Skydrol and Oil Resistance: Why Ordinary Foam Fails in a Hangar

The most common way a foam installation disappoints has nothing to do with the layout. The layout is usually fine. The material was chosen from a price list rather than from where the drawer lives.

What Skydrol is, and why it is different

Skydrol is a phosphate ester based hydraulic fluid, and variants of it are the standard hydraulic fluid on transport category aircraft. It was adopted because it is far more fire resistant than the mineral oil based fluids it replaced, which on an aircraft is a very good reason.

The trade-off is that phosphate esters are aggressive towards materials that are entirely untroubled by mineral oil. They are known for attacking certain paints, certain elastomers and a range of plastics. This is not obscure: anybody who has worked around hydraulics knows what it does to the wrong glove.

So the useful mental model is not "oil resistant foam". Oil resistance and phosphate ester resistance are different properties, and a material can have the first without the second.

What failure looks like

It is gradual, which is why it gets missed until it is thoroughly established.

First, surface softening. The cut edges of the pockets lose definition. A pocket that held a tool snugly starts to feel loose.

Then tackiness. The surface becomes slightly sticky. Tools pick up residue. Somebody wipes a spanner before using it and does not think about why.

Then dimensional loss. Pockets widen, the insert no longer holds tools in transit, and the two tone layer boundary blurs so the contrast that made a gap visible fades.

Finally it is a contamination source. Degraded foam sheds, and it sheds onto tools that go into aircraft. At that point the tool control measure has become a FOD contributor, which is a genuinely bad outcome for something bought to reduce FOD.

The dangerous middle is stage two, because the insert still looks broadly right and is no longer doing the job that justified it.

Where the exposure actually is

You do not need resistant foam everywhere, and specifying it everywhere is a waste.

Definitely. Hydraulics trolleys and any kit used on hydraulic systems. Landing gear tooling. Flight control rigging kits. Anything that lives in a bay where fluid is routinely handled.

Probably. Line maintenance boxes on transport aircraft, because a flight line box opened in a bay where a leak is being investigated gets whatever is in the air. General kits shared across a hangar that includes a hydraulics bay.

Not needed. Bench drawers in a clean workshop. Avionics kits, which see solvents rather than hydraulic fluid. Precision measuring and inspection cases.

The test is not what the tools are for, it is where the drawer is opened.

How to specify it once

Three things at the enquiry stage, and it is handled.

1. Say where the drawer lives, not what the tools are. "Hydraulics trolley, main gear bay" tells us more than a tool list does about material.
2. Name the fluids it will meet. Skydrol and which variant if you know it, engine oil, specific solvents, cleaning agents.
3. Say whether it is exposure or immersion. Occasional splash and wipe is a different specification from a tray that sits under a component being drained.

Our Skydrol resistant blank is a chemically resistant closed cell grade in a black over orange pair, at 118 EUR ex VAT for a 600 by 400 mm sheet against 64 EUR for the standard 30 mm two tone. The premium is real and it is smaller than recutting a drawer in eighteen months. Both are in shadow foam inserts.

Handling, which extends any grade

Even a resistant grade lasts longer if fluid is not left sitting on it. Wipe a spill rather than letting it soak. Let tools drain before they go back into pockets, which is a habit worth having anyway. And inspect inserts at the same time as anything else in the crib: an insert going soft is an early warning, and replacing one is cheap compared with the alternative.

If you are having foam cut, say what it will meet before it is cut. The layout stage is when material is decided, and changing your mind then costs nothing.