Aviation Topic of the Week
By Michael Oxner, January 11, 2004


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This week's topic:
Icing

Rolling right along like snowball downhill, winter topics abound. This week we'll look at something a little more deadly than sliding off the end of a runway at slow speed. Many people have lost their lives to airframe icing, so we'll have a look at some of the effects. I haven't gone into the utmost detail, but rather just skimmed the surface. The AIP has a wealth of information on icing and aircraft in AIR 2.12 and this should really be consulted for the balance of information that I don't bring up here. This page is meant to be an overview with more general information.

How Ice Can Affect Aircraft Performance
Types of Ice
Types of Anti-Icing Systems
Reporting of Icing Conditions
How can You Deal with Icing?
    On the Ground
    In the Air

How Ice Can Affect Aircraft Performance

Ice has many effects on aircraft performance. Ice or snow on the wing having the thickness and texture of medium or coarse sandpaper can reduce lift by as much as 30% and increase drag as much as 40%. If it's on the leading edge, it may affect both of these to a greater degree. It may even change the point at which the airflow separates from the wing, altering the stall speed and stall characteristics of an airfoil. Reread the first sentence. This may amount to no more than a layer of frost.

Whether the ice accumulates on the ground or in the air, the effects on an aircraft are the same. Consider the following points out of the AIP, AIR 2.12.3:
  1. Ice accretion on lifting surfaces will change their aerodynamic properties resulting in a reduction of lift, increase in drag and weight with a resultant increase in stalling speed and a reduction in the stalling angle of attack. Therefore, an aerodynamic stall can occur before the stall warning systems activate;
  2. Ice adhering to propellers will drastically affect their efficiency and may cause an imbalance with resultant vibration;
  3. Ice adhering to rotor blades will degrade their aerodynamic efficiency. This means that an increase in power will be required to produce an equivalent amount of lift. Therefore, during an autorotation this increase can only come from a higher than normal rate of descent. In fact, it may not be possible to maintain safe rotor RPM's during the descent and flare due to ice contamination;
  4. Ice on the windshield or canopy will reduce or block vision from the flight deck or cockpit;
  5. Carburetor icing, see AIR 2.3; and
  6. Airframe ice may detach and be ingested into jet engine intakes causing compressor stalls, loss of thrust and flame out.
Other imaginable effects include ice chunks flying from the airframe and actually damaging other parts, such as fuselage ice breaking off and hitting tail surfaces, or even setting a jet engine compressor's blades off balance and then the engine vibration could actually tear the engine apart. I've seen photos in the past of aircraft landing after severe icing and being considered lucky by all to have made it back alive. Just the increase in weight as even as little as 1/4 inch of ice accumulates on the airframe may be enough to drag light aircraft out of the air. The increase in power required to keep altitude may exceed the engine's output for small aircraft. Another effect which is common is ice accretion on the source of static air pressure for aircraft instrumentation being covered, and the pitot tube as well. If these conditions happen, the aircraft's altimeter may read in error, as will the vertical speed indicator, and most importantly, the airspeed indicator. Yet another occurrence with icing can occur in the intake ducts of turbojet engines, and this type of icing can, reportedly, occur when the skies are mostly clear and temperatures are marginally above freezing. This is similar to carburetor icing in concept, and can lead to a loss of power.

While all of these effects are not simulated in Flight Simulator, some of them are. The most common one I can think of is ice on the pitot tube which causes the airspeed indicator to fail and read zero.

