How Aircraft Deicing Equipment Works

John Peltier

Aircraft have sure come a long way when it comes to all-weather capability! One of the biggest advances is how we can deal with ice that can potentially form, or has formed, on the aircraft with aircraft deicing equipment.

There are two types of systems with drastically different purposes to keep you safe. Anti-ice is used before flying into icing conditions, to keep ice from forming. Deice is designed to remove ice after it has already formed. The systems, in general, have many similarities but not all of them are actually approved for flight into known icing conditions.

Anti-icing Systems

Anti-icing systems (preventive aircraft deicing equipment) usually involve some sort of heat. Heating these surfaces keeps water from freezing, and thus, ice from forming. Critical areas that are heated in some aircraft include at least the pitot tube, and sometimes the propellers, windshield, wings, and engine inlets.

Heat for these systems comes from two sources. The first is from the engine, known as bleed air. Turbine aircraft commonly employ bleed air to heat up engine components like compressor blades and inlets. Ducting to engine components is less complex, though sometimes this bleed air is also routed to leading edges of wings and windshields. Carburetor heat on piston aircraft is another form of bleed air anti-icing / deicing systems. In general, however, larger bleed air heating systems are not commonly found on general aviation aircraft. Just about all of these systems introduce excessive noise and rob the engine of some power.

Propeller with electric deice detail. Photo by: YSSYGuy

Propeller with electric deice detail. Photo by: YSSYGuy

The other method of heating critical surfaces is with electricity, much like your toaster uses. This method of heating is usually applied to pitot-static systems, propellers, and drains. It is important to activate these systems before ice buildup starts, as they may not get hot enough to melt thick ice. It operates by simply applying electricity to a closed circuit. The new Boeing 787 Dreamliner uses electro-thermal systems for deicing rather than bleed air like its predecessors.

Deicing Systems

Additionally, some aircraft are equipped with dispensers to apply deicing fluid to the wings. Deicing fluid has a very low freezing point and delays ice formation. Some of these fluids not only prevent ice from forming but they also inhibit the formation of new ice except in the most extreme circumstances. These systems are known as “weeping wings” and their drawback is the limited supply of fluid that they can carry. Many a pilot has left these systems activated for too long and run out of fluid!

Pneumatic Deicing boot on leading edge of wing, Photo by: YSSYGuy

Pneumatic Deicing boot on leading edge of wing. Photo by: YSSYGuy

The only other reactive form of aircraft deicing equipment commonly used is slightly more complex. Removing thick ice can be tricky especially given the uneven surfaces. Knocking the ice off of leading edges of control surfaces is the only other way if it’s not done with heat or fluid. This isn’t done by making your passenger get out of the aircraft with a long pole. No, strips of rubber are used instead. These rubber boots inflate, slightly changing the shape of the wing and breaking the ice free from the aircraft. The rubber then returns to its original aerodynamic shape. These too have their drawbacks, adding extra weight and power requirements to the aircraft.

Heat tape is the next great thing to happen in regards to aircraft deicing equipment; this lightweight graphite foil can melt ice that forms on the leading edges of wings and tail surfaces without adding much extra weight at all, without altering the shape of the airfoil, and without requiring a lot of extra power. NASA has been extensively testing these systems.

Regulations Regarding Aircraft Deicing Equipment

So, with all of this fancy aircraft deicing equipment, do you think you’re safe if your aircraft is equipped with deicing systems? You better check your Pilot’s Operating Handbook for the answer. Many general aviation aircraft that have these systems installed are not legally allowed to fly into known icing conditions. This goes for both factory-installed equipment and for retrofitted equipment.

Anti-icing and deicing equipment need to go through a rigorous testing process in order to be certified for flight into known icing conditions. These tests are twofold: first, the airframe is tested to determine which flight regimes will put it at the greatest risk for ice formation. Then the aircraft is tested in this flight regime and all systems are subjected to the worst-case scenarios. The autopilot, engine intakes, ice detection systems – all of them are subjected to harsh conditions to ensure operability. These tests have to show that these systems provide some degree of preventing ice formation or shedding ice. This does not mean that continued flight into icing conditions would be smart, or even safe.

