Aircraft and engine ice protection systems are generally of two designs: either they remove ice after it has formed, or they prevent it from forming. The former type of system is referred to as a de-icing system and the latter as an anti-icing system.
A de-icing system has two very attractive attributes. First, it can utilize a variety of means to transfer the energy used to remove the ice. This allows the consideration of mechanical (principally pneumatic), electrical and thermal methods. The second attribute is that it is energy efficient, requiring energy only periodically when ice is being removed, with some mechanical designs requiring relatively little energy overall. This is a significant consideration when designing ice protection for aircraft with limited excess power.
The principal drawback to the de-icing system is that, by default, the aircraft will operate with ice accretions for the majority of the time in icing conditions. The only time it will be free of ice accretions will be the time during and immediately after the cycling of the de-ice system. This requires an understanding on the part of the designer and the pilot of what effects the ice accretions will have on aircraft performance, both prior to and during system operation.
Anti-icing systems reverse this paradigm. Properly used, they prevent the formation of ice continuously, resulting in a clean wing with no aerodynamic penalties. An anti-icing system must have a means of continuously delivering energy or chemical flow to a surface in order to prevent the bonding of ice. The typical thermal anti-icing system does this at significant energy expense. The concept is not viable for aircraft that do not have the requisite excess energy available during all flight phases. An exception to this is the use of a chemical systems.
It is not uncommon for a system that is designed as an anti-ice system to be used initially as a de-ice system. For example, the manufacturer may recommend that the wing thermal ice protection system be selected on when ice accretion has been detected, thus initially bypassing the anti-ice capability. Once selected on, the system is usually left on until icing conditions have been departed, allowing the anti-icing capability to function as intended.
The selection of system design and the determination of operating procedures are based on the manufacturer’s understanding of the tolerance to ice accretion exhibited by the particular aerodynamic surface. For example, turbojet/turbofan engine inlets are almost universally protected by thermal anti-icing systems. These systems are nearly always used in an anti-icing manner, which is to say they are selected ON upon encountering visible moisture and crossing below a temperature threshold. This approach is due to the intolerance of the compressor inlet to ice ingestion; an imprecise de-ice cycle would lead to damage and/or loss of power.
On the other hand, the same airplane may use a thermal anti-ice system for the protection of the wings, but the manufacturer may recommend that the system not be activated until ice accretion is noted on some representative surface. The judgment here is that the aerodynamic penalties associated with such “pre-activation” ice are acceptable and pose no safety hazard.
Any time a design utilizes an ice detection system as a primary and automatic means of operating the ice protection system, the system becomes a de-ice system. An automatic means of activation will necessarily have a threshold for triggering both activation of the system and de-activation of the system. This is almost universally accomplished by means of an ice detector, which, as the name implies, must have some ice present to detect. Thus, the system is not activated until ice has accreted. Once the ice has been removed, the system automatically terminates, and awaits another ice detection trigger before cycling again. This is the de-ice cycle.