then there is only one-half-square inch. Not enough force can be applied to the edge of the ice to peel the ice from the boot by the airflow. To complicate things, the ice could be thin enough to be flexible and just move with the boots or shatter into tiny pieces without peeling, making the ice flexible. You have not lived until you see the leading edges turn white and you turn on your security blanket switch and you see black lines in the leading edge but the leading edges stay white as the boots collapse. Automatic deicing switch positions have been installed for years on multiengine airplanes, but the techniques passed along from Captain to young first officer had warned against using it except in the once in a lifetime severe icing episode. As an old-timer, I worry how we can continue to fly safely as we lose the intellectual property developed over years of aviation passed on from experienced to novice as we enthusiastically accept dependence on technology.

–Paul E.

12 Responses to “Ice-bridging brouhaha”

  1. Chris Says:

    I was a captain on a Saab 340 turboprop for a regional airline. The FAA requires the use of the auto position of the boot switch when in icing conditi0ons, so this was standard operating procedure.

    My experience is that when there is very little ice on the boot, the inflation rarely, if ever, breaks off all of the ice. That being said, I’ve NEVER seen “bridging”. Instead, what happens is that when the icing severity picks up a little and there is a little more ice, it mostly breaks off at that point. In my experience, it is rare for any boot cycle to break off all of the ice in one cycle, but it is also my experience that I’ve never seen any significant ice on the boot after a few cycles. I’ve had COUNTLESS experiences in which a boot cycle finishes with ice still left on the wing only to have a later cycle blow it all off once the icing severity picks up a little.

    I guess my point is that people maybe shouldn’t go into ice, blow the boots twice and still see ice there and say “wow – bridging is for real.” Just because some ice remains, doesn’t mean it’s going to bridge. However, it would really be awful if you had an asymmetric stall in some minor turbulence because you were worried about blowing the boots too soon due to “bridging”. It seems like most of the “experience” in these letters are “I tried it once that way and I was scared after seeing some ice.” I suggest they try ithe boots on auto for a significant length of time and see if it changes their minds about bridging.

    NASA pays a lot of people a lot of money to study this type of thing (and in real aircraft, too – not just the laboratory) – they’re usually right.

  2. John S. Says:

    I have watched this debate about icing very carefully. I fly a 421 and get into ice frequently day and night. I always was taught to let the ice build up before useing the boots. That method does not work for me. I’ve tried cycling the boots with different amounts of build up and what I’ve found best was, about 1/8 inch. If I let it go to 1/4 or more, the ice never seems to leave even after cycling several times. Cycling at a trace, or first indication of ice is a waste of time but 1/4 inch or more as most pilots suggest is too much ice. I believe the shape of the wing has a lot to do with ice accumulation and shedding capability. The 421 seems to be an ice magnet. I dont’ think it flies as well with ice as my Bonanza. I will keep using my tried and tested method of cycling when I get 1/8 inch. One final note no one has mentioned and is important. Keep the boots in good shape and treated properly when the icing season comes around..

  3. Jason A Says:

    Call it what you want, ice bridging or rubber ice, But I can tell you if you don’t allow the ice to build up at least 3/4-1 inch thick on a Merlin, you’ll never get a clean wing. Same goes with 210′s and barons. I think it’s just the inefficiency of boot systems. Boots work fine when operated correctly for your particular airplane. Most I’ve found do require a fairly significant amount of accretion to effectively shed when you pop the boots.

    However, we can’t even get the FAA to make up their mind about a definition of “known icing!” Best defense for pilots is when you find yourself in the ice, DO SOMETHING ABOUT IT NOW. Don’t hold and don’t screw around with the stuff. Get out of it as soon as you can. And if you fly a van, leave the plane on the ground PERIOD!

  4. heavy metal pro air Says:

    NEVER, EVER inflate the boots until u have at least a quarter inch of ice on them@! I have 24,000 hours (how much time do you have Mr. FAA/NASA?) and that is in everything from Cubs, Navajos, biz jets, to 767′s. I flew air taxi in piston twins for years with boots and I can tell u if u inflate boots prior to ice buildup u will die. Are u gonna believe NASA who has to scrap 26% of their shuttle missions and has a horrible safety record (blown up shuttles, fires, etc) or are u gonna believe the “voice of experience”. NASA and the FEDS are totally irresponsible for publishing so called “facts” when they have very little experience in the real world of ice flying freighters 6 nites a week for 30 years like some of us real timers have! By the way, my accident record is flawless, NASA’s is horrible!

