The Cold Shoulder

Winter doesnt have to put all of your flying plans on ice


Freight dogs are a special breed. They are coming up the hard way. Their focus is on the left seat of a large air carrier – eventually. The standard model is a young guy who has put up with miserable pay as a flight instructor, battled his way on nickels and dimes through college and is accepting jobs as a pilot of a weary light twin hauling light freight and canceled checks.

The outfit he flies for is underfunded, with all that goes with that. He is paying a big price to realize his dream. But he keeps the faith, he hones his instrument skills as best he can, without the luxury of paid simulator courses. Freight dogs squeeze all the experience they can out of every multi-engine flying hour.

Here is one, enroute to pick up cargo at a small airport in Pennsylvania on a dark February night. His airplane was a Beech E18S. He had received three preflight briefings earlier that indicated snow, mountain obscuration and low visibility. Enroute to his destination, he reported seeing the lights of University, and requested a contact approach. He also said that if he could not get in, he would make an ILS approach. When the airplane was four miles south of the airport, the air traffic controller cleared the pilot for a contact approach to runway 24.

A witness reported hearing a low-flying airplane. He said that it was snowing heavily and that he saw the tail light of an airplane briefly through the snow. Shortly afterward he heard what was later determined to be the sound of the collision. Searchers located the wreckage of the airplane, 3.3 miles south of the airport, on top of Mount Nittany, which is about 2,000 feet high.

The NTSB gave as a probable cause: an improper flight planning decision by the pilot and his failure to maintain proper altitude during the approach. Factors related to the accident were darkness, adverse weather conditions and the high terrain.

Winter has a variety of obstacles to flight to throw at a pilot. They can be dealt with – with some care in planning. But disregard of the risks can result in tragic consequences.

Ice and Airplanes
Whats the problem with ice in flight? A number of things. To lead off, ice can stop the engine by icing up the carburetor. If you have a fuel-injected engine, ice can block off the air source to the engine. It doesnt help the airframe much, either. Accumulated ice on the airfoils and the undersides of the wings and fuselage causes a disruption of airflow.

Ice adds weight, but the primary danger is that it upsets the smooth flow of air across the airfoils and increases drag. As the additional drag is added, an increase in angle of attack and power is required to hold altitude and speed.

Ice can accumulate on every exposed part of the airplane. It builds up on the propeller, the windscreen and the antennas. The intakes, cowlings and vents dont get missed either. In-flight icing can build up where it cant be reached by de-icing fluid, boots or heat. Icing has caused antennas to vibrate to the point where they break off. A light airplane, regardless of how it is equipped with de-ice and anti-ice equipment, can only handle a little icing before continued flight is impossible.

On landing approach and landing -where most accidents occur – an iced-up aircraft can roll and pitch out of control or stall and spin. Once the wing shape has been changed by the ice into something designers never intended, the airplane almost certainly will stall at lower angles of attack and higher speeds than normal. NASA wind tunnel and flight tests have demonstrated that ice, snow and frost accumulations as mild as coarse grain sandpaper can increase drag by 40 percent and reduce lift by 30 percent. Ice and airplanes are not the best of friends.

Icing accumulations generally collect into four main types, frost, rime, clear and mixed.

Frost forms when moisture in the air freezes onto the cold aircraft surface. The result is an innocent-looking thin coat of frozen moisture. When overnight radiation cooling is at its greatest and conditions of high pressure exist, frost will occur.

Rime ice looks like a thin line of frosting on the wings leading edges when it first forms. It starts to collect when the airplane collides in-flight with water droplets that are below freezing temperature but still in liquid form.

Rime ice is similar to the texture of a coarse grade of sandpaper. It is usually slow to build. Once the rime accumulation is in warmer temperatures, it can melt away rapidly. It forms mostly in stratiform clouds at temperatures between -10 and -20 degrees Centigrade.

Clear ice forms into a clear sheet of tough, tenacious ice. Clear ice builds up very quickly. Once formed it takes a long time to melt away. It forms primarily in freezing rain. In just a few minutes, the buildup of ice can be catastrophic. No light planes anti-icing or de-icing equipment can cope with it. Clear ice forms most often in the tops of cumulus clouds at temperatures between 0 and -10 degrees Centigrade.

Mixed ice is a combination of clear and rime ice that eats lift voraciously. Mixed ice can take on a variety of shapes that destroy the airflow over the lifting surfaces and create great amounts of drag. It creates large cone-shaped ice formations on spinners, nose cones and other pointed objects. The wing leading edges collect shapes that resemble a double horn.

