Aircraft Icing

Introduction

This Safety Advisor discusses:

Icing accident statistics
Deicing and anti-icing equipment
Kinds of ice
Ice flying strategies and tactics
Tailplane Icing

Why Ice Is Bad

Ice in flight is bad news. It destroys the smooth flow of air, increasing drag while decreasing the ability of the airfoil to lift. The actual weight of the ice on the airplane is secondary to the airflow disruption it causes. As power is added to compensate for the additional drag and the nose is lifted to maintain altitude, the angle of attack is increased, allowing the undersides of the wings and fuselage to accumulate ice. Ice accumulates on every exposed part of the airplane ? not just on the wings, propeller, and windscreen, but also on the antennas, vents, intakes, and cowlings. It builds in flight where no heat, boots, or deicing fluid can reach it. It can cause antennas to vibrate so severely that they break. In moderate to severe conditions, a light aircraft can become so iced up that continued flight is impossible. The airplane may stall at much higher speeds and lower angles of attack than normal. It can roll or pitch uncontrollably, and recovery may be impossible.

Ice can also cause engine stoppage by either icing up the carburetor or, in the case of a fuel-injected engine, blocking the engine's air source.

Ice: The Rime and Reason ? Why It's Freezin'

Structural ice is the stuff that sticks to the outside of the airplane. It is described as rime, clear, or mixed.

Rime ice has a rough, milky white appearance and bumpy texture. Much of it can be removed by anti-ice and deice systems but not all. Clear ice is smooth and follows the contours or the surface more closely. It is hard to remove. Mixed ice is a combination of rime and clear ice. Ice distorts the flow of air over the wing, diminishing the amount of lift and significantly increasing the drag. Wind tunnel and flight tests have shown that frost, snow, and ice accumulations (on the leading edge or upper surface of the wing) no thicker or rougher than a piece of coarse sandpaper can reduce lift by 30 percent and increase drag by 40 percent.

Some unwary pilots have, unfortunately, been caught by surprise with a heavy coating of ice and no plan of action. However, pilots can learn enough basic meteorology to understand where ice will probably be waiting. The pilot can then formulate an ice-avoidance flight plan before ever leaving the ground.

Ice forms at 0 degrees Celsius (32 degrees Fahrenheit) or colder.

Even the best plans have some variables. Although it is fairly easy to predict where the large areas of icing potential exist, the accurate prediction of specific ice-filled areas and altitudes poses more of a quandary. Mountains, water, wind, temperature, and moisture all play ever-changing roles in weather-making.

All clouds are not alike. There are dry clouds and wet clouds. Dry clouds have relatively little moisture and, as a result, carry little ice. North Dakota, because of its very cold winters, is often home to dry clouds. However, winter in the Appalachians in Pennsylvania and New York often brings a tremendous amount of moisture with the cold air and lots of wet clouds that, when temperatures are freezing or below, are loaded with ice. The Great Lakes are a great moisture source. The origin of a cold air mass is a key to how much supercooled water vapor the clouds will carry. If the prevailing winds carry clouds over water, they will probably be wet.

Fronts and low-pressure areas are the biggest ice producers, but isolated air mass instability with enough moisture can generate enough ice in clouds to make light aircraft flight inadvisable.

As a general rule, flat stratus clouds tend to have rime ice in them, and the icing layer will be relatively shallow ? typically a few thousand feet. The amount of moisture in stratus will determine how bad the ice potential is. If stratus occurs over mountains, close behind a cold front or in the lee of lakes, the ice can be severe.

Cumulus clouds generally tend to have clear ice, and it may be well dispersed within the cloud. Flying through winter cumulus for any period of time is apt to result in moderate ice, or worse. Supercooled water and, therefore, icing, can occur well below the freezing level in cumulus clouds.

Stratocumulus clouds, as the name implies, are hybrids, and the ice can be a mixture as well.