  5. Gordon Graves Says:

    Icing makes for an interesting debate. Let me preface what I am about to say with this: I have a great deal of respect for my compatriots at NASA and the FAA. However, experience has taught the effect of political expedience on otherwise impeccable scientific research and/or technical data. I know they have our best interests at heart. But, I have come to question the motives of any government bureaucracy, because I should… For that matter, how do we determine if the change to selecting “Auto”, by itself, will have been effective, when included with the dissemination of other knowledge gained from testing, evaluation, and accident investigation?

    So often we hear “after extensive testing”, or “after extensive evaluation”, “it has been determined that…” How extensive is extensive? Could it be that one of the many considerations for this determination was the knowledge that, at the time some accidents forced the issue into the lime-light, the “manual technique” (after a certain amount of accretion) was the most espoused? Certainly science and technology could not produce the urgent fix-all that could assuage the threat of a Congressional hearing, the way a simple change in technique could. Irregardless, due to the insidious nature of ice and the plethora of lawyers and a legal system that favors their litigious aims, we must now add the de-ice switch to “AUTO” to the expanding list of switches that must be selected for optimal affect in court, should an event occur. And, in the event of litigation, that we are told to select “Auto” would, at least, afford the government some relief, if not the pilot or crew. As I said, “I do know they have our best interests at heart”.

    Experience is a hard teacher. She gives you the test first, then the answer. We’ve all learned some things from experience. As a fledgling flyer, I had great opportunities to gain knowledge from mentors (Chef Pilots and Captains) who survived some of life’s tests and shared their experience.

    During my career spanning 40 years as a professional aviator I have come to know that life is a test. If the test were an FAA test, the correct answer to the question of which to select for de-icing is “Auto”. However, icing is insidious, always leave yourself an out. Fly as though Anti-ice/De-ice equipment will fail; where are you going? Developing this attitude is no different than if an engine fails; where are you going? Just different ingredients for a different soup. I’m sure NASA and the FAA didn’t intend “Auto” to be a fix-all. Neither is it a panacea, even when you are reaching the limits of task saturation, ice doesn’t care! “Manual” is still available and always will be. If “Auto” doesn’t provide the desired result or doesn’t work, switch to “Manual” as often as necessary. Either way, I know of no aircraft certified for flight into heavy or severe icing. For that matter, I have yet to fly an aircraft certified for flight into known icing conditions that the Flight Manual does not recommend leaving the icing environment, as soon as possible, when icing conditions are encountered.

    It’s not so much a question of which as it is when. Now that I think about it we had “Anti-ice (De-ice)” inserted at several locations on our (FAA approved) checklist. Interesting how few times the need for anti-ice (de-ice) corresponed with the checklist call-out. Icing is a fact of life. If you’re in it, it should be a part of your scan.

  6. Paul Buller Says:

    My experience is that it is more a matter of the treatment applied to the boots than exactly when you cycle the boots. Our airplane detailer used to apply a floor wax to the boots of our Turbo Commander to make the boots look shiney. They looked great but the ice would stick like glue to the boots. After some investigation I was told about B.F. Goodrich Icex treatment for their boots. I was told that it looks ugly but it works well. I can verify that statement is true on both counts. The ice would come off completely with just one inflation of the boots. I have always let a little ice build up before activating the boots. That seemed to work fine. As far as waiting until an 1/8 or 1/4 or 3/4 inch builds up all I can say is while flying hard IFR during an approach in the mountains I would find it very difficult to determine the thickness of the ice buildup by looking out the window. I think it is best to cycle the boots when you see ice on them. If you can see the ice it is probably thick enough to break off when you cycle the boots.

  7. Jack Webb Says:

    I have not experienced ice bridging but I have had ice that did not want to release from the boots. Later we discovered, doing icing tunnel tests in Cleveland, that using boots not cleaned and dressed, it took more force tangential to the boot surface to break the ice loose. This probably translates to the ease with which the airstream would help the ice depart the aircraft. Boot dressing is important!
    Additionally I had a problem holding in LearJet where the ice melted from the leading edge but flowed back and refroze in the gap between the aileron and the wing. This was nasty and was most probably caused by the low speed for holding of about 190 indicated. It took over 35 pounds of pressure on theh control wheel by both the copilot and me to free the ailerons. There other problems caused by the hold and we told Center we needed immediately another altitude or clearance from the holding pattern. We increased our speed to over 320 knots which eliminated the problem.

