Planning on flying this winter? Thought so. Now is the time to prepare yourself for those weather reports warning of "icing conditions in clouds and precipitation above the freezing level." How you respond to this warning will depend a great deal on your ratings, experience level, and time spent flying in clouds — make that ice-laden clouds.
Most low-time pilots respond by standing down and waiting for their routes to clear of any ice-bearing fronts or other systems. Pilots with more experience, and instrument ratings, may elect to fly when an icing alert has been sounded. Those who have under their belts a number of icing encounters — successful ones, that is — are most likely to launch into situations conducive to icing conditions. One wag on the Pilot staff used to call these types of pilots "ready, rated, and willing to take off into known insanity."
Hold on a minute, I hear you say. Federal Aviation Regulation Part 91.527 prohibits flying into known or forecast icing conditions unless your airplane has approved ice protection equipment. Am I suggesting that pilots are out there droning around in ice-laden clouds, blasé to danger, literally flying in the face of both the regs and common sense?
Yes and no. While no one wants to run into icing — and most pilots do excellent jobs of avoiding ice — there's always the chance of an inadvertent encounter on any winter flight in instrument meteorological conditions. Unforecast situations can and do arise where icing conditions suddenly materialize, and even the most cautious pilots can find themselves unwittingly caught in icing situations. So let's just face the facts and say that inadvertent icing encounters can and do occur, and move on to the tactics for dealing with them.
One tactic has already been mentioned: Stay on the ground until conditions improve and icing is not a factor. This is the safest advice, but it is of little help to a pilot who is in cloud and beginning to take on ice.
The best advice for any pilot who notices any ice forming — and this also holds true for pilots flying with ice protection equipment — is to come up with an escape plan. It's important to emphasize that even the most powerful, best-equipped airplanes can be felled by heavy ice accretions. The crash of an American Eagle ATR-72 at Roselawn, Indiana, in October 1994 drove that point home, as did the crash of an Air Florida Boeing 737 after a 1982 takeoff from Washington's National Airport.
During the course of the ATR crash investigation a new, especially dangerous type of icing condition was identified (see " Wx Watch: The Worst Ice," December 1995 Pilot). The loss of control that preceded the Roselawn crash involved the airplane's entering a zone of very large supercooled water droplets, dubbed super cooled drizzle drops, or SCDD. It's believed that SCDD caused ridges of ice to form aft of the ATR's wing leading edge deice boots, which in turn caused uncommanded aileron deflections and a fatal roll and dive. So icing affects all types of airplanes, not just those of the general aviation fleet.
Another upshot of the ATR investigation is the institutional recognition of SCDD — itself an admission that there's still a lot we don't know about the icing environment and that our current icing certification rules may have to be adjusted to reflect newly discovered realities.
Ranking the risk
Back to the escape plan. The first part of forming an escape plan is to consider your flight environment before you take off. This plays a big part in determining the risk level you could find yourself confronting, and lets you mull over your options should icing become a factor. Let's rank some arbitrary risk situations by geography and weather conditions, from least risky to most dangerous:
- Comparatively flat terrain; low minimum en route altitudes; air mass cloud formations with high cloud bases and low cloud tops, comprising thin stratus layers or scattered-to-broken benign cumulus clouds; above-freezing temperatures below or above the clouds; and daylight. That is the combination of circumstances that allows the most favorable outcome of an icing encounter. If there's icing in the clouds, you can climb or descend quickly to escape it. Flat terrain, low MEAs, high cloud bases, and warmer-than-freezing temperatures below the clouds mean that you'll lose ice as you descend — a necessity for a safe landing — and that even if ice denies you the ability to maintain level flight above the clouds, you'll still be descending to ice-free altitudes. Finally, thin cloud layers are less subject to the lifting forces present in fronts and lows and are therefore less likely to contain large water droplets of the type that cause the worst clear-ice accretions.
