What is CAT duct? What’s the difference between CAT, CEET, SCAT, and SCEET duct?
These are designations for the special air duct material used in many aviation applications. The black CAT and CEET duct does not have sufficient heat resistance to use it forward of the firewall, with the possible exception of unique factory-engineered placements. While it literally has what sounds like a high-ish temp rating (300-350 degrees), in reality heat hardens it up too fast due to being neoprene-impregnated. It even hardens over time when used in cabin air duct applications. The silicone-based SCAT and SCEET does not harden much with age. The SCAT and SCEET also does not absorb moisture like aged CAT and CEET. This latter characteristic is what has led to so many duct-related corrosion problems in older airframes. The SCAT and SCEET can still cause fretting corrosion if the stainless-steel reinforcement wire wears through against aluminum, but that is not nearly as serious as the piano wire or sometimes copper-plated steel wire (both typically non-SS) in CAT and CEET, coupled with moisture in the duct fabric. The resulting rusty wire against aluminum really eats things up. The older CAT and CEET was worse than today’s impregnated fiberglass fabric, but both will absorb moisture as they age. Both old and new SCAT and SCEET are permanently moisture-repellant.
The actual temperature ratings are CAT-300; CEET-350; SCAT-450; and SCEET-500 degrees F. Remember that it is not simply the ambient air temperature that matters. The radiant heat from the cylinder heads and exhaust system are very hard on any soft parts that are exposed to it.
I do my best to use SCEET in all applications. I have never found an application in which SCEET was “too stiff” to work. The only exception might be a very tight location for which the factory part has a tightly molded curve in a specialty duct. Even those can sometimes be found in molded SCEET. I also try to get my hands on ducts that have molded collars on both ends. I sometimes find suitable ducts at events like Sun-N-Fun and Airventure/Oshkosh. There is a company called Custom Ducts that will make molded ducts to your type and size specs (http://www.customducts.net/index.html); they give us a BAC discount. Duct stock comes from the factory in full lengths that have a molded collar at both ends (length varies with size). I always order duct by the full length; that gives me two molded ends. When cost is a major factor, sometimes I can use a length that has one molded end and one cut end. I install the cut end on a location that never (or rarely) needs to come off, and I use the molded end on the location where it may require periodic removal (such as the heater shroud connection, removed at every Annual to inspect under the shroud).
There is one cautionary note. Duct that is used in the normal engine induction path (as opposed to the alternate air path) comes with a risk. If the inner liner or any of the inner reinforcement comes loose, it can block the inlet or be ingested into the engine. It is a remote risk, nearly always related to poor maintenance practices, but it has happened. Factory ducts made for this application are virtually always specially molded, one-piece ducts, with molded collars. An example is the filter housing to injector servo duct used on Bonanzas. I always try to use only the molded factory part (or the equivalent FAA-PMA part) in this particular application (induction air). If you do use SCEET or any other lined duct, make certain that the inner liner overlaps the end tubes, and is captured under the clamping area. Better yet, order a duct with molded collars from Custom Ducts.
CAT is undesirable under the cowl; it just can’t take the heat. I wouldn’t expect any overnight failures from use of CEET. If you already have CAT or CEET in place, you can use it “on condition”. Just feel it each time the cowling is opened up, and make sure it is still resilient. When you feel it getting stiff, just plan on replacing it relatively soon.