Airframe icing has been a safety hazard since the first pilots flew in cold clouds, or in freezing precipitation. Can you imagine those early mail pilots trying to get across Pennsylvania in the winter with its lumpy terrain, perpetual cloud cover and freezing temperatures? With all of those leading edges, struts and flying wires to collect ice, it’s a wonder any pilot made it.
In case you have forgotten somehow, icing is caused by supercooled water droplets freezing to the surfaces of an airframe. The word “super” means the droplet temperature and the air it is suspended in is below freezing, but the droplet remains liquid until disturbed by impact with the airplane.
This concept of supercooled appears to defy what we all learned in elementary school science class. That is, water freezes upon reaching a temperature of 0° C. Like most of nature’s phenomenon, that is true — but not always, and that’s why icing can occur.
The potential for airframe icing is present whenever there is visible moisture and air temperature is at or below freezing. That is the definition used often by regulators, and you can find it in airplane operating manuals. And it is accurate because the “potential” for icing is present under those conditions, but actual icing occurs only a tiny percentage of the time when moisture is visible and air temperature is below freezing. That’s what is so frustrating about ice. Only a tiny minority of cold clouds contain icing conditions, but there is no way to know with certainty which clouds will create ice, and which won’t.
There is some potential confusion about icing with air temp above freezing, but the issue there is the so-called “ram rise” experienced by old-fashioned air temperature probes when an airplane is flying at high airspeeds. An air temperature reading that is not corrected for the effects of airspeed is called RAT (ram air temperature). At high speeds RAT can be as much as 10° C above static air temperature, so that’s why some turbine airplane manuals advise of possible icing conditions with RAT above freezing.
In order for a drop of water to become supercooled and remain liquid it must be transported from air that is above freezing to cold air rather quickly. If a droplet moves gradually from warm to cold air it freezes into snow, or ice pellets, or even sleet. Snow isn’t much of an issue for airplanes in flight because — except for a thin white strip on the leading edges — it doesn’t stick. The same is true for ice pellets. Sleet, which contains a mixture of ice and liquid water, is a different matter altogether because the liquid water can freeze on the airframe, causing the already frozen ice pellets to adhere, too.
In a typical icing scenario the liquid water is moved from warm air to freezing air by some lifting force in the atmosphere. Convection — the natural lift of warm air into colder air above — is the most common means for getting a liquid drop of water into freezing air without actually freezing the water. The most extreme cases of convection are, of course, thunderstorms, and that’s where you find the most extreme icing. But if you fly into a thunderstorm, ice is only one of your problems as you deal with severe turbulence that can break the airframe, the possibility of hail that can destroy the airplane, and the chance that rain and or hail will be so heavy that it drowns the engines. Avoiding thunderstorms is a different topic and one for which no pilot needs a reminder.
But there are mild forms of convection that are almost always present in the atmosphere, so air of various temperatures and moisture contents is always lifting or sinking. It is that almost continuous convection that can lift warm water into freezing air quickly enough to supercool the droplets and cause ice. That’s why every cold cloud has at least a tiny chance of containing icing conditions.
Another atmospheric means of supercooling water is mechanical lifting. Air moving over rising terrain is lifted rapidly and water can easily become supercooled. Big mountains like the Rockies can lift droplets quickly and generate severe icing conditions in their lee and pilots of all categories of airplane are wisely alert to those conditions. But the lower mountains of the eastern U.S. are actually more consistent and threatening ice producers, because they have a great moisture supply upwind in the Great Lakes and from low-pressure systems that circulate moisture up from the Gulf of Mexico. If there is a place most likely to have icing conditions it is over the mountains downwind of the Great Lakes because there is mechanical lifting, a moisture source and long periods of cold temperatures.
Among the many frustrations when it comes to forecasting the presence of icing conditions is the very local and confined area of the special conditions it takes to make airframe ice. If the moisture travels from warm to cold too slowly, there is no ice. If air temperature changes only a couple of degrees, the unique conditions for icing are removed. And if the moisture source does not produce as predicted, icing won’t occur either.
If you want to know how uncommon icing conditions are, talk to any airplane manufacturer who is conducting a flight into known icing conditions certification program. Special meteorological consultants advise test pilots on where icing is most likely to occur, but they are often wrong. Test programs can stretch out over weeks or months, as test pilots try to find icing conditions that match certification criteria and then fly in those conditions long enough to collect ice and data. Often the test airplane has to circle in a very small area to remain in icing conditions that a normal flight would exit in just a couple of minutes.