- VFR or MVFR conditions during climbout from departure airport; cloud bases at MEA; freezing level just above MEA; frontal weather conditions and IMC en route; cloud tops to 9,000 feet msl or higher; airplane without ice protection or turbocharging; destination forecast to experience VFR conditions. Under these conditions, risk levels increase dramatically. Instrument flying will be required, and exposure to icing conditions in frontal clouds is a virtual certainty. Without deice boots or anti-ice equipment, ice will continue to build. Without turbocharging, a climb to on-top conditions probably will not be possible or safe. Climbing in icing conditions invites ice accretions on the underside of the wings, which can quickly ruin whatever aerodynamic efficiency might still be present on already-iced-up wings. Still, there are some outs. A return to the departure airport could be a good strategy. Descending to the MEA could also work, since we've stipulated that the freezing level is just above that altitude. Finally, there's the prospect of progressing to VFR conditions as the destination nears, which can be a great comfort after enduring even a small bout with icing clouds. So turning back or descending and continuing could save the day — but you'd probably pick up some ice and have to ask ATC for a lower altitude to escape from icing conditions. If a weather briefing mentions this type of scenario, it's time to think about delaying the flight or, if circumstances and your risk tolerance permit, flying an airplane with ice protection equipment and turbocharging. In that way, you'd be both prudent and legal.
- IMC from departure to destination; frontal weather with low cloud bases and high tops; high terrain and high MEAs; freezing level at the surface; night; no ice protection; and no turbocharging. This is a setup for disaster. For the safest flight, you'd need an airplane certified for flight into known icing conditions, the ability to outclimb any icing situations, and sufficient range to be able to divert to an alternate airport with suitable and legal weather minimums should a missed approach or diversion need to be performed. You'd also need enough instrument time and weather flying experience to allow you to maintain your wits.
Climb or descend?
There are no hard and fast rules for answering that question. Descending to warmer temperatures is a great idea, but wind and temperature aloft reports and forecasts can be off by a few critical degrees, and this can make all the difference in the world. If the freezing level is forecast to be 3,000 feet and the MEA is 2,000 feet, it should be OK to descend. But what if you reach 2,000 feet and you're still in cloud, and the OAT is 0 degrees Celsius? The forecast was wrong, and you're still building ice. Now it's time to find a nearby airport and land while you can. The moral: Before descending, learn from pireps or other sources as much as you can about the temperature and cloud conditions down low. And be prepared for an approach if things aren't as advertised.
Climbing can work, as long as you don't have far to climb. As mentioned earlier, climbing in icing conditions can cause extremely dangerous ice accretions to form. The steeper the climb attitude, the greater your exposure to this risk. That's why some manufacturers of known-icing-certified airplanes publish a recommended climb speed for use in icing conditions. This climb speed is designed to produce a lower angle of attack, so it's an airspeed higher than VY. Manufacturers don't want you building ice on the underside of the wing — even in airplanes certified for flight in known icing. Why? The portions of the wing aft of any areas protected by boots or heated leading edge panels are as prone to collect ice as those of an unprotected airplane.
Perhaps the best strategy is to climb as high as you can, as soon as you can (assuming ice-free conditions in the climb, of course) so that you can remain on top of any clouds for most of the duration of the flight. This works well only if icing conditions are not present during the descent and landing at your destination.
Icing in VMC?
You bet. Some of the worst ice happens in advance of warm fronts, where warm rain aloft falls into the cooler air of a retreating cold air mass. This is a setup for freezing rain, which is a fast-forming coating of clear ice that can cripple the biggest, most powerful airliners in minutes. One minute you're flying along under a high overcast in VFR weather — and an OAT of 0 degrees Celsius — and the next you're in freezing rain. So take forecasts or reports of freezing rain seriously and stay away. Also, be alert any time you fly in advance of a winter warm front.
Approaches and tailplane stalls
The strategy here is to keep your angle of attack low and your power up, so that you preserve what lift your iced-up wings are producing. This means flying a faster-than-normal approach (but not too fast, because this increases the tailplane's angle of attack) and using no flaps, or minimal flap settings, for landing. Icing increases stall speed, so maintain a healthy margin of airspeed above your airplane's published clean stall speed. How much of a margin? Who knows? If you're flying a non-ice-protected airplane in icing conditions, you're a test pilot. The manufacturer didn't have to fly the airplane in this condition, so you're plowing new ground. It's not a good feeling.