Types of Ice

There are three types of ice encountered in aviation. I believe the AIP says it best, so I'll quote AIR 2.12.3.1:
  1. Rime ice commonly found in stratiform clouds in granular, opaque and pebbly and adheres to the leading edges of antennas and windshields. Rime ice forms in low temperatures with a low concentration of small super-cooled droplets. It has little tendency to spread and can easily be removed by aircraft de-icing systems.
  2. Clear ice commonly found in cumuloform clouds is glassy, smooth and hard, and tends to spread back from the area of impingement. Clear ice forms at temperatures at or just below 0°C with a high concentration of large super-cooled droplets. It is the most serious form of icing because it adheres firmly and is difficult to remove.
  3. Frost may form on an aircraft in flight when the descent is made from below-freezing conditions to a layer of warm, moist air. In these circumstances, vision may be restricted as frost forms on the windshield or canopy.
The first two types of icing may significantly contribute to the overall weight of the aircraft. Clear ice in particular. It is also quite common for a mixture of rime and clear ice, simply reported as "mixed icing". Frost by comparison seems pretty benign. Never underestimate its effects, though. As mentioned above, just a thin layer of frost contamination can be enough to significantly alter aerodynamics to raise the stall speed and increase the amount of power required to remain in flight. Frost formation, as mentioned above, is common as an aircraft cruises at high altitude in cold temperatures and then descends into milder, humid air. The cold airframe may warm up fast enough to prevent frost accumulation, but often the fuel is still chilled. Where most aircraft carry at least some fuel in the wings, this provides a cold source to remove heat from the wing surfaces, above and below the wing tanks, and frost may accumulate in these areas if nowhere else. Of course, frost may also occur on the control surfaces while an aircraft sits on the tarmac, just as it does on your car's windshield.

Types of Anti-Icing Systems

There are several systems in use today that are designed to either remove ice from control surfaces, or prevent it before it accumulates. Some of the most common ones are:
  1. Electrical heaters are on some surfaces to heat the affected area. These are often found on windshields much like the rear defrost that is common in cars. Leading edges of wings and stabilizers may be warmed this way, but this requires a large electrical current to be effective over a large area. Often, pitot tubes are heated electrically. Their small areas are good candidates for this type of heating. Pilots must remember to turn them off, since a pitot heater left on when no longer necessary can deform the pitot tube, rendering the pitot static system useless. This effectively kills the airspeed indicator, an important gauge onboard an aircraft. Often these systems are meant to be used to prevent ice buildup, rather than to be used after ice accumulation. Many propeller driven aircraft will have electric heating systems on the propeller blades as well.
  2. Pneumatic systems are quite common as well. Many turboprop and piston engine aircraft come equipped with black "boots" on the leading edges of wings and stabilizers. These rubberized boots will alternately inflate and deflate in cycles of a few seconds, flexing the accumulated ice in hopes of cracking and deforming it.
  3. Many jet aircraft have ducts throughout the leading edges of wings which take hot "bleed air" from the engines. When this hot air is circulated through the ducts, the heat warms the ice and melts the ice in contact with the wing in hopes of removing it. The areas around jet intakes can be similarly heated in some engines helping to reduce the susceptibility of the engine to duct icing. Note that such a system also steals some power from the engines while in use.

Reporting of Icing Conditions

Like turbulence, there are some descriptors which are pretty much standard so pilots have an idea what previous pilots have encountered based on previous reports. Here is a table with some definitions of icing intensity that may be used as a guideline when reporting ice, or when receiving reports of ice.

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

As with turbulence, the pilot should include the time of the icing, both when first encountered and last encountered, aircraft type, altitudes involved, position relative to a readily discernible fix (ie 40 west of YQM) whether in could or not (for example when flying below cloud in freezing precipitation), and the type of icing (rime, clear or mixed), and the intensity, based on the above table. The aircraft type involved is important, since moderate icing to a Piper Navajo might be insignificant to a B747, while moderate to a B747 would certainly be of concern to a Piper Navajo. Also, different types of aircraft respond differently to icing. ATR42s don't react to favorably to moderate+ icing conditions, while I've rarely heard a Dash 8, which is of very similar configuration, report severe icing. You know if a Dash 8 calls it severe, it's pretty bad. Other conditions can be reported as well that might help pilots. For example, if icing was encountered on departure while climbing through a cloud deck, the base of the cloud deck could be important to someone else flying in the same area as his only way of getting out of the ice might be to get underneath it. The outside air temperature during the icing encounter could be important as well as it may give a pilot an idea of where the freezing level is. See below for more.

How can You Deal with Icing?

