From the Flight Deck: Freezing Level Boots, Bleeds, Benefits and Beliefs

From the Flight Deck: Freezing Level Boots, Bleeds, Benefits and Beliefs

From the Flight Deck: Freezing Level Boots, Bleeds, Benefits and Beliefs

You’ve heard it before and have likely said it yourself: being on the ground wishing you were in the air is better than being in the air wishing you were on the ground. Inflight icing can make us wish we were on the ground. Structural icing occurs when supercooled water freezes on impact with any part of the aircraft during flight. Airframe icing can lead to reduced performance, loss of lift, altered controllability and ultimately a stall and loss of control. These are very bad things and wishing won’t help.

Pilots that lean more towards VMC flight planning may shake their heads and wonder why anyone would launch into known-ice in the first place. There are two classifications of deicing systems in GA: those for flight in known-icing and “non-hazard” systems that provide time to escape from icing. Known-ice certification is rigorous. In addition to protecting a dozen or so surfaces and components that include the obvious leading edges, windshield, engines and air data probes, the manufacturer must evaluate aircraft tolerance to accumulation on unprotected surfaces such as antennas, landing gear, nose cones, leading edges of flight controls and tip tanks. Most T&T readers operate aircraft in the first classification– certified for flight into known-ice. The decision to fly into ice lies in our confidence in the system and experience with ice. The worst icing is common in just the top 1,000 feet of cumulus clouds when the temperature is 0°C and lower and ice normally resides in a layer of cloud only two or three thousand feet thick; it can usually be transited quickly. That being said, those that fly in icing  know to get out of it as soon as possible, to always use deice and anti-ice equipment and to have a plan if the equipment can’t keep up or if it fails.

img_1754-1We Have a Problem

Remember the scene in the movie Apollo 13 when the switch is activated to stir the O2 bottle? The camera follows the electrical current along the wire to the tank. When it reaches the O2 tank, it explodes. When I actuate a control that is seldom used like the alternate air, alternate static or the boots, that scene plays through my mind. You likely won’t explode when you hit the deice switch but it only takes one pneumatic hose or a bleed-air duct coming unfastened for you to lose some, or all, of your system. If the deicing system fails or if it can’t keep up, consider this: In NTSB studies, a pilot diverting due to icing was effective in less than 25% of the cases that reached the required threshold of an accident or reported incident. The predominant sequence of events involves a stall followed by loss of control and impact with the ground. Of the less than 25% that made it to an airport, sufficient performance was lost during the approach so as to force descent below the glide path. Of those that made it to an airport and to a runway, a large number resulted in hard landings. These statistics are why we should avoid or exit all types of icing– even when our system is working properly.

Varieties of Ice

img_5143Clear ice is both clear and smooth. Supercooled water droplets or freezing rain strike a surface but don’t freeze instantly. It generally conforms to the shape of the airfoil. Rime ice is rough, opaque and formed by supercooled drops rapidly freezing on impact. Often “horns,” “bowls” or other protrusions are formed and project into the airflow. Rime ice appears white in color. Mixed ice is a combination of clear and rime ice. Frost ice is the result of water freezing while the aircraft is stationary. Frost with a surface texture similar to forty-grit sandpaper is enough to disrupt an airfoil’s boundary layer airflow causing increased drag and a premature aerodynamic stall.

SLD ice (Supercooled Large Droplet) is similar to clear ice but because droplet size is large, it extends to unprotected parts of the aircraft and forms larger ice shapes. Runback ice forms when supercooled water moves aft of the surfaces beyond the protected area and then freezes as clear ice. If we encounter any of these types of inflight ice, a pilot report is mandatory and should include the type of ice and level of intensity: trace, light, moderate or severe. ATC will also want to know the OAT at the time of the encounter. The gamut of systems to combat these types of icing runs from liquid, to pneumatic boots, to electrically heated components, to bleed air systems. Each system presents varying degrees of weight, cost, effectiveness and reliability.

Thermal Systems

A thermal system (bleed-air or electric) may operate one of two ways: fully evaporative or wet. In the evaporative case, sufficient heat is provided to cause supercooled water to completely evaporate. This has the advantage of protecting the aft, unheated portion of the airfoil. A wet thermal system can only prevent water from freezing. It requires less energy but it can fail to prevent runback ice, which forms when the running water passes aft of the protected surface and freezes. Even a fully evaporative system may transition through a wet phase as it heats and cools. The ideal method for operating a fully evaporative system is to activate it prior to entering potential icing conditions, thus allowing the surface to stabilize at the required temperature.


Electro-thermal systems use resistive circuits buried in the airframe, windshield or propellers to generate heat. The system only needs to melt the contact layer
of ice for wind-shear to then shed the remainder.

TKS (Tecalemit – Kilfrost –
Sheepbrige – Stokes)

The TKS system uses a glycol-based fluid which exudes through 0.0025-inch-diameter holes in panels on the leading edges of the wings and horizontal stabilizers. The fluid lowers the freezing point of water preventing it from freezing and adhering. The system can be installed as a known-ice or non-hazard system depending on redundancy, additional components and type of aircraft.

Pneumatic Boots

img_5220-1The most common deicing system for GA aircraft, including some jets, uses pneumatically inflated rubber boots on the leading edges of airfoil surfaces. This normally includes the wings and empennage, but may also include struts and cargo pods. The system uses relatively low pressure air to rapidly inflate and deflate the boot. The principal drawback to boots is the aircraft will operate with ice accretions for the majority of the time in icing conditions and it provides no protection from runback ice. Also, the only time it will be free of ice will be immediately after cycling the system.

Ancestor Worship: Ice Bridging

Early pneumatic boot designs had relatively low volume air supplies to draw from, and were slower to inflate and deflate. A phenomenon which was thought to be occasionally observed with these systems was known as “ice bridging,” in which the boot expanded under the ice and stretched it without breaking its structure. This led to a space beneath the ice shape which allowed the boot to subsequently inflate and deflate with no effect. The problem was addressed by allowing a particular thickness of ice to develop before inflating the boot. Once the requisite thickness was attained, the boot inflation would shatter the ice and clear it off the surface. With the current, rapidly inflating systems, there is almost no evidence which supports the existence of this phenomenon.

From the NTSB in 2008: “For 60 years, pilots have been taught to wait for a prescribed accumulation of leading-edge ice before activating the deice boots because of the believed threat of ice bridging. In theory, ice bridging could occur if the expanding boot pushes the ice into a frozen shape around the expanded boot, thus rendering the boot ineffective at removing ice. The Safety Board has no known cases where ice bridging has caused an incident or accident……” Leading-edge deice boots should be activated as soon as icing is encountered, unless the aircraft flight manual or the pilot’s operating handbook specifically directs not to activate them.

Exit Strategy

Icing accidents result from a combination of increased weight, increased drag, loss of lift, and a decrease or loss of thrust caused by induction air blockage and propeller or compressor blade contamination. Whether thermal, “weeping wing” (a.k.a. TKS) or pneumatic, deice systems give your aircraft more utility and safety but are designed to get you out of a bad condition. Avoid ice as much as possible (standard temperature lapse rate is 2.0°C / 3.5°F per 1,000 ft.) and exit it promptly when encountered. Because of accumulation on unprotected areas, consider adding a few knots to your configuration and approach speeds. Know your system, test your system and don’t hesitate to use it early. If you demand one iota more than it can give, you may find yourself wishing you were on the ground.T&T

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