FAQ: What happened to anti-icing fluid minimums?

We’ve often been asked during training sessions and audits why the anti-icing fluid minimums were removed from the various service provider and air carrier deicing programs.

This question is reasonably difficult to answer as the issue is complex and is one for which dissenting opinions exist. Although SureConsult is in favour of maintaining anti-icing fluid minimums in the various programs, the objective of this post is not to point fingers at those with differing opinions, but rather to try to examine the issues and arguments at hand, and to answer the question in a rational manner.

Anti-icing fluid minimums have been an important part of the various deicing programs for two decades. Yet for reasons that remain unclear, anti-icing fluid minimums have largely been removed from deicing programs and industry standards due to concerns about the published minimum spray amounts by some, and this despite almost unanimous pushback from most industry stakeholders wanting to keep the minimum amounts.

On one side of the argument, opponents wanting to eliminate the minimum spray amounts say the numbers are too low and there’s a safety issue. On the other, proponents point to a lack of evidence to support the safety claim, as no accidents related to the improper anti-icing fluid volumes sprayed on aircraft have occurred during the past two decades. So, who’s right?

Minimum anti-icing fluid amounts, for those who don’t know, are the minimum amounts of Type 2, Type 3 or Type 4 fluid that need to be applied after deicing to protect those surfaces and to provide a protection time to treated surfaces, which is referred to as holdover time. Historical anti-icing fluid minimums were developed by taking the surface area of the horizontal surfaces of a given aircraft, then applying a theoretical 1 liter of anti-icing fluid per square meter of surface area plus an overspray percentage depending on the size of the aircraft. The goal was to achieve a minimum 1 millimeter layer of thickness of anti-icing fluid on the horizontal surfaces.

Opponents to this approach claim that when anti-icing fluids are tested for holdover times, the test surfaces are flooded with fluid resulting in a greater thickness of fluid than 1 millimeter for some fluids. Proponents, on the other hand, claim that anti-icing fluids do not pool and that the stabilized thickness over time is relatively independent of the applied amount for a suitable application. Proponents also indicate that this is just another example of the operational world needing to replicate the results of scientific tests, as opposed to science replicating operations. In the testing world, anti-icing fluids are poured onto clean and bare aluminum plates, while in the operational world, anti-icing fluids are pumped and sprayed with pressure through nozzles onto aircraft surfaces that have been previously deiced with heated glycol-based deicing fluids. The scientific and operational procedures are very different and would surely result in holdover time variance. So, what’s with the focused scrutiny only on the minimum spray amount?

Opponents claim they don’t want service providers to rely solely on minimum spray amounts for determining whether an anti-icing spray is suitable or not and we concur with this, as it is possible to apply all the minimum spray amount on one section of the aircraft and none on the rest of the aircraft, which would obviously be inappropriate. Proponents claim the minimum spray amounts should only be treated as targets which, if employed as a final quantitative check along with a series of visual and qualitative checks, is a very valuable tool.

The confusion created by the elimination of the minimum spray amounts has been significant. Experienced operators who had used the minimum spray amounts for many years were being told that they needed to apply more without a strong explanation as to why. To further complicate matters, when anti-icing operations occur with Type 1 fluid, there remains a regulatory requirement to apply a minimum of 1 liter of heated Type 1 fluid per square meter during the second-step anti-icing. So now trainers are required to explain that we still have the 1 liter per square meter requirements for Type 1, but no longer for Type 2, Type 3 or Type 4.

Applying more anti-icing fluid than required has numerous negative effects as well. Fluids are expensive, and applying more fluid than required is costly for the carriers and flying public. Applying more anti-icing fluids is also contrary to modern environmental mitigation strategies being implemented globally. Excessive thickened fluid use may also result in increased fluid residue issues, and we’ve heard stories over the past few years of aircraft experiencing elimination issues during the takeoff roll and climb-out phase of flight, and this is partly being attributed to excessive thickened fluid use.

So why were the anti-icing fluid minimums removed? Basically, there is no strong explanation, despite arguments for and against. What is clear is that there remains a vast discrepancy between the two camps and no compromise has ever been reached, which is unfortunate, because the vast majority of industry stakeholders want to continue to use these tools in operations.