A Little Scare!

This morning I took off for "The Other LOE" fly-in; the EAA one at Moriarty, NM. I was about 80 miles out, cruising along at 10,500 MSL, sort of planning the descent, when I glanced over to see my engine monitor flashing 12 PSI on the oil pressure. I immediately used the nearest function on the Garmin 496 and found that I was only about 9 miles from Roswell, NM, fortuitously an airport with facilities.

A lot was going through my mind, and in fact I am surprised at how I got sort of overloaded in a very simple situation. I found out that even though I have been to Roswell quite a few times, when you are not planning to go to a place it takes you a while to get up-to-speed. Where is the airport (it was below a cloud layer)? What are the frequencies? Should I call approach (I didn't)? Should I listen to the ATIS (I did but shouldn't have wasted my time)? Did I leave the dipstick out (no)? What should I do about the engine? I will be castigated for this statement, but yes I did consider stopping the engine but did not, based on the fact that it was showing some oil pressure, was running fine, and I was not familiar enough with the area. Close in, over my home airport, I probably would have stopped the engine.

I called the tower and told them I had had a loss of oil pressure and wanted to land. A different (more seasoned) controller came on immediately and cleared me to land on any runway. Not much point in declaring an emergency in this situation and this pretty sleepy airport. I would have if it would have looked like there was going to be any kind of a delay at all.

During the descent, at idle, the oil pressure indication came up some, to sometimes as high as 25 PSI. After landing the indication was reasonable and I taxied to the FBO.

It took a couple of hours for a mechanic to come out. The maintenance hangar is about a mile away and they said they couldn't tow the airplane. I couldn't believe they couldn't come up with a bit of rope from somewhere, but there you go. The mechanic and I decided to try starting up to taxi over. The oil pressure was entirely normal.

We decowled and looked the engine over. Everything looked fine and they didn't have any kind of oil pressure gauge to check against mine, not that there were have been much point, as mine was now showing normal.

We buttoned everything back up and took a test flight. Everything looked normal for about 15 minutes then the gauge started showing lower pressures in kind of a down then up way. Must be the transducer. We couldn't think of anything else that would cause these symptoms. The indication got as low as about 40 PSI from a normal 70 or so.

The young mechanic was pretty cool. He is a glider pilot that flew with the Alamogordo, NM club. He had never been in an RV and after I let him fly it and showed him the maneuverability he said "I'm, going to build one of these."

So we both agreed that a ferry flight back to Carlsbad would be safe and that is what I did, with the same patterns of normal pressure indications, dropping later in the flight.

I am using a the modern/obsolete Vision Microsystems VM-1000 and don't like what I have been hearing about the company. I sure hope I can find a new transducer.

How do these transducers work anyway, and how could one fail in this way?

Parked next to me at Great Southwest Aviation were a couple of Eclipse Jets that seemed to be very much still in test mode, which was surprising to me, because I haven't heard of any more problems since the freezing static? lines and electronic panel problems.



Two experimentals, parked side by side.

If you go by the unpainted parts, they were doing something with the tip tanks and rudder trim.


What is with the little wing deal at the top of the trim tab? Was that there before and what does it do?

Lots of old decrepit airliners at Roswell. My mechanic was telling me how fun it is to taxi a 747. He is not a power pilot.


Small engine but a big bulge underneath for something?

Larry, we are all glad that it appears to only be a transducer.

I have written numerous times before that it is important to have a mechanical backup oil pressure gage when using an electrical gage. IMHO, it is the only engine parameter that truly needs a backup. It is for precisely the sort of situation you were in. I have the 1" round UMA mechanical gage backing up my electrical one in the monitor. If (when) my monitor tells me that there is no oil pressure, I want to KNOW if it is telling the truth. Transducers have proven to not be too reliable.

I do not see a need for backing up any of the other engine instruments, as none of the other parameters have such immediate consequences as oil pressure. Hot oil - well, a low power or idle descent to the nearest airport will show if that it true. Fuel pressure - if the engine is running ignore a low fuel pressure indication. High cht on one cylinder - idle descent and you will know if it is true. RPM - look out the front windscreen. Empty fuel tank? Engine still runs, the gage is lying.

