John & Anne Wiley

Left hand door post trim purchased 12/2010 from P.A.S.T. for 1983 172Q, supplied part listed for 172P,R,S.

LH Door Post P.A.S.T. - inside (click to enlarge image)

LH Door Post P.A.S.T. - outside

Installing this door post trim piece can be difficult since it requires removing the vent tube cap (knob) that you use for pilot overhead vent air in flight. Older model 172 vent tubes could easily be removed from inside the aircraft, but on this 1983 172Q (and presumably similar vintage P models) the tube cannot easily be removed. By first removing the co-pilot (right) side vent cap, and breaking the brittle old pilot side door post trim piece to remove it, I was able to figure out how to remove the cap. I plan to swap the co-pilot OAT cap to the pilot side so that it’s easier to read. Here are some pix with notes (click the images to enlarge them and read the notes).

Right Vent Cap Removed

The vents have two tubes, one inside the other. The inner tube is the one that slides in and out to control air flow in flight. Both tubes are narrowed at their inner ends (inside the cabin), and the inner vent tube also has an expanded ring that stops against the narrowed outer tube end. The expanded ring on the inner vent tube prevents the tube from being removed from inside the cabin as was easily done on earlier model 172s. The right side outer tube end inside the cabin on this aircraft had what appears to be black electrical tape installed, probably in an attempt to make the vent seal better when closed.

OAT Vent Cap Inside

Because the OAT prevents the threaded shaft from turning inside the cap (knob), I was able to remove the cap by turning it while holding the nut with my fingers inserted through the inner vent tube opening (where air comes out when the tube is pulled into the cabin in flight). I learned that there’s a large washer that goes inside the narrowed end of the inner vent tube, followed by a rubber washer and a metal washer that the nut tightens against. In this way, the cap (knob) is held on the end of the inner vent tube. It is this large cap (knob) that prevents easy installation of a new door post trim piece, because the hole in the piece is smaller than the cap (knob). This could be solved by cutting from the inside of the hole in the door post trim piece, but that would create a finish problem because the cut edges would not stay together unless glued back together somehow.

Outer Vent Tube Flange

Because my A&P was repairing a minor fuel leak (fuel smell inside the cabin at the right door post when flying with more than 1/2 fuel), the wing root fairing was removed. I took the above photo standing in front of the plane, looking for a stop screw on the inner vent tube. I found none. It appears to me that the only way to remove the inner vent tube from the outer one, is to remove the entire vent tube assembly from the wing and then slide the inner vent tube out (toward where it had been mounted on the inside of the wing). It is clear to me that there is no way to remove only the inner vent tube from inside the aircraft.

Pilot Side Vent Tube Stop

Above is a photo of the expanded ring on the inner vent tube that stops against the narrowed end of the outer vent tube, thus preventing removing the inner vent tube from inside the aircraft. There is no stop screw to remove, nor any other way to overcome this stop other than destructively cutting away or prying open the narrowed end of the outer vent tube.

Pilot Vent Tube Base

While it would probably be possible to remove the inner vent tube by removing the entire assembly from the inner surface of the wing where it meets the cabin, my goal was only to install the door post trim piece. So naturally, I tried holding the nut inside the inner vent tube end and turning the cap (knob). Alas, the screw turns with the nut. The head of the screw is hidden behind a lovely aluminum disc inside the cap (knob). Next I tried prying off the metal disc in the hope of gluing it back into place afterward. It showed no signs of being removable without mangling it considerably. Instead I was able to puncture the center of the disc with a small slot screwdriver, expand the hole with a larger slot screwdriver and then a phillips screwdriver.

Pilot Vent Tube Cap

It was still quite difficult to hold the nut, and of course the phillips head tried to strip. With the help of a deep socket and ratchet on the nut and considerable pressure on the phillips, I was finally able to remove the nut and thus the cap. Like the co-pilot side, it is mounted with washers and is apparently interchangeable so I plan to swap them to make the OAT more easily readable from the pilot seat. After doing this I was at last able to place the new door post trim piece in position without cutting it to get past the vent tube cap (knob).