Freezing rain and drizzle are the ultimate enemies that can ruin airfoil shapes and make flight impossible in a matter of a few minutes. These conditions occur when precipitation from warmer air aloft falls into below-freezing air underneath. It freezes on impact or may flow back behind the area protected by surface deicers.

Warm fronts in icing season can be very dangerous and should be flown with caution. As you approach the front, the clouds build quickly and the clear air between layers quickly disappears. All ice is dangerous, but freezing rain is the granddaddy of troublemakers, even for aircraft approved for icing flight. Normally, a climb will solve the problem temporarily because it has to be warmer above to create this kind of condition. If you choose to fly through the front, be sure you are flying through it via the shortest route and not flying along the front.

From the NTSB Final Report

On October 31, 1994, an ATR 72, American Eagle flight 4784, crashed in Roselawn, Indiana, during a rapid descent after an uncommanded roll excursion while on autopilot. The airplane was in a holding pattern in freezing drizzle and was descending to a newly assigned altitude. The NTSB determined that one of the probable causes of this accident was "loss of control, attributed to a sudden and unexpected aileron hinge moment reversal that occurred after a ridge of ice accreted beyond the deice boots.... Had ice accumulated on the wing leading edges so as to burden the ice protection system, or if the crew had been able to observe the ridge of ice building behind the deice boots...it is probable that the crew would have exited the conditions."

Structural Ice

How quickly a surface collects ice depends on its shape. Thin, modern laminar wings will be more critical with ice on them than older, thick wing sections. The tail surfaces of an airplane will normally ice much sooner than the wing; this is particularly important to pilots. If the tail stalls due to ice and the airflow disruption it causes, the down force is suddenly removed and the aircraft will pitch down rapidly. At low altitudes, recovery is unlikely. Several air carrier aircraft have been lost due to tail stalls. It also happens to light aircraft but usually isn't well documented.

Induction and Carburetor Ice

Not all aircraft ice is structural ? induction icing is the cause of many accidents. Induction icing most commonly affects carbureted engines but can affect fuel-injected engines as well, by icing over the air intake vents. In the AOPA Air Safety Foundation's Weather Safety Review, carburetor icing accidents topped the charts as the number-one cause of icing accidents, comprising a whopping 51 percent of all icing accidents. Carb ice can form in a wide range of air temperatures (from 20 to 90 degrees Fahrenheit) and thrives when the air is humid. There is a 3O- to 40-degree temperature drop from the outside air temperature (OAT) to the coldest part of the carburetor. If the air is moist and the relative humidity is 50 percent or more, carb ice is likely to form, particularly when engine rpm is low. This is why, when flying airplanes with carbureted engines, students are drilled to turn on the carburetor heat before making a significant power reduction. Carburetors will even ice up at cruise power when flying in cloud.

Carburetor ice decreases the engine rpm and may make the engine run rough. The pilot should notice the decrease in power and may hear the engine roughness. Turn on full carb heat, then wait. The engine may run even rougher for a short period while the heat is melting the ice in the carburetor. It will take a little time to clean itself out. When the engine runs smoothly, turn off the heat. The engine rpm should return to its original setting. Now be prepared for carb icing, and if the rpm drops again, fly with carb heat on. The hot air is less dense so the mixture becomes richer, and as a result, the rpm will drop a bit further. Lean the mixture, and most of the rpm loss should return. Keep an eye on your fuel consumption when using carb heat. A number of fuel-exhaustion accidents have resulted from miscalculations.

The "Haves" and the "Have-Nots"

There are two kinds of aircraft ? those that are approved for flight in icing conditions by the FAA and those that are not. Icing approval involves a rigorous testing program, and relatively few light aircraft carry this approval. From a legal perspective, aircraft that do not have all equipment installed and functional are prohibited from venturing into an area where icing conditions are known. There are some legal issues beyond the scope of this advisor as to what constitutes "known" ice. We will focus on the operational and safety issues. Partial equipage, such as a heated propeller or windshield, do not prepare an aircraft for flight in icing conditions ? they only make the escape a little easier.