Obviously, there are exceptions. But, it is not uncommon to hear about erroneously low oil pressure indications. Think about if you were IFR above low ceilings and your electronic gage said you lost oil pressure. What would you do?

AlexPeterson said:

...I have written numerous times before that it is important to have a mechanical backup oil pressure gage when using an electrical gage....

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Alex, you make a good point. With my setup the transducer screws into the engine and wires come out of it. How do you do the backup? Do you have some sort of adapter machined that duplicates the engine port?

Larry, that is interesting that your transducer is directly mounted. I don't think transducers like being mounted directly on the engine (vibration, heat). I have a 1/4" (aircraft) hose from the engine to the firewall, with a little manifold. The transducer connects to the manifold, while a 1/8" aluminum line runs to the panel where the mech. gage is.

I believe I made the manifold from a block of aluminum, but various manifolds are available for this purpose. Have a look here:
http://www.aircraftspruce.com/catalog/hapages/alummanifoldfit.php

I have a quick-connect air fitting with a 1/8" pipe plug union that I have used to check oil pressure senders. Simply connect it to a compression tester and dial in whatever pressure you want to check the transducer. Since compression testers are calibrated from the factory you will have an accurate pressure source.

I have a simple back up of sorts...

The electronics international gauge which I run (which has a very high quality transducer) is backed up with a seperate pressure switch which does dual duty as an oil pressure light on the panel and hobbs switch. Not a gauge perhaps, but the red light on the panel driven from this back up transducer will confirm pressure, or loss of pressure as the case may be.

A good time to remind everyone if this happens to watch the oil temp and see what it does. If it is above normal and/or going up then it confirms the low pressure reading is the real deal and your engine's most likely terminal. (You may have done this but didn't mention in your post.)

These anomalous readings are fairly common in the Cirrus -- it sure gets your attention!

Be sure to tell us what the cause was.

- Brett

rocketbob said:

I have a quick-connect air fitting with a 1/8" pipe plug union that I have used to check oil pressure senders. Simply connect it to a compression tester and dial in whatever pressure you want to check the transducer. Since compression testers are calibrated from the factory you will have an accurate pressure source.

Click to expand...

AlexPeterson said:

Larry, that is interesting that your transducer is directly mounted. I don't think transducers like being mounted directly on the engine (vibration, heat).

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I don't think mounting on the firewall is so much to keep the transducer happy/healthy, but rather to keep it from snapping right off. Lots o' vibration + concentrated mass = breakage + spewing oil .

szicree said:

I don't think mounting on the firewall is so much to keep the transducer happy/healthy, but rather to keep it from snapping right off. Lots o' vibration + concentrated mass = breakage + spewing oil .

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I'll have to check, but I think I mounted the transducer where it is because the instructions said to. It is a very light and wide and low profile aluminum deal, threaded to go right into the port. I can't imagine it ever snapping off. Other transducers I mounted on the firewall but if this one was supposed to go there, I will be very disappointed in my judgment.

szicree said:

I don't think mounting on the firewall is so much to keep the transducer happy/healthy, but rather to keep it from snapping right off. Lots o' vibration + concentrated mass = breakage + spewing oil .

Click to expand...

First off, I wonder if it's the oil temp sender, which also is connected to wiring, & not a weightier object such as a pressure transducer?

Anyway, here is an example of heavier objects hanging off vibrating engines. This plane came down just a hundred yards behind my house, as I live next to the airport.