With all the attention given to iced-up wings, don't forget that the horizontal stabilizer or stabilator is also subject to ice accumulations and tailplane stalls. When the tailplane stalls, its loss of lift pitches the nose of the airplane down. Because flap deployments increase the angle of attack on tailplanes, avoid using flaps to keep the tailplane as far from a stall as possible. Should a tailplane stall occur, the recovery procedure is exactly the opposite of conventional recovery techniques: Apply aft-stick pressure on the control column. When there is a warning, onset of a tailplane stall can be announced by a change in pitch forces, buffeting, or nose-down pitching motions.
If you have a choice, select an airport with long, wide runways served by ILSs for flying an iced-up approach. You'll be flying at high airspeeds and may need every bit of runway available. These kinds of airports are seldom found in mountainous areas — another reason to avoid flying over high terrain when the chance of icing is present.
The bottom line
Stay away from ice. That's the safe and legal way to fly. Take forecasts of icing seriously and plan your flights so as to minimize your risk of icing.
The information presented here is for use in cases where icing is encountered inadvertently, and it does not constitute an endorsement to fly in icing conditions. It's for use in helping you to carry out tactics that, it is hoped, will maximize your safety if all else fails and you find yourself in ice. The goal, as always, is to exit icing conditions as soon as you notice them.
Those with the need to fly often in winter weather systems should seriously consider flying airplanes that have been certified for flight in known icing conditions. Even then, the rule for flying in ice is the same as it is for airplanes not certified for flight in known icing: Escape at the first sign.
E-mail the author at [email protected].
Icing Action Plan
The worst has happened. You're in icing conditions. What do you do?
- Turn on the pitot heat — if you haven't already. You should have the pitot heat on anytime you're flying in cloud or precipitation, regardless of the OAT.
- Select an alternate source of static air. This may mean opening a dedicated valve to admit an alternate source of pitot-static instrument air, or it could mean breaking the glass face of the vertical velocity indicator if you don't have such a valve.
- Select alternate engine intake air. Some airplanes have spring-loaded alternate air doors that open automatically when the normal source of engine intake air is blocked by ice. Others require that the pilot manually open alternate air doors. Check your pilot's operating handbook for your airplane's alternate air setup — if there is one.
- Call ATC. Let them know you're having a problem so that you can receive better handling and vectors to any nearby airports. Controllers understand icing, too, and can be very helpful in providing you with ways out of your problem.
- Prepare to declare an emergency. If things are falling apart fast, don't hesitate to exercise your emergency powers. You want to turn around, descend, or land as soon as possible, and declaring an emergency will open these alternatives immediately. Fly the airplane first, and worry about the paperwork — if any — later.
- Prepare for iced-up approach procedures. This involves flying at a faster-than-normal approach airspeed and using little or no flap deflection.
Types and temps
Rime ice — Frosty, milky-looking surface that can build to a thick, rough coating. Caused by small supercooled water droplets. Found most often in stratus clouds and at temperatures in the plus 20 to minus 10-degree Celsius range.
Clear ice — Transparent, smooth, extremely adherent coating that can run far aft of leading edges. A large-droplet phenomenon. Found most often in cumulus clouds and temperatures in the minus 10 to plus 2 degree Celsius range. Freezing rain is a form of clear ice.
Mixed icing — A combination of rime and clear ice that forms large, pebbly accretions. Can form when temperatures are anywhere between minus 15 and 0 degrees Celsius.
Certified vs. STC'd
It's one thing for an airplane to be fitted out with deice boots, heated windshield plates, heated propeller elements, and other ice-protection paraphernalia under terms of a supplemental type certificate (STC). It's quite another for an airplane to have earned certification for flight into known icing.
STC'd installations do not make flight into known icing conditions legal. An STC'd ice protection installation meets fairly loose standards. It need not prove its worth in a wide range of icing conditions, for example. The manufacturer must prove only that the installations work and that they do not adversely affect the airplane's handling characteristics, procedures, limitations, or weight and balance.
Certification for flight into known icing, on the other hand, requires an extensive series of flights in a variety of actual and simulated icing conditions as set down in federal regulations. These tests prove that the airplane's ice protection systems and unprotected surfaces can successfully shed ice and otherwise withstand icing encounters. While earning this certification makes it legal for you to fly in icing conditions, it's still no guarantee of safety.
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