Getting de-ice fluid off before it sits through a summer

Not all of the de-ice fluid leaves with the ice. What stays behind runs into the places you cannot see, dries there, and then sits through a summer of heat and humidity. March is the month to deal with it, which is why we are writing this in March.

This is one of the few genuinely seasonal jobs in this business, and it is the one most likely to be skipped because the aircraft looks fine.

What is actually left on the aircraft

Type I fluid is a heated, dilute glycol solution used to remove contamination. Type IV is a thickened anti-icing fluid designed to stay on the surface, which is the entire point of it. It contains thickeners and a surfactant package precisely so that it does not run off immediately.

That property does not switch off after rotation. Fluid sheds from the surfaces it is meant to shed from, and the rest of it goes where liquid goes on an airframe: aft, down, and into gaps. It collects in flap tracks, in hinge lines, in the cove areas behind control surfaces, around the wing root fairing, in the empennage and along the belly.

Then the water evaporates and the glycol and thickener do not. What is left is a residue that is hygroscopic, which means it draws moisture out of the air and stays wet longer than the surface around it.

Why sitting through a summer is the problem

Three reasons.

It becomes an electrolyte that will not dry. A hygroscopic residue in a lap joint or a hinge is exactly the water entrapment situation FAA AC 43-4B describes as a corrosion mechanism, and unlike rainwater it does not evaporate away.

It gets harder to remove with time and heat. Fresh glycol residue is water soluble and comes off with a flood and a mild cleaner. Residue that has been through a Florida or Texas summer is baked, and now you are into stronger chemistry and longer dwell, which puts you closer to the limits on everything else nearby.

The rehydration and drying cycle is the worst case. Residue that gets damp overnight and dries in the afternoon, every day for months, is a cycling corrosion cell rather than a static one.

There is also a documented operational issue with dried and rehydrated thickened fluid residue accumulating in aerodynamically quiet areas around flight controls. That is a flight safety subject that belongs to maintenance and the airframe manual, not to us, and we mention it only because it is another reason the answer to residue is removal rather than tolerance.

Where it collects, in the order worth checking

• Flap tracks and flap cove areas, particularly the inboard sections
• Aileron and elevator hinge lines and the coves behind them
• The wing root fairing and anywhere a fairing overlaps structure
• The belly aft of the wing, where everything on an airframe eventually goes
• Gear doors and the well, which catch spray from the ramp as well
• The empennage, especially where a horizontal meets a vertical
• Static ports and drain masts, which is a maintenance concern rather than an appearance one

Notice that almost none of these are places anybody looks at when deciding whether an aircraft needs washing.

How to take it off

Glycol is water soluble, which is genuinely good news. The correct process is unglamorous.

1. Flood, with volume rather than pressure. Warm water and a lot of it. The residue wants dissolving and carrying away, and high pressure into a hinge line pushes contaminated water further in rather than out.
2. A mild aviation cleaner with real dwell time. This is a dwell job, not a scrub job. Time is the variable that costs nothing.
3. Get into the geometry. Extending flaps where that is appropriate and permitted, so the tracks and coves are reachable. A wash that does not reach the collection points has not addressed the problem.
4. Rinse thoroughly and then rinse the low points again. Residual cleaner left in a joint is its own version of the same problem, which is the exact scenario ASTM F1110 sandwich corrosion testing exists to evaluate.
5. Dry it, and check the drains. AC 43-4B notes that low-point drains are frequently blocked by sealant, debris and grease. Water going in and not coming out is the whole failure mode.

A dry wash is the wrong tool here and it is worth saying plainly. Encapsulation chemistry handles what a towel can reach, and none of the list above can be reached with a towel. This is a flooding job. The general version of that argument is here.

When to do it

After the last de-ice event of the season and before sustained heat. In the northeast that is generally March into early April. In Tennessee it can be earlier. The point is that the window is between the end of the icing season and the start of the summer, and it is not very wide.

We run this as a distinct seasonal job at Teterboro, White Plains, Allentown, Philadelphia and Knoxville, and the aircraft that get it are noticeably easier to work on the following spring. The ones that do not get it are where we find staining in the coves and residue you can feel with a fingernail.

It is also a good moment to look at the boots, since they have just done a season of work and are due for preservative anyway. The boot schedule is here.

If your aircraft flew a northern winter, it has residue on it, whether or not you can see any. Getting it off is a normal exterior service done in the right order with enough time allowed for the parts nobody photographs.