But these tests only occurred after 1977. For aircraft certificated before 1977, these systems were only checked to see if they had any kind negative impact on aircraft performance. There was no guarantee that these systems could handle ice at all.

Even after 1977, not all aircraft go through these tests. The tests are expensive! So manufacturers (mostly general aviation) have these systems installed as “emergency” equipment, much like a parachute. They’re only tested to make sure that they won’t affect aircraft performance so that they can get certified for installation. Flight into severe icing is never legal under any circumstances.

Photo by: Anton Dit

Photo by: Anton Dit

Unfortunately, many pilots see that their aircraft has deicing equipment and believe that means that they can fly into icing conditions. But this equipment usually isn’t certified for that, at least in general aviation! Just remember next time you try to fly into icing conditions that these systems most likely weren’t tested with real ice!

Flying with anti-ice and deice systems, even if they’re certified for flight into icing conditions, does not make you invincible. Especially during freezing rain – this can accumulate ice rapidly, without many visual cues to the pilot, and spread beyond regions that are protected by this equipment.

As with every other system, it is critical to preflight and test operation on the ground. It can be as simple as turning on the switch and making sure circuit breakers don’t pop and turning on the ice detection light and guaranteeing that it illuminates. Check the manuals for the proper procedures.

Next time you hop into an aircraft that has deicing equipment, check the POH, AFM, or the cockpit for placards indicating whether or not flight into ice is legal!

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Featured Image: Mazaletel

How Not To Impress a Friend With Carburetor Icing

Shawn Arena

Welcome back for another installment of one of my ‘lessons learned’ stories from my personal flying experiences over the years. This particular story, about carburetor icing, could have just as well been sub-titled: “How do you un-declare an emergency?”

A Beautiful Flying Day with a Beautiful Friend

Our story this time takes place in the summer of 1986. I was living in a one-bedroom airport in Costa Mesa, California, about one and one-half miles from John Wayne/Orange County Airport (SNA) in southern California. By that time I had my private pilot license about two years and enjoying every venture I took to the air – but today’s venture was more than what I was expecting.

A very beautiful young woman Abby had moved in to the same apartment complex and we became good friends – not dating or anything, but more than ‘Hi, how are you?’

She would come over to my place, or I would visit hers and we would talk about the day’s events or just chit-chat. One day I got up enough nerve and asked if she would be interested in going flying with me the next weekend to do some sightseeing at Catalina Island (AVX).

Catalina was one of those island locations you hear about in the movies or read in travel magazines. It is part of the Channel Islands chain off the coast of southern California, crystal clear lagoons and flora, and Avalon (the only city) was a tourist’s paradise. Oh, and by the way, their claim to fame (among other things) were the buffalo burgers they served at the airport café. So the time and date were set to meet at SNA to begin our journey.

Some Unexpected Carburetor Icing

The day had come and it was spectacular. In a pilot’s vernacular it was CAVU (i.e. clear and visibility unlimited). I rented a Cessna 152 from the flight school where I learned to fly and off we went. Geographically, the statute distance is 26 miles and about 2 hours by ferry (Readers note: in 1958, the group the Four Preps released a hit song in California whose opening lyrics were- ’26 miles across the sea, Santa Catalina is waiting for me…”) , but even in a two-seat underpowered Cessna 152, it took only about 20 minutes.

About mid-channel, the ‘fun’ began (let me preface this ‘fun’ by saying air temperature at sea level was 95 degrees, but at 5,500’ MSL it was about 70-75 degrees or so – keep that in mind, as it plays a very important part in our story). I suddenly noticed the propeller beginning to feather and the RPMs were dropping. Up to that time in my brief flying career, I had not experienced anything abnormal, like carburetor icing, in any flights. All at once I had my flight instructor Lance in my ear, “Start a descent and push in the carb heat.” Well I started my descent (but did not instinctively push in the carb heat for some reason) – I guess some first time “Oh, Oh’s” took over.

