Airplanes and ice were not a good match from the start. But limiting our annual aviating by somewhere between three and six months — and more in some parts of the country — was never a good solution either. So, ice protection, as it became known, was a requirement for many year-round operations. Pneumatic boots were introduced in the early 30s, and weeping wings in the early 40s. The advent of jet engines in the late 30s eventually led to the introduction of bleed air systems. And now, more robust electrical systems allow for new solutions.
As far as how the FAA arrived at introducing regulations on icing protection systems, there is some interesting history rooted in WWII strategies. According to a NASA special publication, Engines and Innovation, military planners initially suggested an offensive through Alaska and the Aleutian Islands as the most direct approach to Japan after the attack on Pearl Harbor. This led the Army to request research from General Electric, Massachusetts Institute of Technology (MIT), and all three laboratories at the National Advisory Committee on Aeronautics (NACA, NASA’s predecessor agency) to improve protection against aircraft icing. Ironically, a more southern approach was chosen, and the Pacific theater became known for the exact opposite conditions. Although the Japanese would go on to occupy Attu and Kiska for more than a year before they were dislodged, the Aleutians would not be the main thrust for the Allies. This research had limited value during the war, but it would provide data to the Civil Aeronautics Administration for regulation.
Let’s take a look at the history of some ice protection systems, where we are now with this technology, and what options might be right for you.
Dictionary of Terms
Before we dive into the details of icing protection systems, let’s first define some terms. There are anti-icing and deicing systems. Anti-icing systems are designed to prevent ice from accumulating on protected surfaces while deicing systems are designed to periodically remove accumulated ice from protected surfaces. Some systems can have aspects of both, but some only have one, so it’s essential to understand what exactly yours is designed to do and what limits it can handle.
Another definition to understand is FIKI, an acronym for flight into known icing conditions. This is an approval to operate in icing conditions with certain systems that are certified. It’s critical to know whether your system has this approval or not because many unapproved systems appear similar to approved systems and are often made by the same manufacturers. The difference often lies in some combination of endurance, redundancy, and completeness.
Inadvertent systems, sometimes called escape systems or non-hazard systems, are intended to allow pilots who encounter unforecast icing to escape with some additional safety margin. They do not allow for flight into forecast or existing icing conditions, or for continued flight in those conditions. The best way to think of them is like the instrument-based training requirement from the private pilot certificate. Those tasks are there to give you a basic ability to recover from an inadvertent instrument weather encounter, not to allow you to file and fly under instrument flight rules (IFR). It is critical to know the capabilities of your system before you need to use it.
Burn, Bust, Weep, and Zap
There are several ways to approach ice protection. Still, they all perform a similar function: prevent or eliminate (or both) ice accumulation on protected surfaces of an aircraft with a minimal performance impact. NACA developed a test airplane during the war, a Lockheed 12, that used engine exhaust gases to heat the leading edge. The solution worked, but the increased mechanical complexity meant that it was not feasible to implement in production at a time when industrial capacity around the world was at an extreme premium. The concept would become more viable with the arrival of jet engines. The jets provided an abundant source of hot air courtesy of bleeding it from the compressor stage of the engine. By preventing any ice from adhering to the heated surface, this anti-icing solution is very effective and allows the wing to maintain optimal shaping, but it is costly in terms of performance. Bleeding that air off the compressor reduces thrust, meaning that either higher thrust settings are required, or performance is reduced when the system is in use.
Small general aviation (GA) aircraft must pursue other options since bleed air systems are generally incompatible. The first option was pneumatic boots, which were invented in the late 1920s by researchers at BF Goodrich and originally called rubber “overshoes.” Pneumatic boots work by using a series of rubber bladders that lie flat when not in use but can be periodically pumped full of air to change the shape of the leading edge of the wing to break up any ice accumulation. Boots are very effective and allow for practically unlimited usage in an icing encounter. The downsides include additional weight from all the boots, pumps, and electrical capacity needed to run the system. Also, pneumatic boots are strictly a deicing system, and the rubber material’s condition will need to be monitored as it will degrade over time, especially when stretched routinely.
One of the most popular GA icing protection systems is what is referred to as a weeping wing. These systems use a leading edge with micro-perforations that “weep” a fluid to lower the melting point of water/ice. The British invented them during WWII. It works similarly to how salt is used on the road or antifreeze is used in automotive cooling systems. This gives the system a few key advantages. First, it functions in both an anti-icing and deicing capacity. It also provides coverage beyond the leading edge as the fluid will be carried by the slipstream across the surface. This addresses one of the weaknesses of boots in that it prevents/eliminates ice accumulation behind the leading edge. Weeping wings also require less electrical energy to run their pumps than boots. But every system has its limitations. Weeping wings are time-limited by the amount of fluid on board. And that fluid is a variable weight penalty and a possible supply issue at some airports if you need to refill while flying cross country.
