Aerotech

 

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Pneumatic Instrument System Considerations

Copyright 2000. Permission hereby granted for publication in the World Beechcraft Society magazine

Recently I’ve installed a CV1J4 “yellow Aerotech Clearview” pneumatic system filter purchased at an earlier WBS meeting in line in my instrument air system.

Soon afterwards (less than 20 hours) its pleats matched the darkest patch in the graduated scale on the filter plastic housing indicating that it was time to replace it and a possible impending pump failure of our almost new pump.

Concerned more about the later I called Raph Heysek of Aerotech and discussed this matter with him. Ralph has addressed the WBS membership in the past at some of the meetings in Arizona and New Mexico and was most helpful. He was the Airborne factory technical representative for many years and is a great source of knowledge concerning the instrument pneumatic system used in Bonanzas and Barons.

He very patiently coached me through a series of troubleshooting steps, loaned us Airborne’s test kit 343 and help me resolve this matter while I learned a whole lot from his wealth of knowledge in the process.

Since the pump had been replaced recently the theory of its impending failure became questionable. We started by verifying its output and proper operation of the in-line regulator utilizing the part of the kit which using compressed air generates simulated pump operation saving having to operate the engine during this troubleshooting. With the penetrating test needles in the kit we carefully recorded readings at various locations and immediately found an unusually high pressure before and after the regulator. The regulator was not operating properly and was creating a very large demand on the pump to overcome its resistance. It was obvious that the pump was working itself to death generating a large pressure but we did not know why.

The investigation was moved from outside the firewall to the inside cabin, beneath the panel. One possibility was that the secondary filter was plugged or there was some sort of restriction in the line leading to the instruments. Utilizing the test kit supplied I methodically proceeded downstream from the firewall penetration point. After the second filter (you DID know that Bonanzas and Barons have TWO in series filters, didn’t you?) I came to a small cylinder connecting the hose going towards the instruments in the panel. Before it I read very high pressure but after it the normal 5” Hg (approximately 2.5 PSIG) was indicated. I removed it and found this Variable Orifice element almost totally closed.

That is when I realized what took place in the past:

Apparently the regulator began malfunctioning and instead of replacing it someone attempted to correct this situation by closing down the Variable Orifice element in order to obtain the desirable 5” of Mercury in the panel indicator.

The problem with this incorrect fix is two fold:

• 1. as the pump wears down the indicated pressure decreases linearly and most importantly
• 2. is that the pump has to generate unusually high pressures to achieve the desired end pressure which results in its premature wear.

Until using the Ralph’s Clearview filter we did not even know that we had a problem but never logged more than 400 hours from one pump (and as little as 25 hrs.). He later indicated that he’s seen a lot of misdiagnosing in the past by people that are not very knowledgeable of the Bonanza’s Pneumatic system or do not have proper testing equipment.