But it is the very capriciousness, and even rareness, of icing, that makes it a genuine threat. Because it can occur anytime there is cold air and moisture, we have to have a solution to escape before the drag of the ice accumulation overwhelms the ability of the airplane to fly.
Though there have been advancements in ice protection technology, there are really only two ways to handle airframe ice — break it off after it forms, or prevent formation in the first place. And each technology has its tradeoffs.
The earliest effective ice protection system was deice boots, which were developed before World War II. The concept is simple. Flexible tubes are attached to the leading edges of an airframe and, after ice forms, are inflated to crack the hard ice and allow it to blow away. Goodrich pioneered pneumatic inflating deicers, and remains the leader in that technology today.
Preventing ice formation — anti-ice technology — operates by warming the surface to prevent freezing, or by distributing an anti-freeze chemical that prevents the supercooled droplets from freezing on contact.
Chemical anti-ice protection in the form of a TKS system has gained popularity over the past 20 or so years, but the technology was developed during World War II by the British who needed ice protection for bombers that flew missions in IFR conditions. The concept is simple, but the hard part is distributing an effective flow of the anti-ice chemical evenly over all of the surfaces to be protected.
Even though TKS dates back more than 60 years, the technology has not stood still. Early TKS systems used a mesh of fine wire to distribute the flow of fluid over the leading edges. The mesh worked, but the flow was not as uniform as desired, nor was it possible to shape the mesh precisely to the design airfoil shape.
The solution was to create a leading edge from a rigid metal such as titanium and then use a laser to drill tiny holes to allow the fluid to flow. New materials also allowed the membrane that is behind the holes to be more accurately formed so that fluid flow is more uniform.
Little has changed, nor does it need to, in TKS protection of propellers. Fluid is fed into a ring mounted inside the propeller spinner. Centrifugal force spits the fluid out along the prop blade root and ribbed surfaces on the blade guide the flow out along the span of the blade.
Like TKS, pneumatic deice technology has not stood still. The biggest change has been in new materials that can be much thinner and more resilient. That has allowed newly designed boots to have many individual inflation tubes spaced closely together, while the total thickness of the boot is a fraction of what it was years ago. The multiple tubes snap up quickly, breaking even very thin layers of ice. And the thinness of the boot structure allows it to conform precisely to specific airfoil shapes so aerodynamic efficiency is not compromised.
Deice boots had received something of a bad name, particularly among pilots of piston-powered airplanes, but they deserve a new look. One issue was that the older boots just didn’t work all that well with their large inflation tubes and slow inflation cycles. Pilots were instructed to allow a significant amount of ice to form before activating the boots to create a more efficient removal. But with newly designed boots that issue is gone. A modern boot can remove tiny amounts of ice and can be set in an automatic mode where it cycles on a predetermined schedule, removing whatever ice has formed with no pilot intervention required.
Another, perhaps even more important worry for piston pilots and deice boots was the link between the boots and the critical gyroscopic flight instruments. In a piston airplane, air pumps provide the pressure to inflate the boots and to spin the gyros. Cycling the boots, with the extra demands they place on the pump, could possibly cause a pump to fail. That left a pilot in icing conditions and potentially without vacuum to spin the artificial horizon. That’s why TKS, when it was rediscovered by piston airplane pilots, thought it was such a safety advancement, and it was.
However, the link between the air pumps that power a deice boot and critical flight instruments has been severed in all new production piston airplanes I can think of, and also in many existing airplanes that have been retrofitted with electronic attitude heading reference systems (AHRS), which are part of every glass cockpit. The AHRS needs no vacuum to operate so there is no connection between the deice system and flight instruments.
Pneumatic deicers also have advantages over TKS in terms of weight and initial cost. A deice boot system with its hoses and valves weighs about the same as a TKS installation with its leading edge flow devices, hoses and pumps. But TKS is at a considerable weight disadvantage because you must carry the anti-ice fluid. And for a certified flight-in-icing system, the FAA demands that a lot of fluid be carried so the airplane can fly through a lengthy ice encounter.
Boots have finite lives depending on how much you fly, if the airplane is stored out in the sun, and how well they are maintained, with small holes being patched immediately and preservatives being applied routinely. A deicer can easily last 10 years before replacement, and the boots on the wings of my Baron are now 18 years old and doing okay with a few patches. It’s impossible to say exactly how the cost of periodic boot replacement will stack up against the higher initial cost of TKS and the expense of buying and carrying fluid, but it is clear that TKS does not have the price advantage that, at first glance, may appear. Expect to see pneumatic deicers on airplanes of all types far into the future.