The non-approved airplane and ice: Most light aircraft have only a heated pitot tube, which will limit their cross-country suitability in cooler climates during late fall, winter, and early spring. Although ice forecasts are notoriously broad and, in some cases, inaccurate, the pilot needs to plan for a guaranteed escape route if ice is encountered. This will be addressed in the tactics section of this Safety Advisor.

In addition to the wings, where ice is the most visible, other parts of the aircraft can suffer quickly. A completely blocked pitot tube due to an inoperative heater will cause the airspeed indicator to function like an altimeter. As the aircraft climbs, so does the airspeed. As the aircraft descends, so does the airspeed. A Boeing 727 crew that neglected to turn on pitot anti-ice stalled and crashed when they thought the jet was going into an overspeed condition because of the high indicated airspeed during climb-out. In heavy ice, control surfaces may bind or jam when the pilot really needs full control authority. Approved aircraft have been checked with significant ice accumulations on all control surfaces to ensure no binding. If you look closely on some approved aircraft, you will see significant gaps where space has been provided to allow for ice buildups without interference.

Unheated fuel vents can become blocked, which may lead to fuel starvation. Fuel tanks, especially bladder types, may collapse because air is unavailable to replace the used fuel. The engine may stop.

A number of accidents occurred when flights had successfully negotiated the enroute phase and approach but the pilot could not see well enough to land through an iced-up windscreen.

Without an approved icing package, you become a test pilot. To be sure, there are light ice conditions that do not pose much of a hazard, but it's a fine line between light and moderate. The pirep system, which is inexact at best, can be misleading at worst. Invariably, the question comes up as to how much ice a particular non-approved aircraft can carry. The answer is no one knows because it has never been tested. We don't recommend betting your life on the local airport sage who may have been in ice a few times and is prepared to dispense all the free advice you're willing to gamble on. You and your passengers deserve better. Weather conditions that look the same may not be.

Deicing and Anti-Icing Equipment

Listed below is equipment that can be used to combat ice. Many aircraft will have some, but not all, the gear required. In some cases, the equipment has been added as an aftermarket modification. Although it may give the pilot more time to escape icing, it has not been tested in the full range of conditions and, therefore, will not change the aircraft's limitation prohibiting flight into icing. Use the equipment to buy time to escape icing conditions ? do not depend on it for prolonged periods, particularly in moderate or heavier ice.

Anti-icing is turned on before the flight enters icing conditions. Typically this includes carburetor heat, prop heat, pitot heat, fuel vent heat, windscreen heat, and fluid surface deicers (in some cases).

Deicing is used after ice has built up to an appreciable amount. Typically this includes surface deice equipment.

Propeller Anti-icers ? Ice often forms on the propeller before it is visible on the wing. Props are treated with deicing fluid applied by slinger rings on the prop hub or with electrically heated elements on the leading edges. Ice prevention on props is preferable to deicing because the ice may not come off the blades evenly and the unbalanced prop may vibrate badly.

Wing Deicers ? There are presently three major types of wing deicers: boots, weeping wing systems (fluid deice systems), and heated wings. For the most part, general aviation aircraft equipped to fly in icing conditions use boots and, to a lesser extent, weeping wings. Hot wings are typically found on jets and will not be discussed in this Advisor.

Boots are inflatable rubber strips attached to the leading edge of the wings and tail surfaces. When activated, they expand using air pressure to break ice off the surfaces. When using boots, it is important to stay above the minimum speed, as specified by the airframe manufacturer, for flight in icing conditions. The process is repeated as the ice builds up again. Cycling the boots before there is enough ice buildup may cause it to break off unevenly, or perhaps not at all. BFGoodrich, presently the only supplier of boots in the United States, has introduced a new system that incorporates ice detection and deicing into one package. The SmartBoot System advises the pilot when to cycle, confirms boot inflation, and detects any residual ice, making the SmartBoot System easier to use. and more effective than conventional boots.