KAVX Catalina Airport from the air

KAVX, Photo by Ravi Komatireddy

By that time we were close to the airport and I radioed the Unicom operator I wanted to declare an emergency. They immediately waved off any / all aircraft in the vicinity of the airport and I was cleared to land Runway 26. Since Catalina is an island airport, it is surrounded by cliffs on both sides of the runway. And as I was concentrating on putting this puppy on the ground, I realized I needed to listen to Lance’s second half of his imaginary message to push in the carb heat. I did, and the engine started back up and RPMs returned to normal. BUT, I was too high and was not wanting to make a bad situation worse.

Aviate, Navigate, Communicate – and Aftermath

I passed over the airport about 2,000 feet above pattern altitude and as I was about to start my ‘stairway to heaven’ climb I heard myself thinking: “How do you un-declare an emergency?” and Lance’s voice came back and said two things: ’aviate, navigate, and communicate’ and ‘there is no substitute for altitude.’ ‘Fly the plane, Shawn,’ I told myself and kept on climbing.

By the time I was assured of a landing by gliding if I had to, I was at a comfortable 8,000’ MSL and headed back to SNA. Poor Abby, all through this she did not say a word, but I noticed that her fingernails had made an indelible impression in the passenger armrests. We landed safely and (figuratively) kissed the ground. And though we remained friends, Abbey never flew with me again, nor did I mention that three-letter word again to her.

In the weeks that followed, I did my best private investigator impression and asked as many mechanics and flight instructors as I could about my experience and all said the same: “Son, it looks like a prime case of carburetor icing.” So it was, a BIG lesson learned for a still-green-behind-the-ears pilot but a valuable one at that, and one I’m glad it happened. So, in closing, be careful out there and remember to ‘aviate, navigate, and communicate’ (and hopefully the girl will want to go on another flight with you!)

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Related:

Quiz: How Much Do You Know About Aircraft Icing?

Quiz: How Much Do You Know About Aircraft Icing?

Dr. Mary Ann O’Grady

How much do you know about aircraft icing and the conditions that cause it?  Take this quiz, and see how well you do.

Aircraft Icing Quiz Questions
  1.  _____ ice is lighter than _____ ice, has an irregular shape and a surface roughness that reduces aerodynamic efficiency.
  2. The three negative outcomes of aircraft icing on the airplane are: _____, ______, and _____ .
  3. The three types of structural icing are _____, ______, and _____ .
  4. The two ingredients for structural icing are ______ and _____ .
  5. The most important effect of ice on the wings and tail is _____ .
  6. The three types of icing intensities are _____, _____, and _____ .
  7. When this type of icing intensity occurs so that deicing or anti-icing cannot reduce or control the accumulation, the pilot’s only option is to _____ .
  8. The type of cloud that produces the most severe icing is a _____ cloud.
  9. If a pilot encounters ice in cumuliform clouds during the winter, he or she should _____, or ______ immediately
  10. The first places that a pilot should look for the formation of ice on the aircraft are _____, or ______.
Aircraft Icing Quiz Answers
  1. Answer: rime, clear
  2. Answer: decrease in thrust, reduction in lift, and an increase in drag
  3. Answer: rime, clear, mixed
  4. Answer: presence of visible moisture, temperatures at or below freezing
  5. Answer: reduction of lift
  6. Answer: trace, moderate, severe
  7. Answer: get out of the icing [conditions]
  8. Answer: cumulous
  9. Answer: divert, descent into warmer air
  10. Answer: leading edges of the airfoils, any objects that protrude into the air flow, such as antennas, OAT probe, etc.
Discussion: Aircraft Icing Conditions

