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1.59 Cent Pneumatic System

When was the last time you pulled the instrument panel out of your plane? MY WHAT !!! You know, the flat black thing at the front of the cockpit with all the round glass widgets in it. It's not like I'm asking you to perform a prefrontal lobotomy or something. It's designed to be pulled out. There's stuff behind it you should look at once in a while.

We're not really interested in the panel or the glass widgets here. What we're looking at is the tubing that attaches to most of that glass. Have you ever considered what makes instruments work? The combination of the pressure from the pitot tube in the nose, the neutral from the static ports on the sides of the fuselage and the suction from the total energy probe in the tail. That's not exactly a Niagara Falls of airflow to work with. It wouldn't take too many leaks to make hash of whatever sense the glass widgets are trying to make out of that airflow.

Which brings us to the point. Remember how tight all those plastic tubes were when you put them on? They didn't need to be safety wired did they? Have you looked at them lately? It's amazing what a few seasons of heating and cooling will do to that springy plastic. Those tubes just slip right on and off now don't they? Do any of them leak? Just a little? Do your varios drop when you pull up in a thermal? Are you blaming the increase in your stall speed on the dust on your wings?

Before you reach for the safety wire and the pliers, let's think about this for a minute. The plastic tubing grows less elastic over time and stretches because the brass tubing it's pressed onto expands and contracts more than the plastic tubing does. Do you want to put the same sort of material on the outside of the tubing as well? Have you ever seen metal wire cut plastic tubing?

There is an alternative to safety wire that is easier to install and isn't as hard on the tubing, Wire Ties: Available at your local friendly electronics supply house in shrink wrapped packages of 30. $1.59 per package. They come in varying thicknesses and lengths. The 4û size is enough to double wrap each fitting and put the plastic buckle on top of the first wrap. Buy two packages. You will be amazed at how many connections you will find when you follow all the tubes.

Having combination instruments really pays off when you do something like this. I don't. My old fashioned panel connects one pitot, one total energy probe and two static ports to three bottles, one netto bottle, one airspeed, one altimeter and three varios; electric, netto and speed to fly. By the time I was done with the cross connections and the panel quick disconnects I had 52 connections to back up and none of them were even loose.

Was it worth it? My varios used to drop to negative and take a long second to recover when I pulled up into a thermal. Now they drop 50 fpm for a moment. It was worth it. Besides, it was raining when I pulled my panel out. It was a good day.

While you're in the electronic supply catalog, you'll find all sorts of handy gadgets for neatening up tubes, cables and wires. There are Peel and Stick Wire Markers for identifying wires and tubes, various sizes of Adhesive Backed Cable Clips for holding things out of the way, Split Loom Tubing and Spiral Wrap for gathering wires into neat bundles, and all sizes and shapes of Wall Feedthrough Bushings and Strain Reliefs for protecting fragile stuff as it passes through bulkheads and other sharp things. And if you really get into neatness, there are bunches of Plugs, Terminal Strips and Chassis Tie Points that are handy for bringing some sense of order to your electrical system. But that's another story.

Posted: 2/24/1996 By: Peter King


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