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
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.
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.
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.