January 1, 2003
By Thomas A. Horne
In the past two issues, "Wx Watch" has delved into two aspects of the general aviation icing problem. In the November issue (see " Icing on the Internet," p. 161), there was a discussion of the importance of preflight information, with the emphasis on a new icing-depiction product on the Internet, the Aviation Weather Center's Current Icing Potential (CIP). In the December issue (see " Winterwise," p. 131), we talked about icing situations, and about which variables made for the safest winter flights.
Now it's time to shake hands with the devil. You're picking up ice.
The National Aeronautics and Space Administration (NASA), the FAA, and the AOPA Air Safety Foundation (ASF) have teamed up to produce a highly informative 55-minute DVD on icing. NASA's Icing for General Aviation Pilots delves into several operational scenarios involving flights in unprotected and known-ice-approved airplanes. An added bonus is a special section on tailplane icing. The DVD (item D410A) can be ordered from Sporty's Pilot Shop (800/SPORTYS; www.sportys.com) for $5.
Let's say your careful planning has gone awry. Perhaps the forecasters got it all wrong, and dense clouds have moved in. The briefer said it would be 4,000 scattered, but that was long ago. Now it's looking like 1,000 broken, with tops reported at 12,000 feet. You try to weave around the clouds, but staying clear of them is fast becoming a losing battle.
Maybe you've been pressing on through worsening conditions, telling yourself that the weather will get better any minute now.
Perhaps you've been flying at night, then suddenly realize that you've been flying in cloud for?how long? All you know is that when you turned on your landing light, its beam lit up an opaque white cone. You're in solid IMC.
Tensions build. You check your outside air temperature gauge. It shows minus 2 degrees Celsius, but it tells another, more disturbing story: Its probe is covered with ice. Now you look around the rest of the airframe, and there it is — ice on the wing leading edges, on the protruding rivet heads, creeping up the windshield.
You've iced up, and now you have to deal with it. Here are some guidelines to help ensure the flight's safe outcome.
Whether you're flying a Skyhawk or a big twin that's certified for flight in known icing conditions, the number-one rule is the same: Escape icing conditions at the first sign of ice accretions.
You have several options, all of them hinging on the information you've learned before and during the flight. You can climb to on-top conditions or to an ice-free area between cloud layers. Hopefully, you'll have some idea of the vertical limits of any cloud layers or tops along your route. This is where pireps can come in very handy indeed.
A descent might be in order, assuming that you'd be descending into above-freezing temperatures, and that terrain or minimum en route altitudes (MEAs) aren't factors. Diverting to an alternate airport with better weather, or performing a 180-degree turn, also can be wise options.
Air traffic control can be a great help when you start collecting ice. Don't hesitate to ask controllers for altitude or route changes, and be assertive if they are uncooperative. If an altitude change would create a conflict with other traffic, tell ATC that you'd be willing to take a heading change to help out with traffic separation.
Picking up ice at an alarming rate? Then don't hesitate to declare an emergency and get the kind of high-priority ATC handling that can go a long way toward the flight's safe outcome. How do you judge the rate of ice accretion? It's beside the point, from a tactical perspective — because you should carry out your escape plans the moment you first notice ice. But for the curious, experts now say that "light" icing is characterized by one-quarter inch of ice accumulation in a 15-to-40-minute time frame. "Moderate" icing is defined as one-quarter inch of ice within five to 15 minutes; "severe" icing means one-quarter inch of ice buildup in less than five minutes. Now that would get your attention fast.
It's important to remember that these definitions vary according to type of airplane. A Boeing 757 may report light icing, but what's "light" for an airliner could well be "moderate" (or worse) for a light single. Bear this in mind when checking pireps, and remember also to check the dates and times of any icing or other pireps. Often, pireps can be hours old by the time you read them. For more information on filing pireps, see the AOPA Air Safety Foundation's SkySpotter program online ( www.aopa.org/asf/online_courses/skyspotter/).
When climbing or descending through icing layers, fly at higher-than-normal airspeeds. Your pilot's operating handbook (POH) may say that the most efficient climbs are made at V Y (best-rate-of-climb speed), but in icing conditions a V Y angle of attack could expose the underside of the wings well aft of their leading edges. Climbing like this can spoil lift quickly and easily lead to a stall at airspeeds well above those published.
Some POHs publish minimum airspeeds for flight in icing conditions. Check your POH to see if these apply. If they do, avoid flying any slower.
Use what you've got. For light singles and twins without equipment packages certified for flight in known icing, your ice-fighting arsenal may consist of pitot, stall-warning vane, and carburetor heat only. Turn pitot heat on prior to entering icing conditions; it takes a while for the pitot probe (static ports and fuel vents, too, in some airplanes) to reach working temperature. By the way, you did check to see if the pitot heat worked during the preflight inspection, yes? Turning up the windshield defroster can help clear ice from small patches of the windshield, and while this would be a desperate move, it may give you a better view of a runway on an iced-up approach than opening the small hinged "storm windows" on some airplanes.
Carburetor heat is another important weapon. Using it not only clears ice from the carburetor venturi, but it also bypasses the intake air filter, which can ice over. Most pilots don't think of carburetor heat as another form of alternate engine intake air, but it is. By the way, once you've turned on carburetor heat, leave it on — even though the engine may run disturbingly rough. It's melting ice that's causing the engine to falter, and there's a natural temptation to turn the heat off to return the engine to its "normal" sound. Don't do it. Leaving the heat on assures that all the ice will melt — assuming you turned it on in time. If there's any subsequent roughness with carburetor heat on, this often can be eliminated by leaning the mixture.
If you have a full complement of ice-protection equipment, lucky you. But don't get cocky. Some of the worst icing accidents have happened to high-time pilots flying large, powerful, known-ice-approved airplanes. Ice protection equipment simply buys you more time to fly away from ice. It's not a guarantee you'll survive a protracted session in icing.
In days gone by, the standard advice was to wait for ice to accrete to a certain level — say, one-half inch — before inflating deice boots. No more. Now, research has shown that inflating the boots at the first sign of ice is the best strategy. So if you have boots, inflate them as often as necessary to pop ice off the leading edges. The old idea that inflating boots too often would create a shell of ice at the limit of boot inflation, and leave the boots pulsating helplessly beneath that shell — a phenomenon known as ice bridging — has been proven false with modern systems. Early deice boot designs of the 1930s and 1940s, however, could experience ice bridging.
Anti-ice equipment, such as heated windshields, heated propellers, and weeping-wing and bleed-air systems, should be turned on prior to entering icing conditions. Deice systems are for removing ice once it has formed.
When your airplane ices up, its handling and stall characteristics change for the worse. Knowing how to cope becomes critical when landing, whether it's out of a visual or instrument approach. Here are the key points:
A tailplane stall, or stall of the elevator or stabilator, calls for a completely different recovery procedure. Most often preceded by a lightening of pitch feel, pitch oscillations, or an uncommanded lowering of the nose, tailplane stalls require that you pull aft on the control yoke and reduce power. That's because horizontal stabilizers and stabilators generate negative lift, which acts in a direction opposite to the wings' lift. High power settings, high airspeeds, and extended flaps all increase a tailplane's angle of attack. Raising the nose lowers the tailplane angle of attack, and breaks the stall.
Use power, as necessary, right into the flare. This will prevent a hard landing and preserve what control effectiveness you may have.
Don't like the sound of all these precautions and strategies? All the more reason to plan your flights so as to remain ice-free from beginning to end. If that means waiting three days for a frontal system to move away, so be it.
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.
Safety and Education,
FAA Information and Services,
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