  8. Andrew Stryker Says:

    Heavy Metal Pro:

    Please stick to hauling freight. The idea of the captain of my next flight having an attitude like yours frightens me more than the prospect of getting out on the wing with a ruler to see if that’s 1/8″, 1/4″, or some other arbitrariy thickness of ice out there.

  9. M.Bolski Says:

    My experience is small 1500h. I ferry airplanes across country incl. Alaska, North Atlantic, over Artic circle, Europa and not always have the pleasure of using any de-ice or ant-ice equipment. Well, the FAA jurisdiction don’t go over international water, but it doesn’t mean I would not follow FAA or NASA recommendations. I learn from experience of others fellow pilots (dead and alive), FAA & NTSB statistic, recommendations and so far I’m still alive getting my own-experience flying in very un hospitable environment . I think the subject of Ice-bridging is more in theory than in real life. Yes – if boots are activated to often or to early, they will not clean the edges so good , but this got nothing to the subject. If boots become stuck in inflated!!! position! for too long in a freezing condition, then we can talk about ice-bridging. My personal limits -get out of ice condition ASAP at all cost. Doesn’t make a difference if I’m flying with de- or ant–ice equipment. I think more important is: ice accumulation on high performance wings (C400, Cirrus, Extra 300) vs. old timers (C182, C206) I know for a fact, that C182 with no ice protection, almost 2 in. rim ice and even more on landing gear will fly fine. Cirrus with TKS activated too late! will scare the live out of you with just some ice left on edges and probably a lot on landing gear. There is something call wet wing effect (don’t mistake with TKS) where water contamination (rain only) can increase stall speed. Flying Extra 300 in heave rain on approach close to stall speed and max weight ??? I know people who end up on a wheelchair. With respect to all heave metal pilots with zillion hours, it’s easy to drink coffee at FL300, flying C172 non-stop for 8 hours over North Atlantic with Ice –X blown from leading to trialing edge in unpredicted ice condition— that’s what’s call experience. Coffee guarantee, but in a diapers. Sorry for my English –but it is to me, a second language.

  10. Andy Reardon Says:

    Bridging most definitely happens! In the American Bonanza Society Magazine in January, 2005 (might have been 2006. I’m on vacation now and don’t have the copies with me), I published an article regarding my icing experience (one of them) on a flight in a Beech Baron 58 in March 1989 from Cincinnati to Waukegan, Illinois. The flight was terminated at Palwaukee due to closure of Waukegan caused by a corporate aircraft skidding off of the runway at UGN due to ice.
    After a missed approach due to low vis at PKE, I landed after a second trip through the 4,000 foot inversion layer. On the second approach,the Baron picked up substantially more ice than on the first approach. I had the boots operating frequently–too frequently. When I landed and Priester Aviation towed me into a hangar, there was ice on the leading edges of the wings and on the outer edges, the boots were covered over, with a large hollow area between the boot and the ice. My only question was how the aircraft still flew with all of the ice, both weight wise and aerodynamic interference. (However, I landed with nearly full power and did NOT TOUCH THE FLAP SETTING after the ice began accreting, consistent with my military instructor’s advice from years earlier.) Another observation: the ice had bridged at about the same distance on both wing leading edges. I could not tell on the stabilizer, though I looked; however, the ice had dropped off of the horizintal stabilizor (but not the wings) by the time I had the opportunity to inspect it. I even tested the boots in the hangar, thinking that perhaps there had been a malfunction. They operated perfectly. Then, after my knees quit knocking, I called my wife for a ride home. Yes, we stopped and had a pop on the way…
    Bridging? You bet. Do not ever think that it cannot happen. I know.
    By way of background, I am 63, started flying in 1967. Yes, I still fly a Baron 58 ( a different one).
    In case one may wonder why I waited from 1989 to 2005 to write the article, it was because I figured that after 16 years, the Fed would not pursue me!
    All the best,
    Andy Reardon

  11. Jim Harris Says:

    Ref. Ice Bridging.
    Jack Webb indicated (14-Mar-2009) that he had to use 35 pounds of force in a Learjet to overcome melted ice that ran into the aileron gaps.

    I experieced a similar situation in a Lear-36, BUT I attributed it to the maintenance department powerwashing the gap seals and dislodging the silicone “grease” on the “Scrub Brush” like material in the gap seal. We duplicated this later at a high altitude that was well below freezing and then went back the same day when the proper “grease” was applied had no problem. (It does get your attention when you think the ailerons are jammed, but we had warm temperatures when descending through the mid-teens.)

  12. FAA test Says:

    Regarding ice bridging, I have never seen it.Ice bridging is extremely rare, if it exists at all.

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