Mixed ice shows up wherever stratus and cumulus clouds coexist. The most likely areas for mixed icing are low pressure centers and occluded fronts – places that are loaded with moisture and low temperatures.

Finding Ice
There are many variables in the weather-making factory, which makes ice forecasting a high-stakes crap shoot. Moisture, temperature, wind, water and mountains all play a part in inducing icing conditions. While it gets closer to a science every year, predicting specific ice-loaded altitudes and locations is still not that accurate. The potential for icing over large geographical areas is often forecast, which tempts many pilots into taking a look because the forecasts are so often wrong.

The moisture content of the clouds is an important clue as to what icing potential they may have. Cold spots such as North Dakota or Wyoming normally have clouds with a relatively low moisture content. Pennsylvania and New York can count on moisture-laden clouds much of the winter. It follows that when temperatures drop below freezing there, the clouds are ice factories.

How much supercooled water vapor can clouds carry? That depends on where the cold air mass originated. The Great Lakes are a good example. If the winds carry clouds over those great expanses of water, they will probably be wet clouds, with the highest icing potential.

Low pressure areas and fronts are the largest of the ice producers. There is also isolated air mass instability and the right amount of moisture, which can develop enough ice in clouds to be disastrous to light airplanes.

Heres where your weather forecaster comes in. TV weather channels and a quick DUATS briefing probably arent enough.

The question to answer is how bad the in-flight icing potential is. If there is heavy moisture with freezing temperatures, and the stratus is over mountainous terrain, on the lee side of lakes, or close behind a cold front, these stratus clouds can produce severe icing. Icing dynamics are complicated, making it crucial that pilots avail themselves of the many weather forecasting facilities available.

Finally, the icing season also carries warm fronts. While that may seem like a sure-fire cure for icing conditions, they usually arent warm enough. Extreme caution is the rule when flying winter warm fronts. The clouds in warm fronts can develop the worst kind of enemy – freezing rain and drizzle. These conditions develop when precipitation from the warm air above falls down through freezing air below. The supercooled moisture freezes on impact with the flying airframe, and usually flows backward beyond where leading edge de-icers can reach. Sometimes, if you still have climb capability, climbing will be a temporary solution to further icing. In any case, flying through a warm front as directly as possible is the safest method, if you must do it at all. Flying along the front, looking for a hole, is the worst way to go.

The Hardware Solution
The first thing to understand about de-icing and anti-icing equipment, certificated or not, for general aviation aircraft is this: It was never designed to make a safe operation out of a dangerous prolonged flight in icing conditions. What you are getting is some minimum protection from the worst effects of structural icing.

De-ice equipment is designed to eliminate some of the ice – after it has formed on the airframe. Anti-icing equipment is designed to prevent the formation of ice. There are several different kinds of this equipment in use on general aviation light aircraft.

Thermal equipment uses compressor, electric or exhaust heat to keep static lines, fuel vents, carburetors, pitot tubes, windshields and propellers free of ice.

Chemical equipment uses anti-icing fluids or paste that is used on windshields, propellers and leading edges.

Mechanical equipment are mainly pneumatic de-ice boots on the wing and tail leading edges. Sometimes you might find them on antenna masts.

Even the best equipment cant reach all of the areas on the airframe and airfoils. The design concept behind anti-icing and de-icing equipment is simple: to buy a little time so you have a chance to get out of the icing.

Although most pilots think of wing leading edges, propeller blades and windshields as the primary spots for ice accumulation, there are others where danger also lies. Pitot tubes protrude into the air stream where the supercooled moisture is located. Structural ice forms first right there. If the pitot static pressure source gets ice-blocked (or freezes) you can say goodbye to accurate airspeed, rate-of-climb and altitude information.

Of course using the pitot heat before icing conditions are encountered is one way to prevent it from happening. I have always found it safest – in a below-freezing temperature situation – to use pitot heat from takeoff to touchdown. There are some precautions to be taken with the pitot heat system. On preflight you need to be sure that you check carefully to see that it operates as advertised. The system was designed for use in a cooling airflow, so you should not operate the pitot heat for long periods of time on the ground.

Pitot heat is not a cure-all, however. Research shows that a buildup of ice at the base of the pitot tube, away from the heat, can be enough to change the pressure pattern around the tube and cause gross airspeed errors.

