Deicing and Anti-icing Fluid Residues
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A larger number of deicing/anti-icing fluid residue problems have occurred in Europe compared to North America and Asia. Industry experts agree that one of the reasons for this is the difference in deicing and anti-icing practices between the continents.

In Europe, a one-step deicing/anti-icing process is commonly used. This process involves the application of deicing/anti-icing fluids in a single application, using a heated mixture of Type II fluid and water, usually in a ratio of 75/25.

In North America, a two-step process is commonly used. This process involves deicing with heated Type I fluid, or a heated mixture of Type I fluid and water, which is followed by an application of Type IV anti-icing fluid. Experience and testing has shown that deicing with heated Type I fluid will help clear away residue from previous anti-icing fluid treatments.


Early research also indicates that the interaction between airplane deicing/anti-icing fluids and runway deicing fluids may contribute to the formation of residue gels.

Airplane deicing/anti-icing fluids typically comprise glycols with thickening agents (polymers). Runway deicing fluids contain potassium acetateor potassium formate-based fluids (deionizing salts). When these fluids combine, the separation of the anti-icing fluid thickeners may be enhanced, leading to a more rapid formation of the residue.

The fluids can be mixed together in two different ways when the airplane fluid flows off the wing during the takeoff roll and goes onto a runway that has been treated with deicers, or when the engine thrust reversers send runway fluids up onto the wing during the landing roll and they flow into the rear spar areas through the control surface vent gaps. These situations make it possible for residue gel to form on the external wing surfaces as well as the internal quiet areas of the rear spar and the balance bays.

While research is ongoing as fluid manufacturers continue to conduct tests on the interaction between airplane deicing/anti-icing fluids and runway deicing fluids, it is prudent to remove as much of all types of residue as possible.


Boeing issued a multi-model service letter (No. 737-SL-12-014) in January 2000 that advised operators about the potential for deicing/anti-icing fluid residue problems. At that time, the service letter quoted a new caution note that had been added to the Society of Automotive Engineers (SAE) Aerospace Recommended Practice (ARP) 4737 Methods Document. The note was also added to the Airplane Maintenance Manual (AMM) Cold Weather sections at that time.

New multi-model service letters (Nos. 737-SL-12-019 and MD 80-SL-12-104) have been issued that include information about where to inspect for residue and updated procedures for cleaning residue. The service letters also advise of AMM revisions that include more information about where to look for fluid residue.

The service letters and AMM revisions recommend that the inspection and cleaning processes outlined below be followed for all airplanes that are exposed to deicing/anti-icing fluids during winter operations. The frequency of the inspections should be based on each operator’s experience during winter operations. Boeing recommends that all airplanes that have been exposed to deicing/anti-icing fluids should be subjected to the inspection and cleaning procedures both prior to and at the end of the winter season. Boeing also recommends that during the winter season, each airplane should be inspected and cleaned no less than once per month.

This frequency is based upon information from operators that have experienced multiple occurrences of flight control issues due to deicing/anti-icing fluid residues. Some operators perform inspections much more frequently than once per month, and Boeing encourages operators to inspect as frequently as practical until sufficient data has been accumulated to more accurately define the inspection period.

Boeing recommends that the inspection and cleaning be performed as follows:


1. Gain access to the following areas where flight controls and other systems components are located:

2. Visually inspect for the presence of dry or rehydrated residue anywhere in these areas. The residue may be very hard to see, especially if dry. Dry residue will normally be a thin film that may be partially covered with dirt or grease (see fig. 4). Rehydrated residue will often be a gel-like substance of more visible thickness.

Figure 4

Deicing/anti-icing residues gel on a 737-300 flight spoiler power control unit input quadrant.

3. Spray the area with a fine mist of warm water to rehydrate any residue that may be present and to make it easier to identify. In some cases, rehydration may occur quickly, but the process often may be slow, especially if residue has accumulated from multiple applications over a long period of time. Wait at least 15 minutes to allow rehydration to take place.

4. If no rehydrated residue is visible, repeat this step at least three more times, if practical, including the wait time of 15 minutes to allow rehydration to take place. This recommendation to perform repetitive spraying and wait for rehydration to occur is based on the experience of several operators during the previous two winter seasons.

Do not spray the controls with water when the ambient temperature is below freezing unless the airplane is in a heated hangar. Doing so may result in ice that impairs the flight controls.

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