Heat is, of course, the ultimate ice protection. Water, supercooled or not, simply can’t freeze onto a hot surface. But thermal ice protection is not as simple as it may seem.
The biggest issue with thermal ice protection is prevention of runback ice accumulation. If a surface is warmed above freezing, ice will not form, but the liquid water, or melted snow, can flow back over the unheated portion of the wing or tail and refreeze. On some airfoil shapes that may not be a big issue, but on other, typically more efficient, wings the presence of runback can destroy lift and create very undesirable stall behavior.
The typical solution to the runback ice threat is to vaporize moisture, frozen or not, on contact with the hot leading edge. In a typical jet with hot wings and engine air inlets, the surface is heated to several hundred degrees, well above the boiling point of water. Droplets turn to vapor on contact with the hot surface and have no chance to adhere further aft on an unheated part of the airframe.
|** Cessna sprayed yellow dyed water onto the nose of this Citation to test ice formation on the radome. The shape of the radome determines how it will collect ice, which is critical in a jet with aft-mounted engines because ice can break off the nose and be sucked into the engines. Tankers spraying a stream of water are often used to test ice formation on small parts of the airframe.**|
As you can imagine, it takes gobs of energy to heat a leading edge to more than 200° with the wind chill of cold air blowing by at hundreds of knots. Some early piston transports had dedicated fuel-fired furnaces to produce the hot air to heat leading edges, but it wasn’t until the turbine engine with its supply of very hot compressed air came along that hot wings became truly effective.
To heat surfaces in a turbine airplane, very hot air is tapped from the compressor section of the engine before the air enters the burner. The compressed air is many hundreds of degrees, so it has the capability to heat the surfaces. However, the air bled from the compressor is not available to be mixed with fuel and burned to produce thrust, so engine efficiency suffers greatly when the anti-ice heat is selected. This is a particular problem for small turbine engines that have little compressed air to spare. That’s why many small jets and turboprops use pneumatic boots instead of heated leading edges for ice protection. In these airplanes bleed air from the engine compressor is used to inflate the boots instead of requiring dedicated air pumps as in a piston airplane.
Another option for heating a surface is electrical power, which works great for small items such as pitot tubes, static ports, windshields and even a smaller airframe surface such as a horizontal tail. But it takes a lot of electrical power to create enough heat to do the job, so its use has been limited until the Boeing 787, which is to be an all electric airplane. The generators will be so massive on the Dreamliner that electrical power will not only heat the surfaces for ice protection, it will also be used to pump up the cabin, power the controls, and even operate the wheel brakes. Boeing believes that using engine power to turn generators will be more fuel efficient than the lost thrust of tapping compressed air to heat the wings and pressurize the cabin.
Electro expulsive deice technology holds some promise for effective protection with low power demands. A version of the technology is being used to deice the horizontal tail on the Hawker 4000 and the Beechcraft Premier I. Electric actuators are used to “ping” the leading edge from inside the structure. The amplitude of the rap, if you want to think of it that way, on the leading edge is small, but the frequency is high, so the very small deflection of the surface effectively shatters any ice that has formed. A large amount of electrical power is needed to actuate the system, but the powered demand period is very short so the energy can be stored in a capacitor, for example. The situation is not unlike a spark plug where very high voltage is needed to create the spark, but the requirement is for only a very brief interval.
Another area of technological advance is in ice detection. That may not be much of an issue during daylight where, on most airplanes, pilots can see some part of the airframe that is an efficient ice collector. But in large airplanes, or for any airplane at night, it can be very difficult to know if ice is forming, and how much, and how quickly, so automatic ice detectors are an important safety tool.
Among the earliest automatic ice detectors is a probe that has a small serrated wheel that turns continuously. If icing is present it will form almost immediately on the sharp teeth of the wheel. As the wheel turns, a fixed wiper knocks off any ice that has formed. It takes more power for the wheel to knock ice off on the wiper than it does to turn freely without ice, so that increase in power demand is what creates the warning to the pilots that ice is detected.
A newer type of ice detector has a small probe that is continuously vibrated at its natural frequency. The probe is very small and thin so it is naturally an efficient ice collector, and ice will form on the probe before it does on larger surfaces such as the leading edges. When ice forms on the probe it changes the mass of the probe and thus its natural frequency. The change in frequency indicates icing and an alert is sent to the crew. Periodically the probe is heated to melt accumulated ice, and then it quickly cools and resumes its task of collecting ice if it is present.
An even newer technology in development by Safe Flight Instruments uses light to detect the presence of ice. A light source shines on a carefully shaped ice collector and a sensor detects if ice has begun to block its view of the light. Collected ice is removed so that the sensor can remain alert of continued icing conditions, or to detect a new encounter.