Static vents are usually located where they have the least chance of being iced over. Even so, it occasionally happens when the static port gets clogged by water that is splashed there during taxi and then freezes. A good preflight and careful taxiing will prevent that problem. If your airplane is not pressurized, it probably has an alternate static inside the cabin. The pressurized birds usually have heated static systems that are tied together with the pitot heat, to avoid this.

Fuel tank vents can also ice, leading to power interruptions and, if your airplane has bladders, a collapsed fuel tank. Although the airplane may be equipped with electric de-icers for the fuel vents, most are not. Know where the vents are and how likely they are to be splashed with water while taxiing.

Carburetor heat and alternate air are also part of the anti-icing/de-icing team. It should be used in snow or rain, or in clouds at near-freezing temperatures. Carburetor heat should be used before the engine is throttled back from cruise power, for the best heating. Fuel-injected engines need airflow, and if the primary air intake is blocked with structural ice, an alternate air door is either activated by the pilot, or opens automatically, to keep the engine going.

Staying in the Air
Light aircraft owners who can afford it protect their airplanes wing and tail surfaces with pneumatic de-ice boots. The contour of the leading edge of the airfoil is covered by the rubber skin of the boot. The boot is inflated by compressed air, cracking the ice coating, which is blown away in the slipstream.

BF Goodrich also makes a system that combines ice detection and de-icing together. Known as Smartboot it tells the pilot when to cycle the boots, confirms that boots are inflating, and detects any residual ice. It certainly is an improvement over conventional boot installations.

There is also the TKS weeping wing system, which pumps alcohol through titanium panels in the leading edges of the wings and tail. The panels have 800 holes per square inch drilled by lasers. The holes are small enough to be invisible to the air stream but large enough to supply anti-ice fluid to the airfoils. TKS can also be used to clear ice from the propeller and the windscreen.

Windshield protection usually boils down to heated pads or a fluid spray bar. The fluid spray (isopropyl alcohol) is squirted out on the windshield and spread by the air stream. One final note, the windshield defrost system wont help much (if at all) to remove well-developed structural ice from the windscreen. Most of these systems will, if used early enough, keep a small peephole open enough for landing. Youll still need to switch back and forth to the side windows to get a reasonable approach and landing.

Propeller ice is usually the first to form. The first indication is a loss of airspeed and some vibration caused by imbalance. You cant see propeller ice as you can the accumulations on the wings. However, it is a significant problem. It can rob the blade of as much as 20 percent of its efficiency.

The most common propeller anti-icing system in light planes uses isopropyl alcohol. The fluid is located in a slinger ring behind the propeller hub, where small tubes lead to the leading edge of each blade. With the rotation of the propeller, the fluid shoots outward through the slinger ring by centrifugal force.

Propeller anti-icing paste is a thick coating of non-drying material painted on the prop blade. It prevents the ice from sticking to the prop blade. When ice comes into contact with the coating, the paste causes a chemical reaction that lowers the waters freezing point and loosens the ice. Its easily thrown off the propeller by centrifugal force. This paste has a short service life, however – two to four hours in icing conditions. It will also wash off if it comes in contact with rain.

There are also electrically heated leading edges of propellers. A timer is usually attached that supplies current at intervals since the heating element only needs to be kept warm enough to prevent freezing.

For the really low-budget operations, there is an operational method to reduce ice buildup on the prop. First, consult your POH. If it isnt there, this is what I do.

The trick is to produce blade flexing by rapidly increasing engine speed and throwing off the ice accumulation. Have you ever noticed the dents across from the props of a twin-engine airplane that is being used as a freight dog? Chunks of ice, thrown off prop blades, were the culprits.

Its easiest to throw ice with a constant speed propeller. You first reduce the engine speed and then rapidly advance the prop control. It can cause some stress on the engine, but there will be more stress if you develop an ice-covered club out there in place of a propeller. The same kind of condition can be caused by a partially-blocked alcohol slinger tube, or a burned-out element in a prop blade. This kind of unbalanced prop will cause severe vibration, certainly enough to cause internal engine failure.

Detecting prop ice at takeoff is a bit difficult. On damp cold days I watch the prop blast hit the leading edge of the wing during engine runup. If traces of ice show up on the leading edge, I turn on the prop deicing and leave it on. If you dont have any prop de-ice, you may want to reconsider why youre about to embark on a flight in those conditions.

Also With This Article
Click here to view “Boot Brain Before Blasting Boots.”

-by Raymond Leis

Raymond Leis is a CFII and ATP with more than 23,000 flight hours.


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