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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