Weeping Wing System ? TKS is a patented alcohol deicer system that pumps fluid from a reservoir through a mesh screen embedded in the leading edges of the wings and tail. Activated by a switch in the cockpit, the liquid flows all over the wing and tail surfaces, deicing as it flows. TKS can also be applied to the prop and windscreen.

Windscreen Anti-icers ? Because being able to see for landing is critical, there are two systems used in light aircraft. An electrically heated windscreen or plate or a fluid spray bar located just ahead of the pilot's windscreen is used to prevent ice. Another method is the windscreen defroster. This is never acceptable by itself on "approved aircraft," but for the rest of us, it's the only source of ice prevention that will keep at least small area of the windscreen clear enough to peer through.

Carburetor Heat/Alternate Air ? Carburetor heat is used any time the engine is throttled back from cruise power and may be used during snow or rain and in clouds with near-freezing temperatures. Fuel-injected engines depend on airflow as well, and if the primary air intake ices, an alternate air door either opens automatically or is activated by the pilot to keep the engine running.

"Occasionally there are bits of trickery in getting on top. Out of Pittsburgh, Pennsylvania, headed east, the highest tops and toughest ice are located in the mountains east of Pittsburgh, over Laurel and Chestnut Ridges. Taking off and then climbing toward the east means one climbs up through the thickest clouds. To the west of Pittsburgh, of course, the land is lower. Back in DC-2 and DC-3 days, we often got a clearance to take off and climb toward the west. The tops would always be lower that way. Once well on top we'd turn back east for New York. It was a way to get up through the minimum amount of cloud. The tops to the west would often be 7,000 feet while the tops over the ridges might be 12,000 feet." [Just remember that you need to be able to top it all. That may mean turbocharging and oxygen.] ? Capt. Robert Buck

"Ice Flying": The Strategy

Smart "ice flying" begins on the ground. For VFR flight operations, with the exceptions of freezing rain and carburetor icing, staying clear of the clouds by a safe margin solves the icing problem. For pilots choosing to go IFR, it becomes more complicated.

Ask the right questions, and remember that conditions that appear to be similar to weather you've dealt with before may be much different.

Where are the fronts? Know the big picture because most ice is in fronts and low pressure centers. Where are the fronts moving? Where will they be when I depart and when I arrive? Check "upstream" weather reports and trends. If the destination is Cincinnati, what's the weather in Indianapolis 100 miles to the northwest? Remember that forecasts are not guarantees and plan accordingly. Where are the cloud tops? You cannot climb through a front with tops to 30,000 feet. For most light nonturbocharged aircraft, once the tops reach 8,000 feet, climbing is no longer an option. Once on top, can you stay on top? Expect much higher clouds over mountains. Where are the cloud bases? Below the clouds and out of freezing rain, there will be no structural icing. Where is the warm air? If the freezing level is high enough above the IFR minimum enroute altitude (MEA), then the flight may be feasible.

However, air traffic control may not be able to guarantee you the MEA due to traffic or conflicts with other sectors. If it's freezing on the surface and the clouds are close to the surface and more than a few thousand feet thick, it is foolish to attempt to climb through to clear conditions on top. What alternate routes are available? Flying the flatlands with lower MEAs is likely to provide much better weather, a smoother ride, and less ice than the same trip over the mountains. Detour if necessary. Avoid flying south through a front that is 200 miles long when you could fly west and be through it in 35 miles. What pireps are available? Pay particular attention to pireps. They tell you what the conditions really were at a particular time in a specific place. Think about whether those conditions are likely to be duplicated during your flight. How will you handle it? What are your escape plans? What are the escape routes? At any time during a flight where structural ice is a possibility, you need an alternate plan of action. That could be a climb, descent, 180-degree turn, or immediate landing at a nearby airport. It will depend on traffic, terrain, cloud conditions, and availability of suitable airports. Quickly tell ATC you are in ice and want out. Ask for a higher or lower altitude or a 180? turn. Pireps are individual judgment calls, so having several for the same area will usually result in a better picture. Be prepared for surprises if you rely on just one pirep. The type of aircraft making the pirep is critical. If a jet or turboprop reports moderate ice or worse, that is a mandate for light aircraft to plan a different strategy immediately. Turbine-powered airplanes are equipped for flight into icing and have much higher performance to punch through an icing layer quickly. A "light" ice report from turbine aircraft may mean moderate ice for you, because they can change altitude much more quickly. How old is the pirep? Weather moves and changes, so a report more than 45 minutes old will be of limited use.

