Dry wash or wet wash, and when each one is wrong

Soil load decides, not preference. A dry wash on a lightly soiled airframe is excellent. A dry wash on a heavily soiled one is an abrasive process wearing a polite name. And a wet wash in the wrong place is a wastewater compliance problem before it is anything else.

Most of what is written about this online comes from chemical manufacturers selling one or the other. Here is how a service company that does both actually decides.

What each process is

A wet wash is what it sounds like: rinse, apply a diluted aviation cleaner, agitate with clean media, rinse, dry. Water carries the soil away from the surface and off the aircraft.

A dry wash uses a product that encapsulates soil in a lubricating film so it can be lifted into a towel rather than dragged across the finish. No rinse, no runoff, and usually a gloss step built into the same pass. Sometimes called a waterless wash, though most crews are still using a small amount of solution.

Both are real processes. Neither is a shortcut. What differs is what they can carry.

The deciding factor is how much soil is on the aircraft

A dry wash product has a finite capacity to encapsulate. Below that capacity, the soil is suspended in the film and travels into the towel, and the finish is untouched. Above it, the film is saturated, the particles are no longer suspended, and the towel is now a sanding block.

There is no published number for where that line sits, and anyone who gives you one is guessing. But it is not subtle in practice. If a finger drawn across the surface leaves a visible line, the aircraft is past it. If the aircraft has been through rain, or has any grit, salt or ash on it, it is past it.

The honest rule we work to: dry wash maintains a clean aircraft. It does not clean a dirty one.

Where a dry wash is the right answer

• Between-flight touch-ups. A quick turn on an aircraft that was washed recently and picked up light dust and a few bugs. This is most of what a dry wash is genuinely good for, and it is good at it.
• Where a wet wash is not permitted. Plenty of ramps have no wash rack and no permitted discharge. Wastewater from an aircraft wash is a regulated discharge and it is not a gray area. We hold to that at every field we work and it is not negotiable regardless of who is asking.
• Where water freezing is a problem. Cold-weather outdoor work where a rinse would leave ice.
• Inside a full hangar where a wet process would put water where other people are working.

Where a wet wash is the right answer

• Any aircraft with real soil load. Post-storage, post-maintenance, after a period outdoors, or anything with exhaust deposit down the belly.
• Salt. This one is not a preference. Salt is water soluble and it needs water to leave. A dry wash on a salt-exposed airframe moves the visible film and leaves chloride in the seams, which is exactly where FAA AC 43-4B says corrosion starts. At our coastal fields this is the deciding argument.
• Anywhere the water needs to reach that a towel does not. Gear wells, flap tracks, hinge lines, the areas AC 43-4B calls water entrapment areas. A dry wash does not clean anything you cannot put a towel on.
• De-ice residue. Glycol residue is water soluble and it wants flooding, not encapsulation. More on that here.

The pattern that actually works

For most operators the answer is both, on a rhythm. A wet wash at an interval set by where the aircraft lives, with dry washes in between to keep it presentable. That way the water gets into the places water needs to get into, and the aircraft looks right in the days between without anybody abrading it.

Aircraft that live in salt need the wet interval tighter. Aircraft in dry inland air can stretch it. That is the substance of how often should an aircraft be washed, and it is where the honest answer stops being one number.

What to ask a crew before they start

Two questions, and they take ten seconds.

Is a dry wash appropriate for the state this aircraft is in? A crew that answers this the same way every time is not assessing, they are defaulting.

What is your towel discipline? On a dry wash the towel is the entire safety system. Fresh, high quality, folded to give many faces, changed constantly, never dropped and picked up. A crew doing a whole fuselage on two towels is doing something else.

Where the ramp permits it and the aircraft needs it, we wet wash. Where it does not, we dry wash and we say so. What we will not do is dry wash a dirty aircraft because the ramp is easier that way, which is the same principle as everything else in how we run the exterior service. If you want to talk through what is right for how your aircraft is actually used, tell us the pattern and we will tell you what we would do.