"One hundred and twenty miles an hour! Only a few minutes before we were cruising at one hundred seventy ... We must not lose any more ... What the hell is wrong with those fancy deicer boots? They are not performing the task for which they are intended ... The blinking red-light indicator shows the deicers are in operation, but outside there is visible proof that they are lying down on the job. The leading edge of the wing is now one long, unbroken bar of ice. And it is clear ice, rumpled as if there were rocks beneath.
Yes, the boots are working. But they are expanding and contracting beneath the sheath of ice and consequently useless. The ice has accumulated too fast for them ...." — Ernest K. Gann, Fate Is The Hunter
Famed author and airline pilot Ernest K. Gann was the first to dish up this frightening image of a phenomenon known as ice bridging. This is a condition that all aviation texts and flight manuals warn us about so diligently. It occurs when deice boots are inflated too often. This expands leading-edge ice accretions and pushes them forward, leaving the boots to pulsate helplessly in an air-filled gap between the wing and the ice. To prevent this from happening, we're told to inflate deice boots only after a specified thickness of ice has accumulated — say, one-quarter to three-eighths of an inch. Then you wait again for ice to form, and you only blow your boots when there's enough ice to make it worthwhile.
This decades-old advice, prompted in no small measure by Gann's hairy DC-2 flight from Nashville to New York, has faced a recent challenge. Government and industry experts now say that there's no such thing as ice bridging, and that Gann's misfortune was most likely caused by old-technology deice boots, which have comparatively slow inflation and deflation cycles, fewer and larger inflation tubes, and lower system pressures. Today's boots operate faster, have more and narrower inflation tubes, and generate higher inflation pressures, so they're much more efficient at removing ice.
The ice-bridging workshop
In November 1997, the FAA and the National Aeronautics and Space Administration (NASA) convened a workshop to hash out the issue of ice bridging. The safety and operational implications of the workshop findings were significant.
Here was the main problem: Everyone in aviation knows that even the smallest ice accretions can severely impair lift, handling characteristics, and stall speed and behavior. This argues for immediate deice boot inflation. But aircrews — particularly those flying turboprop commuter airliners — fly by flight and operational manuals that urge pilots to wait for ice to build before inflating boots. So crews flew by the book and kept Gann in the back of their minds.
But what is the safest technique, workshop participants wondered? Inflate, or wait? Tolerate performance losses while ice builds to the prescribed level, or risk ice bridging by cycling the boots too much? To answer these questions, the nature of ice bridging had to be examined and quantified. The ice-induced crashes of an ATR-42 in 1994 and an Embraer 120 in 1997 made the issue even more urgent.
The verdict
To make a long story short, experts from the FAA, NASA, BFGoodrich, Cessna, and Embraer concluded that ice bridging is a myth, a kind of urban legend of the aviation world. Embraer officials said that there was no observation of ice bridging events in its EMB-110 and -120 fleet of turboprops. This came after certification flight tests in actual icing and tanker-generated, spray boom icing conditions, and 4,900,000 flight hours in service over 12 years. Prior to 1996, when an EMB-120 rolled over and crashed while maneuvering in the Detroit terminal area, Embraer flight manuals adhered to the traditional advice of letting one-quarter to one-half-inch of ice accumulate before actuating de-ice boots. After 1996, Embraer changed its tune and said to turn on the boots at the first sign of ice.
Cessna also said that it hadn't had any experience with ice bridging. The automatic cycle activation system used on the Cessna 525 CitationJet's horizontal stabilizer, for example, proved effective and demonstrated no ice bridging. Any ice that didn't shed upon initial boot inflation of the CitationJet's three-minute automatic cycle time increased in thickness, but shedded during subsequent cycles.
BFGoodrich (BFG), a longtime manufacturer of deice boots and related equipment, could not duplicate ice bridging in any of its tests, either. These included tests of airfoils in BFG's own icing wind tunnel, and in the harsh natural icing conditions atop New Hampshire's Mount Washington. Airframe manufacturers who use BFG's deice boots also have made no reports of ice bridging.
BFG's new Smartboot deicing system, certified on The New Piper's Malibu and Malibu Mirage, also demonstrated no tendency to bridge ice. Smartboot is an ice-detection system that confirms boot inflation, verifies ice removal, and detects any residual ice after an inflation cycle. The idea here is to lessen pilot workload and enhance safety by avoiding tailplane stalls and ensuring the elimination of all ice from protected leading edges.