If your aircraft is on the ground and has icing on it, deal with it before you take flight. Even if it's "only frost", be prepared to spend the necessary time. Turning the aircraft's tail toward the sun often works for light aircraft in temperatures near zero. Putting the aircraft into a warm hangar long enough to melt the frost and let it dry off is also quite effective. For larger aircraft, these plans will probably not be options. Instead, airliners are often doused with one (or more) chemicals which may include some form of glycol. There are several types of de-icing fluids, but the point is the same: melt the ice or snow. Suring a snowfall, snow may accumulate on the wings and stabilizers after application of de-icing fluid. Since many fluids will adhere to the surfaces for at least a short time after application, this often provides at least a certain level of protection. The idea is to de-ice the aircraft and then get in the air as soon as practical afterward to avoid such a deadly buildup. Many satellite airports will douse the aircraft after pushback, while busier airports may have de-icing stations near the runways to account for considerable taxiing delays during winter airport operations.

There are a few strategies that can be used to deal with icing encountered in flight, too. First off, if you encounter icing in flight, use whatever devices you have against it. Carburetor heat is required early, since the total loss of engine power due to an ice-constricted carburetor will also result in a loss of the heat source. Outside air is directed around the exhaust manifold and then into the carburetor when carb heat is applied. If the engine stops, there will be no exhaust to heat the air and this tends to cool quite rapidly since you're probably in cool or cold air already. Once this heat is gone, you have little or no chance of clearing the ice for an engine restart. Use carb heat early if you suspect carburetor icing.

Many aircraft are also equipped with pitot heat. Turning this on as required may preserve the ability to read airspeed. If the altimeter and vertical speed indicator seem to be reading erratic, try use of the alternate static source, should the aircraft be equipped with one. Every Cessna 172 I've flown has had one, so it should be common on larger aircraft as well. This provides a source for static air pressure from inside the cockpit, protected from icing over. The VSI will lag a little more than usual, but other ill effects are less pronounced. It's good practice to turn the pitot heat off when not in use, especially when on the ground after a flight.

Other anti-icing or de-icing equipment can be used as required. As with pitot heat, it may be a good idea to monitor the success of this equipment as it often draws power, significant power with some systems, and should only be used when necessary. Sometimes the level of output of these systems can be regulated automatically, or applied automatically at intervals as required. In the case of engine-powered bleed-air systems, turbine temperatures should be monitored as well to ensure there is no undue stress on the engines during application.

Often the best advice for dealing with ice is to avoid it in the first place. Heavily active convective cloud is often a good place to find ice since it is comparatively warm, moist air providing the source for the convective forces which cools as it rises. Avoiding Towering Cumulus and Cumulonimbus clouds is good thinking not only for ice but for turbulence, hail and possible lightning as well. In winter, Environment Canada provides forecast information on icing conditions. These should be consulted as a regular part of preflight perparation for any IFR or long range VFR flight so that the pilot is well versed on expected conditions. Nothing like saying to one's self, "If I go this way, I'll probably still get there OK, if only a little late. I'm sure there's no ice over that area..." Are you sure?

Preceding pilot reports are important tools as well. These can be obtained from ATC or from FICs while enroute, if any exist for the area. They'll often provide a more comprehensive, though site-specific, idea of what may lie ahead. As mentioned above, a report including temperature may give a pilot an idea of where the freezing level is, and it may give a pilot an opportunity to descend below that to try to shed some of the ice. For example, if a preceding pilot reported a temperature of -3°C at 8,000 feet, with the normal temperature change (lapse rate) being around 2C° per thousand feet, he might reach the freezing level at around 6,500 feet. If local terrain and obstructions allow an IFR altitude as low as 3,000, the pilot may opt to descend below that altitude in an attempt to let the airflow melt the ice. One thing to consider, though, is that super-cooled droplets from the air above (as may be encountered during freezing rain conditions) may still adhere to the airframe in the above-zero temperature, if only temporarily until they melt. This means the icing problem may not be solved by descending. If the air is cold enough, or if the icing is only in cloud and a pilot report gives information that the top of the cloud deck may be accessible, it may be best to climb out of the area of icing rather than attempting to descend to get clear. If you can't climb out of it, the ice may have you try the descent anyway, willingly or not.




In short, icing is only good on a cake, and then it's dangerous to the blood vessels in the long term. It's best avoided altogether if you don't have much equipment for dealing with it, and even if you do, avoiding it should be strongly considered. Any feedback? Please send me your thoughts at moxner@nbnet.nb.ca. Thanks for reading!