Oil mist on face of Oil Pressure gauge

[Robert Eilers]

I discovered after my last flight a lite oil mist on the instrumant panel cover just below my oil pressure gauge. Has anyone ever experienced an oil leak at the face of the oil pressure gauge?

[GAHorn]

Robert, there are several scenarios I can imagine.

Firstly, the oil pressure guage actually does recieve oil directly from your engine through a small copper oil-pressure line. That copper line becomes brittle due to vibration and can break and you can lose ALL your oil, in-flight. It's happened to many, many, many aviators. (It's the reason the El Paso Airport is where it is.... WW-1 Ace Eddie Rickenbacher landed in the desert in the dark due to a broken copper oil pressure line. Charlie Lindbergh deliberately cut his copper oil pressure line before his famous flight, and patched it back together every few inches with a rubber line. Cleo Bickford made an emergency landing on the way back from Alaska due to his fracturing in flight.)

I replaced mine with a Beechcraft synthetic-rubber oil pressure line. A similar part is used in a Cessna 150. PN 359-2D0150 for about $50 from your Cessna distributor. Try Hill Aircraft: 800-998-7832 and tell them the INt'l Cessna 170 Assn sent you. (We need the Brownie-points for when we request donated items at conventions.) The 150 hose is about 15-inches long, so measure to be certain it will fit your installation. A few extra inches are important to allow movement, but the "coils" found in the copper line are not necessary in the rubber line. Hose shops can also custom-make such a line using No.2 fittings, should you need a different length.

Another DISTINCT POSSIBILITY is that your tachometer drive cable is bringing oil into the cockpit from your engine. I wrote about this back in 2002. It basically results from a failed tach-drive oil seal, and the tach-cable housing acts as a conduit. (Evenually it'll end up with oil in your tachometer and cause it to fail.) I'll locate a link and post it here. (Long read)
http://www.cessna170.org/forums/viewtop ... Vegas#p914

Keep in mind the following: Your Oil Pressure gauge, since it reads oil pressure from oil DIRECTLY supplied to it... may have a failed diaphragm, and may leak oil into the instrument.

So.... it seems you need to inspect the integrity of your gauge, your oil pressure line, and perhaps your tachometer drive cable.

http://aircraftspruce.com/catalog/inpages/oilpress2.php

[170C]

George, I happened to open the link to Aircraft Spruce & saw the oil pressure gauge listings. What in the world makes one of the gauges (Scott I think) cost $215.05 more than the least expensive one

[GAHorn]

Documentation and approval basis. The Scott folks have approval for installation in many types of aircraft, including our own. The gauges are made for Scott by Rochester, who does not have specific approval. (But obviously, except for the gauge-face, is the same instrument. My gauge is a Rochester.... with the original gauge-face installed within it by the instrument shop. I have no qualms at all with Rochester as replacement gauges.)

A similar situation exists with our fuel quantity gauges, made by Rochester for the approved supplier Scott.

This situation has been recognized by FAA in some ways, for example the recent advisory letter which expresses approval for replacement switches, circuit breakers, etc. Where the fine-line drawn is getting fuzzier.

[GAHorn]

In an email exchange from Del Lehmann, Mountain Airframe Services, Mena, AR (who supplied my rubber oil pressure hose):

"I wouldn't recommend getting it from Beech$$$$$.

The generic PN# for the hose is 193000-2D-0160. For your info the 193000 denotes material, 2 inside diameter (in 16ths), D aluminum fittings, 0160 length (16"), -0236 would be 23.6" long. An "F" inserted after the 193 (193F000) gets firesleeve.

My cost ranges from $41-$49. I don't know if any of the above sources will extend any discount (list ranges from $75-90) to a private owner. For $60 check in the mail, I can order and drop ship to members who are interested."

His contact info is:
http://www.mountainairframe.com/
Phone: 479-437-3333

[c170b53]

Had a line under the panel back-off in flight fortunately the oil Px line connection on the engine has a restrictor so that the leak is controlled should a line break. I've also had a Scott that developed an internal leak as evidenced by oil mist on the glass of the gauge. I've now gone to the EI gauge with a transducer. A good shop would replace all those lines at engine overhaul and its not a bad idea to replace the copper fuel /oil lines on both sides of the firewall if its been awhile since overhaul.

[GAHorn]

Anything c170b53 has to say about aircraft maintenance deserves serious consideration. He's an airline mechanic (even tho' in Canada they call them injuneers, heh?). We're hoping he'll write a corrosion manual for us. (hint hint)

[c170b53]

Thanks for the kind words George but in reality the beauty of this organization is the diversity,in skill and knowledge of the people in it. Hopefully Its a great reason for people to join the club and continue to add to that diversity. This board operates much like my workplace. Sometimes a person without hesitation has the answer to a problem and sometimes where the solution is more elusive, it takes a group of minds to sort out the problem. I've been a member for a relatively short time (since 96 I think), I've been to only two conventions of which I keep kicking myself in the pants for not going to one in my first year, to know not to miss any if at all possible and I can't believe how much I don't know compared to many of our members that I've been fortunate enough to meet at those conventions. So I'm certainly no expert but it may have something to do with time in Anyway enough of that, having read my post it may sound that I was saying less than favorable words for the Scott gauge when in fact the gauge had been in service for sometime and in conditions that may have shortened it lifespan prior to my ownership. Thanks I just wanted to clarify that.

[GAHorn]

Earlier I wrote that an oil pressure guage might have a failed "diaphragm" and mist oil internally. Just to explain that a bit further...
... In reality these Scott/Rochester gauges actually are similar to those halloween party or New Years Party toys, where a paper or rubber tube is flat and deflated and rolled up at the end of a wooden or paper tube/mouthpiece. When one blows into the mouthpiece, the flat tube inflates and un-coils...thereby extending itself into a longer tube.
Similarly, the mechanical oil pressure gauges have a coiled tube that pressure causes to extend and operate the needle-movement which indicates relative pressure. That tube is usually made of brass or copper, which we already know can become brittle over time, break, and leak. Or the soldered tube can fracture and leak oil internally. Either way, it's a failed gauge.
I hope this helps explain the mechanism a bit.

[blueldr]

George,
Isn't that properly called a bourdon tube?

[GAHorn]

I thought a bourdon tube was what you drank Kentucky bourdon and Coke through. (And we thought YOU had memory problems.) But as you've so eloquently recalled, your party-pal Eugene Bourdon invented the Bourdon tube pressure gauge when you were both partying in France back in 1849.

[Robert Eilers]

Thanks for the info and advice folks (great forum). I am dropping the airplane off tomorrow at my favorite local Mechanic to have him look at and make any necessary repairs. I dismanteled the instrument panel and poked around behind the panel enough to identify that the leak is most likely at the fitting behind the instrument. My guy tells me that he has dealt with this sort of leak before - usually resulting in clipping the end of the copper tube at the fitting and refitting it. He says the copper tubes tend to split at the fitting. Every time I get upside down in the airplane and seriously examine what is behind the instrument panel I develope a new respect for avionics techs.

[GAHorn]

Robert, please consider replacing that line with hoses as discussed earlier in this thread, both forward and aft of the firewall.

[blueldr]

George,
My party buddy, Eugene, wasn't so goddam smart. He just copied the common anatomical tube that nature pumps up the blood pressure in and it stiffens and straightens out.

[GAHorn]

I was never all that familiar with Eugene.