A newer solution is a rework of the electro-thermal heated wing concept, in which an electrical current provides the heat. This eliminates the need to sap bleed air and, thus, performance from the engines of a jet. It also means that it can be installed on non-turbine aircraft. The downside is that it requires quite a bit of electricity. One system for GA that was spun off from NASA technology required the installation of a very beefy alternator to provide the power needed. There is no broad-based adoption of this approach at the moment, as the company that marketed it for GA no longer lists it on its website.
Another electric-based solution is electromechanical or electromechanical expulsion. These systems use a series of actuators to induce a shockwave in the aircraft’s skin and literally blast the ice off the protected surface. These systems have been proven effective and have much lower electrical operating requirements than heating systems. While this may sound like a perfect solution, there are always some disadvantages. First and foremost, they need to be built into the aircraft, though some systems may be retrofitted. While they are in use in some aircraft, they have generally been limited to business jets or larger. They may prove a viable, well-tested solution for modern electric designs that will be sensitive to the electrical draw of many other technologies. Affordably integrating this solution into GA could be a challenge but would open up ice protection to many more pilots.
The Devil in the Details
When considering ice protection, the first step is deciding whether you want to pursue a system as part of an aircraft purchase or retrofit. Any of the systems described here will come with a cost in terms of weight, maintenance, training time, and money. In some cases, it may be best not to opt for one at all. But if you have a need that justifies ice protection, is an escape/inadvertent system sufficient? As noted earlier, many of these systems function similarly to the FIKI-approved versions, but lack redundancy or additional protections (i.e., backup pumps/electrical equipment or protection on windshields or propellers). These omissions align with the mission of providing an escape capacity while reducing cost, complexity, and weight. This is a good compromise point for many GA pilots. It’s essential, though, to mind the limitations built into the system and use it as intended.
Suppose you have the need and resources to jump into a complete FIKI system that will give you the best capabilities. Just remember that the additional maintenance, weight, and cost of these systems should not be taken lightly. These robust systems also have limits, so, be sure to consider all of the pros and cons when selecting a system. A FIKI approval doesn’t make your plane impervious to icing. Even large aircraft with FIKI systems have been lost to icing accidents. A complete understanding of your icing equipment, in coordination with sound risk management, can help you maximize your winter flying safely.
Don’t Forget to Breathe and Check Your Speed
Induction system icing occurs when ice forms around the air intake of an engine robbing the engine of air to support combustion. Carburetor ice is discussed in our “Breaking the Ice” article, but let’s not forget our fuel-injected friends. Induction icing that occurs around the air inlets and filters of these aircraft can significantly reduce engine power or even cause them to fail if blocked by ice. Knowing your aircraft and using an alternate air source can prevent both conditions. When power loss is noticed, activate the alternate air source immediately if the aircraft is not equipped with automatically activating spring doors. Keep in mind that as with carburetor heat, alternate air sources also use unfiltered air. Use of an alternate air source on the ground should be avoided unless icing conditions are present during taxi operations.
To avoid airspeed errors or a complete loss of airspeed indications, remember that pitot tube and/or static pressure port icing must be considered. During preflight, checking the operation of the pitot heat is always recommended, especially during the winter months or when flying in visible moisture. Several situations can lead to misinterpretations of airspeed that must be understood and avoided.
- Static port is blocked by ice creating a smaller difference between ram and static pressure. As a result, airspeed will falsely decrease during a climb and increase during descent. The opposite is true during a descent. These false indications have led to confusion, incorrect application of power and pitch, and sadly, accidents.
- Pitot tube ram air hole is blocked, but the aft drain hole and static port are open. In this situation, the airspeed would read nearly zero as inaccurate static pressure is introduced and ram air pressure is lost.
- A completely blocked pitot tube (ram and drain holes). In this case, the ram pressure is trapped and as a result, the airspeed remains unchanged so long as the altitude does not change. A climb will indicate an increase in airspeed while a descent will show a decrease in speed. You can think of your airspeed indicator as now acting in the same manner as the altimeter. This is another recipe for problems if not recognized and dealt with by immediately turning on the pitot heat and recognizing the false indications.
James Williams is FAA Safety Briefing’s associate editor and photo editor. He is also a pilot and ground instructor.