"For the second flight of the homebuilt airplane, the pilot/builder took off and remained in the traffic pattern. While on the downwind leg and approaching the base leg turn, the engine began 'misfiring and/or running very rough.' Smoke filled the cabin, and the engine seized shortly thereafter. During the forced landing, the airplane clipped a high stack of PVC pipes and impacted the ground inverted. Postaccident investigation disclosed the pilot had removed the engine oil pressure sending unit from the engine oil pressure port, located on the right side of the engine-driven oil pump housing. He then installed a common automotive brass T-fitting, to which he attached both the oil pressure sending unit and a Hobbs hour meter fluid pressure-activated switch. The T-fitting failed in the pipe thread, flush with the pump housing exterior surface. The arm of the 'sending unit assembly' was about 4.5 inches long, with an estimated weight of about 7 ounces. The path of the engine cooling air directed the leaking oil to the cabin air heat muff, where it was burned by the muffler prior to entering the cabin, accounting for the smoke in the cabin.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:

Total failure of an oil line fitting, resulting in oil exhaustion and subsequent engine seizure. Factors were the poor fitting installation by the pilot, and unsuitable terrain on which to make a forced landing. "

L.Adamson RV6A

Glad to hear your day turned out OK Larry! I have been just where you were a few years back, in my Grumman - looked up and saw my Oil Pressure light, looked over, and the gauge read ZERO! (The first thought I had was "Oh, this is going to be expensive", proving that the most dangerous thing in the cockpit might just be the owner in the pilot seat.....)

In my case, I had ruptured an oil line, and dumped the whole load overboard, but since it had gone down under the belly, I didn't know it. I was in gliding range of an airport, and figured that the engine was going to keep turning if I shut it down or not, so let it run at idle. Long story short, I made the runway, engine checked fine, and I ran it for several more years.

I am not sure that in an oil loss case that an oil temp would be a reliable indication - if no oil is running over it, I don't know what it would do, but it certainly might be a tell-tale.

I also believe in back-up oil pressure indications, and have a dedicated sending unit and light, independent of my fancy electronics. Both the pressure 'ducers are mounted to a manifold on the firewall - the only sensor mounted to the engine is oil temp.

Once again Larry,it sounds like you handled it very well, and glad you and the plane are OK!

Paul

L.Adamson said:

First off, I wonder if it's the oil temp sender, which also is connected to wiring, & not a weightier object such as a pressure transducer?...

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Boy, I just don't know. The mechanic said that is it and it has been so long since I installed it I didn't question it. I'll figure it out tomorrow.

As I think about it I think the pressure transducer is on the firewall because I have a braided line going to a sensor there and that wouldn't make sense for temperature.

I am not sure that in an oil loss case that an oil temp would be a reliable indication - if no oil is running over it, I don't know what it would do, but it certainly might be a tell-tale.

Paul[/QUOTE]


Yes good point -- zero on the temp could also be a corroborator and in that case it's not only happening -- it's already happened! In fact I think I read a story by someone this happened to a few years ago.

CHECK THE WIRE CONNECTIONS....

n5lp said:

This morning I took off for "The Other LOE" fly-in; the EAA one at Moriarty, NM. I was about 80 miles out, cruising along at 10,500 MSL, sort of planning the descent, when I glanced over to see my engine monitor flashing 12 PSI on the oil pressure.

I sure hope I can find a new transducer.How do these transducers work anyway, and how could one fail in this way?

Click to expand...

Might just be a bad connection. Do a jumper from transducer, to ground. And check all of the connections. Any connection or wire end can cause this. I would bet my money on a bad connection............. A resistance change can cause this!!!!

Forgive me if this has already been considered, but I seem to remember a thread a while back about some oil cooler lines that had a manufacturing defect that produced a little flap of rubber inside the line. Possibility?

gasman said:

Might just be a bad connection. Do a jumper from transducer, to ground...

Click to expand...

Thanks for the tip. I always think more clearly with time and am now thinking that the connector near the transducer is likely the source of the problem.

As for going along with the mechanic's idea that the temperature sensor was the pressure transducer, I am baffled by that. I just recently was looking at someone's RV engine installation and criticized the way they ran the line from the engine to the transducer. I must have just been addled by the incident.

szicree said:

Forgive me if this has already been considered, but I seem to remember a thread a while back about some oil cooler lines that had a manufacturing defect that produced a little flap of rubber inside the line. Possibility?

Click to expand...

I don't think a flap would have much effect on pressure like it would for flow. I fabricated the line and it doesn't have right angle fittings so it is easy to just look inside the line. The installation has worked fine for over 8 years.