This chart on NOAA’s ADDS weather site is a forecast of icing at all altitudes from 1,000 to 30,000. The red areas show the chance of large drizzle drops that can create severe icing. The forecast is then broken down into 2,000-foot increments so that you can see if icing is a threat at your expected altitude, and what type of ice is predicted.
A debate among certification authorities is whether ice detectors should be “primary” or “advisory.” A primary ice detection system automatically turns on appropriate ice protection systems when ice is detected. A message is sent to the crew, but no direct switch flipping or button pushing is required to activate the icing protection. An advisory ice detection system sends a message to the crew alerting it to take action.
The FAA and other regulators have gone back and forth on this question, and the pendulum appears to be swinging back toward advisory detection. It would at first appear that a primary ice detection system is optimum since the only crew action would be to arm the system for flight. But the concern is that the pilots become complacent and detached from the ice threat and may not be as alert as they should be if the ice detection system were to fail. Redundant detection systems, of course, are required for a primary system. I can see merits to both certification philosophies, but I must say I do like a primary system, particularly for engine ice protection, so you don’t have to flip the switches every time you fly into a cloud, but can be assured that ice protection will activate if even a trace of ice is detected.
Icing remains a very real threat to all types of airplanes, and is a hot button issue with the FAA and other regulators around the world. The NTSB has elevated the threat of airframe ice high up on its “most wanted” list of safety improvements. The FAA has made certification for flight into icing testing more demanding, particularly when it comes to testing flying qualities with “residual” ice on the airframe.
But technology can only go so far to protect an airplane from an icing accident. The rest is up to pilots. It’s crucial that pilots of airplanes with ice protection systems know the limitations and how to operate the system. All certified airplanes have minimum airspeeds for flight in icing that must be observed, and there are typically other limits on approach airspeeds or flap extension when residual ice may possibly be on the airframe.
For pilots who fly unprotected airplanes in the cold weather months there has been good progress on forecasting of icing. NOAA’s Aviation Digital Data Service (adds.aviationweather.noaa.gov/) has an excellent icing probability and severity forecast that, unlike conventional icing forecasts, shows the chance of icing by altitude bands and over confined areas. While the ever-present icing forecast from the FSS will say there is a chance of light to moderate icing over enormous areas, the ADDS icing forecast maps show you the percentage of probability in 2,000-foot altitude increments for more than six hours into the future. For the short term you can see a prediction of the severity of icing in 2,000-foot increments of altitude, and also the possibility of large drizzle drops that can cause extreme ice formation.
NOAA warns that, in compliance with FAA policy, the icing forecasts presented on the ADDS site are for situational awareness and to supplement conventional FAA forecasts and alerts in the form of airmets or sigmets. That is all well and good, and shows the usual caution, but I have found the ADDS forecasts to be very accurate and the first really useful tool in planning a flight when cold clouds are likely.
But, no matter how good forecasts may be, there is still the chance of icing in a cold cloud, so if you don’t have icing protection on the airplane, you need an escape route in the form of high ceilings that allow a descent to clear air, or reliable reports of low cloud tops that you can reach even if ice begins to collect.
The low cloud tops, though, do have a big caveat when flying over the Great Plains. In that part of the United States, low clouds on a cold day almost always indicate the presence of a temperature inversion with warmer air aloft above cold air trapped at the surface. That means the cloud tops are typically low, and it is clear above. The problem is that the cold air and moisture are near the surface so the only way to land is to descend into the potential icing conditions. More than a few of us have been forced to abandon approaches and climb back into the clear warm air above when cold wet clouds cover the Plains. The conditions can be very widespread with ice-free alternate airports hard to find, so be very alert when it’s cold and overcast on the flat lands of the Great Plains.
Finally, pilots need to be aware that some newer design airplanes may have more unpredictable flying qualities when ice forms because the airfoil shapes are more efficient and lower in drag, but also respond more dramatically to the shape change caused by ice. Many piston airplanes with their design roots traced back 40 and 50 years hold few surprises when the wings ice up. Yes, drag goes up and the airplane slows down, and at some point you won’t have enough power to continue to hold altitude. But as long as you don’t extend full flaps, and maintain a healthy margin above stall, the airplane will fly more or less normally. But other airplanes with smaller wings and thinner airfoils may not tolerate the presence of ice so well. In either case you will become a test pilot once ice forms, so it is essential that pilots of unapproved airplanes avoid ice, and then exit it immediately after an unexpected encounter.