Residual ice
Residual ice is the ice that clings to deice boots after an inflation, and doesn't shed between inflation cycles. Usually, this type of ice is randomly spaced along the span of the boot and has small cracks that open wider and expand when a boot inflates. If residual ice expands and collapses with the boot, this does not constitute ice bridging.
A theory has been advanced that pilots mistake stubborn residual ice for ice bridging. In this sense, pilots may have a semantic problem, with the result that the ice- bridging "problem" has been overrepresented (and maybe even overembellished) over the years. But if the ice moves with the boot, it's not bridging.
Shedding behavior
"Generally speaking," said BFG, the more ice that accumulates on a leading edge prior to boot inflation, the more it sheds when the boots are popped. In one example, BFG said that if one-quarter-inch of ice were allowed to form, then 50 percent of that ice would shed after boot inflation. But if one-half-inch of ice were to accrete, then 80 percent of the ice may be removed. So this evidence favors letting at least some ice build before activating a deice system — at least at cruise speeds and configurations, or in configurations determined to be least sensitive to what could be large ice-induced changes in lift, or pitch and roll motions.
Here are some other BFG findings:
- Clear ice (what BFG calls "warm" ice because of the warmer temperatures in which it forms) has a higher breaking strength.
- Rime ("cold") ice has a much lower breaking strength.
- Ice adheres more tenaciously to rubber as temperature decreases. Practically speaking, BFG says, "the ice layer needs to be thicker at colder temperatures in order to remove the same percentage of ice at warmer temperatures."
- Use of BFG's Icex boot treatment minimizes ice adhesion, and promotes better ice shedding.
Workshop conclusions
The NASA workshop came up with four main conclusions. First, it found no evidence of ice bridging. It also said that ice-protection systems that inflate boots automatically can shed ice successfully at the onset of icing. Any ice that doesn't shed on initial boot inflation continues to increase in thickness and sheds during subsequent inflations. Finally, it determined that activation of deice boots at the first signs of icing was warranted.
How it can happen
That said, ice bridging can happen. In fact, the phenomenon was observed in icing wind-tunnel tests. Bridging took place only when boots were given very long (greater than 10 seconds) inflation and deflation times. This is considered a consequence of a failure in the deice system. Such failures include faulty timing devices (the units that govern the sequence and rates of inflation and deflation of all the booted surfaces), bad or iced-up valves or venturis in the ice-protection system, leaks in the boots, or very low airflow or pressures into the boots — especially in piston-powered airplanes operating at low engine rpm.
New rules, and placards
The FAA took note of this and other findings, and recently issued a notice of proposed rulemaking (NPRM) dealing with boot-inflation technique. As NPRMs go, its enaction as an airworthiness directive (AD) would have a mild effect, just a placard on the panel and a notation in the flight manual. The placard would say to inflate deice boots at the first sign of icing. So that makes it official: The FAA says that ice bridging doesn't exist.
Ice flight
Have boots on your airplane? Have known icing certification? That's great. Keep them in good working order, patch any leaks immediately, and remember to inflate them when you first notice ice.
But don't become complacent. Every icing expert in the world will tell you that ice-protection systems are designed for you to make a safe exit from icing conditions. They don't give you carte blanche to loiter in ice indefinitely. So the advice to turboprop pilots with deice boots is the same as the warning given to those of us (that would be the overwhelming majority) who may fly without any ice protection other than the bare minimum: Escape from icing conditions as soon as you can.
Of course, you've already turned on the pitot heat if there's even the slightest hint of visible moisture; you've opened your engine's alternate air doors (some aircraft types have doors that automatically open when the intake air filter becomes blocked with ice or other contaminants); and you've developed an escape strategy during the preflight weather briefing. Ideally, you'd climb or descend to warmer temperatures, and ultimately fly to an airport without experiencing ice during any descents. Temperatures at and below any minimum en route altitudes (MEAs) along your route of flight are above freezing, of course. If not, you're asking for trouble — whether you have boots or not. Inflating deice boots may save your bacon during an iced-up approach and landing, but ice shapes on unprotected surfaces can increase drag and change the airplane's handling characteristics.
Links to additional information about airframe icing can be found on AOPA Online ( www.aopa.org/pilot/links/links9912.shtml). E-mail the author at [email protected].
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