Wx Watch: Ice bridging redux

The NTSB says one-quarter-inch of ice can be deadly

March 1, 2009

What timing! Shortly after the December 2008 “Wx Watch” (“Icing Facts and Myths”) came out, the NTSB issued a Safety Alert. The topic: ice bridging, which is a controversial phenomenon that may or may not exist, depending on who’s talking. Ice bridging occurs when ice accretions build over de-ice boots. In spite of the expanding and contracting boots, the ice buildup is such that it doesn’t break off. This is most likely to occur when boots are inflated too early and too often during the onset of icing. At least that’s the time-honored advice that persists to this day—in many pilot operating manuals, and in anecdotal yarns. So tradition holds that you wait until one-quarter to one-half-inch of ice forms on airfoil leading edges before inflating de-ice boots. This way, you’re apt to cause a more complete shedding of ice.

But a 1997 NASA ice bridging conference involving government and academic experts determined that ice bridging does not exist. Yes, pieces of ice may adhere after a too-early boot inflation, but NASA said that experience has shown that this residual ice will soon break off in subsequent inflation cycles. That’s the information I passed along in the December 2008 article.

NTSB’s Safety Alert

In a timely move, the NTSB issued its alert. The full document can be called up online. To paraphrase, the alert says the following:

  • “As little as one-quarter-inch of leading-edge ice can increase the stall speed 25 to 40 knots. The danger is that some one-quarter-inch accumulations have minimum impact and pilots become overconfident.
  • “Sudden departure from controlled flight is possible with only one-quarter-inch of leading-edge ice accumulations at normal approach speeds.
  • “For 60 years, pilots have been taught to wait for a prescribed accumulation of leading-edge ice before activating the de-ice boots because of the believed threat of ice bridging.
  • “In theory, ice bridging could occur if the expanding boot pushes the ice into a frozen shape around the expanded boot, thus rendering the boot ineffective at removing ice.
  • “The Safety Board knows of no known cases where ice bridging has caused
    an incident or accident, and has investigated numerous incidents and accidents involving a delayed activation of de-ice boots.
  • “Ice bridging is extremely rare, if it exists at all.
  • “Early activation of the de-ice boots limits the effects of leading-edge ice and improves the operating safety margin.”

The alert goes on to make some recommendations:

  • “Leading-edge de-ice boots should be activated as soon as icing is encountered, unless the aircraft flight manual or the pilot’s operating handbook specifically directs not to activate them.
  • “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.
  • “Turn off or limit the use of the autopilot in order to better feel changes in the handling qualities of the airplane.”

While the NTSB hedged a bit, it seemed to follow NASA’s lead of 12 years ago. So what do our actively flying member-readers say about the subject? Is ice-bridging real, or do the eggheads have it all wrong?

Member response

Almost immediately after the December “Wx Watch” hit the mail, my e-mail inbox went into overdrive. Seems many pilots have experienced ice bridging, contrary to the official advice. Here’s an edited sampling:

Bill Rimer wrote, “I’ll take real-world knowledge any day over limited testing simulating icing in only a few types of airplanes…I 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. I would find the ice didn’t break off cleanly and there would be areas with ice that almost always would stay on the leading edge of the wing for the duration of the flight…with light ice it is almost always best not to inflate the boots at all to leave a rough leading edge and let the air polish and sublimate the ice.”

Jeff Webb said, “Ice bridging does exist. I’ve seen it. Other pilots who fly with me have 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. center of gravity, angle of attack, rate of accumulation, and type of ice. I currently fly a Metroliner and have logged 5,000 hours in this one serial number. 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.”

Terry Biebel e-mailed to say, “I invite you to come along on a typical flight in the Cessna Caravan…ice simply will not shed on the Caravan until there is an 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. Your article is very informative, but I strongly disagree with the theory. I argued with fellow pilots who were senior to me. ‘NASA says,’ I would tell them. They would just say, ‘you’ll see!’”

So the debate continues. I’m willing to accept—especially after this mail—that under certain worst-case, high-accumulation-rate conditions, ice bridging can occur on some types of airplanes.

On the other hand, NASA, NTSB, and other organizations have studied the problem extensively, so I wouldn’t dismiss their findings out of hand. In any event, the old rules still apply, and on that everybody agrees: Avoid icing at all costs, and escape it the moment it’s encountered.

E-mail the author at tom.horne@aopa.org .

Web site of the month

The NTSB has been vocal in its pronouncements on icing. You can download and print out the NTSB’s “Most Wanted List” icing recommendations online. The information makes good instructional material for flight instructors to hand out to students, and serves as a quick summary of the accident-related icing safety recommendations. And in case you want to see the innards of an icing wind tunnel, go online for a look at some of the work the NASA Ames Aeronautical Test Community is doing on tailplane icing. That’s a subject we’ve discussed before in “Wx Watch,” and will no doubt revisit in the future. After all, icing never goes away.

Thomas A. Horne

Thomas A. Horne | AOPA Pilot Editor at Large, AOPA

AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.