Wolfgang Nickel:
I tied N6706D outside for some nights. I had to notice that on the co-pilot’s side rain is getting in. The carpet in the legroom area is wet. What do I look for, and how do I fix it?
Technical Editor:
I haven’t written anything specific for this, as the cause is quite variable. Things to check include:
Door seal condition and door fit when closed.
Windshield perimeter seal. Any apparent cracks in the bead of sealant, even if just a hairline, will wick in a tremendous amount of water. Aircraft Spruce sells sealant in caulking tubes. Make sure that whatever you use is safe for contact with acrylic. The polysulphide 2-part sealants (often called tank sealant) can be had in small tubes, but they do require a special caulking gun. They are the best adhesive sealants for acrylic windows.
Fuselage upper cheek vents. These are the ones with the wire grille over them, on the top right and left corners of the fuselage just aft of the cowling. The obviously take in a huge amount of water when it rains, as they are big funnels. The air from these inlets goes to a distribution box that is mounted above and behind the right and left upholstery kick panels. These boxes have a small drain tube that exits the bottom of the fuselage on the right and left side, just above the door. Two things go wrong with these boxes and their drains. During maintenance the boxes get re-oriented so that the drain is no longer at the lowest point. Or the drain tube connection gets clogged up with debris until water fills the box and escapes through crevices. You can tell by looking at the box whether there is a “high water mark” on the inside, where water accumulated until it leaked out. Be very careful when working with these air boxes. They are very thin plastic, are very old, and are therefore very fragile. Very strong repairs can be made using extremely lightweight model-making fiberglass cloth from a hardware store, along with ABS cement or Marine-Tex marine epoxy. Don’t substitute any other plastic cement for the ABS cement. It can sometimes be found in white, but most often in black (for use with black ABS plumbing parts). The marine-Tex is available in white and gray. You can make your own white ABS cement if you can find some scrap white ABS plastic (like an old bad fairing or tip from one of our planes). You cut the plastic up into tiny pieces (or grate it into granules and slivers), and dissolve it in MEK solvent until you have a thick but spreadable paste.
One of the above is almost surely your problem. By the way, you need to immediately soak the area under the floor, where the wet carpet is, with Corrosion X or ACF-50. Especially if your plane still has the original black CAT air duct. That old duct all became water-absorbent many years ago (from age), and it was made with piano wire (not stainless-steel wire). The wire rapidly rusts, and corrodes any aluminum it touches. Many of the planes have been badly damaged from those old air ducts. I would remove the floorboards and prop them up until the wood and carpet dry out.
Make sure you soak your underfloor areas with the anti-corrosion product of your choice, and check those ducts during the next Annual Inspection. Replacing them in not that expensive. I advise people to use the SCEET lined high-temperature duct (the red stuff with an inner liner). You don’t need the temperature rating, but the stuff is made with silicone rather than the black neoprene-impregnated cloth. The silicone material will never become absorbent no matter how old it gets.