The motivations underlying why rational pilots who avail themselves of all available weather information and data during their flight planning process, yet ultimately decide to deliberately fly into icing conditions are varied. But one key element may be the fact that these PICs lacked sufficient knowledge about icing conditions, and they found themselves navigating into dangerous weather conditions. Aircraft icing has long been classified as one of the greatest weather hazards to aviation. This is because icing is likely to be both cumulative and invisible which can cause the aircraft to slow down, force it downward, and/or make it go out of control. In addition, engine performance can diminish, contribute to false indications on the instruments, and result in a loss of radio communication. It can also freeze the landing gear to a point where it cannot fully extend or retract, and it can prevent the brakes from functioning properly. During the winter months, structural icing is more of a concern for pilots during a flight than induction icing, which is why it’s the focus of this quiz and subsequent discussion.

Only two ingredients are required for structural icing: visible moisture and temperatures at or below freezing. Cooling occurs when lift is produced which can reduce the aircraft surface or skin temperatures to below freezing despite the ambient air temperature being above freezing. Supercooled water is defined as water that remains in a liquid state although its temperature has dipped below freezing. When a supercooled drop of water comes into contact with a cold aircraft, a portion of that drop freezes instantly and adheres to the aircraft’s surface while the remaining portion of that drop is warmed by friction. Aerodynamic cooling can cause that drop to refreeze, however, and it is the manner in which that remaining liquid freezes that determines whether the forming ice is clear, rime, or mixed. If the supercooled large drops flow out and freeze into a smooth sheet of ice, it creates clear or translucent ice that is hard, glossy, heavy, and tenacious. As its accumulation continues, it may build up into a single or double horn-like shape on leading edges of the aircraft which increase drag and a correspondingly inverse decrease in lift.

In contrast, rime ice is created from supercooled small drops where the liquid freezes more quickly before it has had time to spread out over the aircraft’s surface which traps air between the droplets giving rime ice a rough, milky, opaque appearance. Although rime ice is lighter than clear ice, its irregular shape and surface roughness reduces aerodynamic efficiency by reducing lift, increasing drag; and it is more easily removed with aircraft deicing equipment than is clear ice which is heavier and results in a solid sheet configuration. When the supercooled water droplets vary in size or mix with snow or ice particles, a combination of clear and rime ice can form very rapidly into highly irregular shapes that build up on airfoil leading edges. Regardless of which form icing assumes, the amount of the ice accumulation is directly proportional to the amount of liquid water in the clouds with the worst case scenario being a combination of large water droplets, temperatures close to freezing, and clouds having significant water content.

The effects of icing include a reduction in lift, an increase in drag, and a decrease in thrust where the effects of these three factors become cumulative which may require a full power setting and a high angle of attack to maintain altitude. However, this attitude may result in a new problem where ice can begin forming on the underside of the wing which adds more weight and drag so the need to get out of the icing conditions then becomes the prime directive. Depending on the PIC’s experience with flying in icing conditions, any ice may be too much ice but the FAA has categorized icing into three intensities: trace, moderate, and severe. Trace ice is barely visible and is typically not a hazard unless the aircraft is exposed for one hour or more. Trace ice can usually be handled by inflight deicing/anti-icing equipment for durations of one hour or less. Moderate ice accumulates at a rate where even short encounters with it are potentially hazardous, and the use of deicing/anti-icing equipment is definitely required. Severe icing is defined as an accumulation of so much ice that deicing/anti-icing equipment cannot reduce or control its accumulation so the only option for the PIC is to get out of the icing conditions as quickly as possible.

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Reference:

Duncan, P.A. (2016). Rime and Clear and Mixed. Retrieved on March 10, 2016, from
http://avstop.com/stories/rimeandclearandmixed.htm

Additional Quiz:

Do You Know These 5 Aviation Acronyms?

Additional Resources:

Aircraft Icing Safety Advisory – AOPA

Aircraft De-Icing and Anti-Icing Equipment – AOPA

Aircraft Icing Advisory Circular – FAA

The Madness of Icing – Flying Magazine