De-icing:NTSB: 'ice bridging' is a myth

 - January 28, 2009, 7:51 AM
Pilots should “activate boots as soon as the airplane enters icing conditions,” according to a safety alert released in December by the NTSB. The alert (SA-014) is yet another attempt by the Board to persuade pilots that there is no such thing as ice bridging and that pilots should not wait for ice to build to one-quarter to one-half-inch thickness before inflating boots in icing conditions. The NTSB has even added the subject to its Most Wanted List of Transportation Safety Improvements, specifically recommending that for currently certified aircraft, the FAA “require that airplanes with pneumatic de-ice boots activate boots as soon as the airplane enters icing conditions.”

The term ice bridging is used to describe a phenomenon in which ice forms a hollow shell over the de-icer boot. Conventional wisdom suggests that if pilots inflate the boots too early, before enough ice builds, instead of breaking off cleanly the ice can create a hollow shell beyond the extension of the inflated boot and the boot can’t break the shell of ice. The FAA and NTSB do not believe that ice bridging exists, and research conducted in a NASA icing wind tunnel has failed to substantiate the claim that ice bridging is a real phenomenon. The FAA held a conference on the subject in November 1997 to address the issue, and the consensus of NASA, FAA, NTSB, boot manufacturers and aircraft operators was that ice bridging is not real. According to the NTSB alert, “The Safety Board has no known cases where ice bridging has caused an incident or accident, and has investigated numerous incidents and accidents involving delayed activation of de-ice boots.” The NTSB does hedge its statements slightly, however, adding, “Ice bridging is extremely rare, if it exists at all.”

Although the NTSB wants pilots to inflate boots as soon as their airplanes enter icing conditions, instead of waiting for ice to build to a quarter or half inch, the Board acknowledges that aircraft manufacturers’ instructions are the primary reference. At least one manufacturer, Cessna, still recommends waiting for ice to build and specifically mentions in flight manuals for some Citations the possibility of ice bridging as the reason to delay boot deployment.

After the NTSB issued the most recent safety alert, NBAA responded with its own cautionary missive: “NBAA believes operators should continue to base their decisions about de-icing on their experience and judgment, because proving the existence of ice bridging after an accident is difficult, and many documented cases resulted in successful outcomes due to the skill and professionalism of the flight crew.”

AIN has interviewed pilots who claim to have experienced ice bridging. One told AIN that he saw ice build a shell over the inflated dimensions of the boots on a Piper Navajo that he was flying in heavy icing conditions. And NBAA Southeast regional representative Harry Houckes said, “I’ve seen it happen; it’s a fact of life. I’ve seen situations where if you don’t wait for some icing accumulation on the leading edge where the boots are and activate too soon, you push out a slushy mixture and it freezes and you have an icy extension beyond the boots. And you can’t get rid of it. Once it’s formed beyond the boot extension range, you’re [out of luck]. You can cycle the boots as much as you want.”

Houckes’ experience with ice bridging was when flying Beech King Air 200s and 300s, he said. Although the King Air has an automatic mode for de-icer boot actuation, he had a policy of using only the manual mode so ice could accumulate before first deploying the boots. Houckes doesn’t agree that all pilots in all boot-equipped aircraft should follow the advice to inflate boots as soon as icing is encountered. “It seems like the NTSB leaves you some wiggle room by saying ‘follow flight manual procedures,’” he said. “I don’t want to see folks coming into the industry getting lulled into a false sense of security. [Ice bridging] does happen; I’ve seen it.”

It should be noted that amid the continuing debate, the overarching advice to pilots who have to deal with flying in icing conditions is not to plow through the ice debating the finer points of ice bridging but to exit the icing conditions.

The NTSB safety alert warns pilots, “As little as a quarter inch of leading-edge ice can increase the stall speed 25 to 40 knots. The danger is that some quarter-inch accumulations have minimum impact and pilots become over confident. Sudden departure from controlled flight is possible with only a quarter inch of leading-edge ice accumulation at normal approach speeds. Early activation of the de-ice boots limits the effects of leading-edge ice and improves the operating safety margin. If the aircraft flight manual or the Pilot’s Operating Handbook specifies to wait for an accumulation of ice before activating the de-ice boots, maintain extremely careful vigilance of airspeed and any unusual handling qualities.”

Cessna Citation pilots should also know that there is an extra piece of information that affects the ice bridging issue. In the Citation 560 series, for example, the flight manual instructs pilots to wait for ice to build before inflating the de-icer boots. But when in approach configuration, pilots are cautioned to deploy boots immediately upon entering icing conditions. Despite possible confusion due to the differing advice depending on aircraft configuration, Cessna has no plans to change its icing procedures. According to a spokeswoman, “Cessna maintains that 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. At this time, Cessna does not plan any revision to its current flight manuals.”