What's New in Icing?
(continued) 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.
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