Accumulating Knowledge: De-Ice Boots

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A brief history of pneumatic boots, their operation and proper care. 

AS Jimmy Doolittle was demonstrating the technique of blind flying in 1929, work was being done by B.F. Goodrich and the National Advisory Committee for Aeronautics (NACA) to address airframe icing. 

William C. Geer, Ph.D., a retired chemist from the B.F. Goodrich Company, became interested in the problem of airframe icing when it caused a number of crashes of airmail planes. With IMC flight in its infancy at the time, airframe icing was seen as a barrier to progress.

In the early 1930s, work by Geer and B.F. Goodrich focused on rubber coatings to inhibit the development of ice. How to get rid of the ice that did accumulate, despite the rubber boot and the concoction that was smeared on them to prevent the buildup, led to the idea of having inflatable tubes to knock off the ice. The de-ice boot was born.

The upper illustration shows a leading edge boot as it is held against the leading edge contour (its normal in-flight condition). The lower illustration shows a leading edge boot in an inflated condition; the boot is typically in this position only for a short time to remove an accumulation of ice.
Structure, activation and various types

The boots themselves are generally constructed with five or more spanwise tubes. These are created by layers of rubber laid up in such a way as to create the channels which expand when system pressure is applied to them. The inflation pressure is typically around 18 psi.

Once the concept of inflating tubes in the boots was proven to be effective, complete de-ice systems were soon developed. In addition to the boots themselves, a timer, valves and a pressure source were necessary. 

The researchers also discovered that in normal, non-icing flight, the aerodynamic pressure differentials around the wing could allow the boots to expand somewhat without activation of the system. The solution was to apply a small amount of suction to the boots during the times when the boots were not being operated by the pilot.

Modern systems use a pressure pump driven by the engine(s) which in normal operation discharges through a venturi, which supplies the suction to the boots to keep them tight to the airfoils when the boots are not in use. 

When activated, electrically controlled valves switch the pressure from the venturi to inflate some or all of the boots. The timer will keep the boots inflated for several seconds before switching back to the suction mode.

Many common installations will inflate all the boots simultaneously. Examples of this are the earlier 300 series Cessnas. 

With the certification requirements for approval for Flight Into Known Icing (FIKI)—which applies only with aircraft certified after 1973 or if the manufacturers chose to obtain FIKI certification—additional boots were added inboard of the engines, and boots were added to cover all tail surfaces. 

With these expanded systems, it is common for the timing system to inflate the wing boots and the tail boots separately. One example is a later C-421 with the FIKI certification. This is likely due to the requirement that the de-ice system still function after the failure of one of the pneumatic pumps.

De-icing systems are available for aftermarket installation. As each aircraft will utilize differently sized boots, and testing is required to make sure that any installation does not hinder the flight characteristics of the aircraft, most aftermarket installations are by STC. B/E Aerospace and Goodrich hold many of the STCs, but not all.

Proper operation 

There is a certain amount of controversy over the proper operation of pneumatic de-ice boots. The common wisdom—which has come down from the early days of icing flight—is to wait until one-quarter to one-half of an inch of ice has accumulated before popping the boots. The concept has been enshrined in numerous aircraft Approved Flight Manuals and POHs. 

The NTSB has been at war against this mindset for a couple of decades. Based on its research, the NTSB, and to some degree the FAA, have been advocating turning the pneumatic wing de-icing system on at the first sign of icing.

Older-style boots (those dating before the 1960s), may have been prone to a condition called “ice bridging.” This is where ice would build to the point that it formed a bridge over the top of the boots. (Ernie Gann reported on this phenomenon on a DC-2 in his semi-autobiographical book “Fate is the Hunter.”) 

However, the NTSB is adamant that modern boots, (i.e., any that have been installed in the last 50 years or so) will not form an ice bridge. 

Against this, northern pilots do occasionally claim to have seen ice bridging occur in newer aircraft. 

After half a dozen years flying in the ice of the Great Lakes and in Southeast Alaska, I can say that I have never seen ice bridging—but that does not mean I am completely sold on the NTSB’s recommendation to activate the boots at the first sign of ice on the wings.

The other consideration is whether the boots leave less residual ice if they are operated continuously, or if they are cycled after a small buildup. 

I believe that this issue is a bit more nuanced than the NTSB is willing to recognize. When to operate the boots encompasses numerous factors, in my experience. 

It is a fact that the ice sheds better at higher speeds as the force of the airflow on the ice increases exponentially with airspeed. It is also beyond argument that some airfoils are much more sensitive to an accretion of ice than are others. Outside air temperature, the type of ice, and the condition of the boots all affect the ability of the boots to shed the ice. 

The NTSB guidelines seem to assume that all wing de-ice systems can be turned on and that they will then cycle at intervals. This is not the case on most aircraft where you have to individually activate each cycle. When flying single-pilot IFR, I rarely have time to sit and punch the wing de-ice button every few seconds. 

I tend to wait until I have a noticeable accumulation of ice before popping the boots. I don’t wait for one-quarter of an inch, and certainly not a full one-half inch; I generally try to activate them during a high speed descent, and again if I get into air that is at freezing or above. I also try to make a final activation after breaking out of the ice, or on final approach if I haven’t gotten out of the ice. (This is the author’s personal procedure in her own aircraft, and is for readers’ information only. Cessna Flyer urges all pilots to read current NTSB and FAA recommendations. —Ed.) 

Care for pneumatic boots

A set of de-ice boots is very expensive, so they should be cleaned and protection should be applied. 

There are three manufacturers of boot cleaning and sealant products. Goodrich Corp. makes ShineMaster for cleaning and AgeMaster for protecting boots. Jet Stream Aviation Products makes Pbs Boot Prep and Pbs Boot Sealant. Real Clean Aviation Products makes a similar de-ice boot care system called Real Shine. 

In my experience there is one product that needs to be on your shelf, and that is B.F.G’s Icex II. Even after cleaning and sealing, the application of a slippery coating before charging off in the ice is a very good thing. It makes a huge difference in the ability to shed ice. Icex II is expensive, but you won’t have regrets about the cost when the ice is being shed off the wings cleanly. Besides, a quart can last a couple of years for most pilots.

Kristin Winter has been an airport rat for almost four decades. She holds an ATP-SE/ME rating and is a CFIAIM, AGI, IGI. In addition, Winter is an A&P/IA. She has over 8,000 hours and uses her aircraft in furtherance of her aviation legal and consulting practice. Winter also assists would-be owners with training and pre-purchase consulting. Send questions or comments to .

Resources

De-icing equipment
– CFA supporters

B/E Aerospace, Inc.
 
Goodrich Deicer Service Center

De-ice boot care
– CFA supporter

ShineMaster, AgeMaster, Icex II

Goodrich Corp.
(UTC Aerospace Systems)

Other de-ice boot
treatments and protectants

Jet Stream Aviation Products, Inc.
 
Real Clean Aircraft Detailing Products

 

Further reading

NASA SP-2002-4226,
“We Freeze to Please” 

A History of NASA’s Icing Research Tunnel and the Quest for Flight Safety by William M. Leary. NASA History Office, 2002.

FAA-H-8083-31, AMT Airframe Handbook

Volume 2, Chapter 15: Ice and Rain Protection.  U.S. Department of Transportation Federal Aviation Administration Flight Standards Service, 2012.

Available at CessnaFlyer.org/forum under “Magazine Extras”