The Aeronautical Information Manual (AIM) defines how in-flight icing should be reported when filing a pirep:

Trace: Ice becomes perceptible. Rate of accumulation is slightly greater than the rate of sublimation. It is not hazardous even though deicing/anti-icing equipment is not used unless encountered for an extended period of time (over 1 hour). Light: The rate of accumulation may create a problem if flight is prolonged in this environment (over 1 hour). Occasional use of deicing/anti-icing equipment removes/prevents accumulation. It does not present a problem if the deicing/anti-icing equipment is used. Moderate: The rate of accumulation is such that even short encounters become potentially hazardous and use of deicing/anti-icing equipment or flight diversion is necessary. Severe: The rate of accumulation is such that deicing/ anti-icing equipment fails to reduce or control the hazard. Immediate flight diversion is necessary.

Use the many resources available to you: television, the Direct User Access Terminal (DUAT) system, flight service stations, and AOPA Online on the Internet. Continue to request pireps ? and make some of your own ? along your route if you suspect icing to be a potential problem.

Some tips when considering the potential for icing:

Visualize what the weather report is telling you. It may contain 10015G25KT 3/4SM SNSW 12OVC. The ceiling is going to be 1,200 feet, which is okay, but three quarters of a mile visibility, variable in snowshowers, foretells flying in severely reduced visibility When you look down, the ground may be snow covered and look much the same, and navigation will be difficult because forward visibility will be seriously restricted. If the terrain is hilly or mountainous, be extra-cautious. This is definitely not a VFR operation and may not be an IFR operation without an aircraft approved for flight in icing conditions. Know the difference between air-mass clouds and frontal clouds. Of the two, frontal clouds are the ones you want to avoid. Air-mass clouds may have snowshowers but do not have large areas of steady snow. Unless you are flying in the mountains, steady snow or rain means significant weather is building. With the exception of freezing rain, the only way to gather structural ice is in an actual cloud. Flying in snow or between cloud layers will not cause structural ice.

"Ice Flying": The Tactics

Preflight

Carry extra fuel. In icing conditions, extra power is needed because of increased aerodynamic drag or because carburetor heat is needed. Fuel consumption will increase. Contrary to the previous point, keep the aircraft as light as possible ? the more weight to carry, the slower the climb and the more time spent in ice. Remove all frost, snow, or ice from the wings. There is no point in starting the day with two strikes against you. Every winter there are "frost bitten" pilots who crash as a result of guessing how much frost their aircraft will carry. A perfectly clean wing is the only safe wing. Don't count on blowing snow off when taking off ? there could be some nasty sticky stuff underneath the snow. If you think it's light enough to blow off, it should be very easy to brush off before starting. Do it! The propeller(s) must be dry and clean. Consider wiping the propeller with an anti-icing liquid during preflight. Check the controls to be sure there is freedom of movement in all directions. Check the landing gear (especially retractables) and clean off all accumulated slush. Wheelpants on fixed-gear aircraft should be removed in winter operations because they are slush collectors. Be sure to check wheel wells for ice accumulation. This is always a good idea after taxiing through slush. Be sure that deice and anti-ice equipment works. When was the last time you actually checked the pitot heat for proper functioning?