An old trick we used in piston days ( C46, DC4 ) Just cycle the prop if it works you have oil pressure. Bum guage or sender...R Bentley RV4, RV3 almost done...

Oil Pressure Sending Unit

We had a failure of the oil pressure sending unit on our RV. The sender is always the most suspect culprit between a gauge problem and a sending unit problem. Our's was erratic and jumped around quite a bit - a lot like you described. I rigged up an adjustable rheostat and hooked it up as a test of the gauge. I was then able to dial the rheostat up and down and make sure that the gauge was not a problem and worked through the whole range. After eliminating the gauge we ordered a new sending unit and things were back to normal.
We have a small flexible line from the engine to a tee on the firewall. The temp and pressure sending units are mounted on the firewall. This seems to work well. Craig

Bad ground

I had a similar problem, turned out to be a bad ground.

Bryan

Same problem maybe diff cause

I don?t want to make a big thing out of nothing bu___t I had the same thing happen in my PA28-160 some years ago. The pressure would drop after flying for some time. Land and shut down and the pressure would be normal at next start. This went on for a few hours until I lost all oil indications and made an emergency landing. Turns out the piston skirts had broken away due to piston caps wearing and working between the piston and cylinder wall. The remains were collecting on the oil pick up screen and blocking the flow. Shut down and the bits would fall to the bottom of the sump until next start when they would begin to collect again. >>>>>>> Installed FRM and did not have another problem after that. ????? easy check, sample your oil to be sure.

Problem resolved

I did one thing at a time to find out where the problem was with oil pressure indications. First I inspected the transducer and cleaned all the spade terminals. A test flight revealed no difference. Pressure indications were more or less ok, but a bit erratic for the first fifteen minutes or so. They were then increasingly erratic and were going down to around 15 PSI.

Next I tightened the screw clamps on the processor box. The wire just sticks in between two flat pieces of metal and a screw clamps the wire down. I probably tightened the screws about another 1/16 turn. After this the indications were normal. After stabilizing in cruise flight the indication stayed dead on 71 PSI until I reduced power to land.

I got some good troubleshooting advice on this forum but people were assuming that this is a variable resistance transducer; it is not. According to information on the transducer it puts out from 0 to 50 mV for 0 to 100 PSI. It has FOUR wires, presumably two for power in and ground and two for signal out and signal ground. So we have 1 mV per 2 PSI. I haven't done the calculations but that design would seem to be very sensitive to slight resistance changes at the connections. Maybe it is done this way because it is easier for the digital device to deal with small voltages than resistance.

I dodged a bullet on this one. I didn't have to try to find an obscure part for an unsupported device (VM-1000), yet. When I unclamped the transducer the sticker on it stuck to the Adel Clamp so now there is no information on the transducer for a future owner, unless they find where I have taped the sticker into the owners manual.

There is something to be said for putting the most common, lowest tech, most supported stuff in your airplane. Ah, but the lure of the colorful, clever, computerized stuff is great. We just have to be ready to build a whole new panel in the future, I guess.

n5lp said:

Ah, but the lure of the colorful, clever, computerized stuff is great. We just have to be ready to build a whole new panel in the future, I guess.

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Ah....but we're about to put a whole new panel in Louise's airplane because the simple, tried and true stuff is all mostly kaput!

It's OK, she'll still have a bunch of round dials - just a few new square ones!

Glad to hear you found the problem Larry - In general, when I see a sudden, funny indication on a well-running, well-maintained engine (like yours) my first thought is usually "what's wrong with the instrument" - and more often than not, that what it turns out to be.

Paul

instrument and transducer grounds

Just my 2 cents. Important to check grounds to all of the transducers. The biggest bang came from an auxilliary strap from my prop gov. plate nuts to the firewall. From there to a common ground buss for the transducers. Shortening the ground loop seemed to do the trick for accurate readings. Twin Dynons in my 9A, and all is stable and accurate. Reading the posts has revealed a lot of oil temp sender issues resulting from small resistances in the sender circuit to ground. A couple of redundant engine to airframe braids that do not carry starter load seems to have worked for me.

Hope this helps...
Chris 9A almost done
RV6 built and sold