Mechanical fuel/oil gauges

Ed Haywood

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Big Ed
My 78 Super Decathlon has the original mechanical engine gauges. I've convinced myself I need to swap the fuel and oil gauges for electric to get the hot pressurized fluid lines out of the cockpit. I'm also jonesing for 4 cylinder EGT/CHT and fuel flow.

I already replaced the oil temp gauge with electric when the capillary tube broke. Oil P is simple and cheap to replace, but TSO electric fuel P gauges with >30 PSI range are expensive. That is pushing me towards an engine monitor or a combined FP/FF gauge like the EI FP-5.

How realistic are my concerns about mechanical gauges? I know they have been around forever, so they must be pretty reliable. Is there any history of fuel pressure gauge lines breaking and causing fires? My plane has a lot of fuel lines in and around the cockpit, and a header tank for inverted flight hanging over my lap. But those are gravity fed, while the pressure gauge line is at 30 PSI. I keep picturing hot vaporized fuel spraying all over me and the wiring behind the panel.

I noticed FAR 23 requires a check valve. Does that immediately cut off flow if the gauge line breaks? Is there a way to check function? All I see on my parts diagram is a fitting.

Last question: my oil P gauge line has a switch plumbed to trigger the Hobbs. If I have no intention of ever renting the aircraft, is there any reason for keeping the Hobbs or can I toss it?

Screenshot_20211016-124130~2.png Screenshot_20211016-124324~3.png PXL_20210115_171318611~3.jpg PXL_20210724_181705974.jpg PXL_20210115_171318611.jpg
 
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but TSO electric fuel P gauges with >30 PSI range are expensive.
As mentioned in other threads, it depends on your APIA if he needs a TSO instrument.
I keep picturing hot vaporized fuel spraying all over me and the wiring behind the panel.
Buy a glider.;)
I noticed FAR 23 requires a check valve.
I don’t believe that check valve reference applies to your instrument lines. However, Part 23 requires pressurized instrument lines that contain flammable liquids to have restrictive orifices at the source to reduce the volume in the event the line breaks. So there really is no “difference” from a gravity fed leak, in my opinion. My guess the restrictor orifice is item 74225 in your attachment.
Last question: my oil P gauge line has a switch plumbed to trigger the Hobbs. If I have no intention of ever renting the aircraft, is there any reason for keeping the Hobbs or can I toss it?
The only requirement is to have a Time in Service record as stated in 91.417. Hobbs meters and recording tachs are convenient methods to maintain that record however the device its self is not a requirement. Most people stick with a hobbs/tachs because they always forget to write down the additional time in the record.
 
Lines like the fuel pressure gauge line are supposed to be checked at annual inspections. Mechanics need to get under the panel and have a good look. Cessna has numerous mentions of lines needing inspection for chafing by control systems or wiring or whatever. This stuff is not failure-prone and tends to start dripping long before it fails, but I have found fuel lines and fittings stained blue (and/or red, sometimes, in old airplanes) indicating that they've been seeping and nobody caught it for a long time. It stinks and they think nothing of it. I once found a blue stain under the floor of a 180, under a fuel line tee, that was a sixteenth of an inch thick on the belly skin. That one had been seeping for a very long time and no mechanic found it in all that time. You should not be smelling fuel inside your airplane.

When I installed a Subaru engine in a Glastar, I put a fluid-blocking device in the fuel gauge line, on the firewall. It had a diaphragm in it and the fuel pressure worked against one side of that diaphragm, and the other side was filled with oil (5606) right up to the gauge. No fuel in the line in the cockpit, and if the line broke in there it would just spill a small amount of oil. Not applicable to any TC'd airplane as far as I know, unless someone has an STC for it. Can't remember where we got it. Too many years ago.
 
The mechanical gauges read more than actual with increased use and there is the fluid in the cockpit risk. I believe the risk of a sender failing in the under the cowling is greater than a gauge or line failing in the cabin.
 
Ed, just go with an engine monitor, it will tell you a myriad of information about your engine and provide you with the gauges you are looking for.
 
I can't speak to the gauges. But I'd have concerns about the tubing, too. I had similar concerns when I started my flight training in J3s, where the fuel tank is kind of on top and in front of you, and tubing runs into the cockpit. What I did was go through all of the ntsb reports available online, and looked for info on cub fires. Turns out, it statistically isn't an issue...maybe because they don't have an electrical system. So maybe go through the reports and see what you have?

For me, funny thing was that I was concerned about fires in the cub, where statistically it seems a lot more likely in the much newer planes I fly more now. Maybe because they have electrical systems.
 
I've had the EI fuel pressure, fuel flow, oil temp, oil pressure, and hydraulic pressure for a couple of years now. Frankly, if anything is going to blow it's the hydraulic sender which is at 1200 psi. I've had mechanical gauges blow out before. Nice to have everything on the other side of the firewall now. The EI is way more accurate and reliable than the tractor gauges you're flying behind now
 
The EI is way more accurate and reliable than the tractor gauges you're flying behind now
Yeah, they're nice. I've installed some systems, both JPI and EI, and as far as reliability? Nope. As with anything electrical, there are problems. The connections between probes and leads are the biggest, and the manufacturers have tried numerous different ideas to get better reliability. The G1000 stuff has small two-pin plug-in connectors, with plastic housings, for the CHT and EGT, that are all near the exhaust system by necessity, and they give constant hassles. Oil leaks, exhaust heat, vibration, moisture and cold weather all take their toll and they need cleaning and the application of an expensive contact solution to keep them going. Sometimes I've had to replace the plastic connectors themselves.

None of these connectors are hermetically sealed. That allows stuff to get into them and cause oxidation, and the factors I outlined above all accelerate it.

I've seen the old mechanical stuff last a LOT longer with no hassles. Decades longer. I'm not anti-technology, and I think that when a mechanical tach is giving problems it should be replaced with an STC'd electronic tach that is always accurate and doesn't suffer all the wear and drive cable problems of the mechanical instrument. And glass-panel nav stuff is nice, replacing the mechanical gyros being a good thing. But most instruments don't involve spinning parts and tiny bearings and dried-up lubricants and weakening magnets; they hardly move at all. For instance, your gyros will pack up far more often than your ASI, VSI or altimeter.

It's nice to have the diagnostic info of an engine monitoring system. Some have complained that it's too much information, and they fret over one cylinder that's running a bit hotter than the rest, something they never noticed before they had the new gadgetry. They think something's wrong and about to fail, while really, these old engines run like that, especially carbed engines.

Like I said, I'm not anti-technology. I learned to fly in airplanes with old radios, some still with vacuum tubes. Horrible stuff, hot, slow to warm up, consuming vast amounts of power. Transistorized radios were better, and repairable. The digital stuff is better yet but not so repairable, but I'd not go back to the transistorized radios. Mechanical switching and all, stuff to oxidize and fail.

Electrical problems are the biggest factor in airplane or automotive repair. On an engine, it's around 90% of the hassles.
 
To me, the capillary tubes going to the mechanical instruments would be of lesser concern than the header tank that resides under the instrument panel of a Decathlon. And that doesn't bother me too much.
 
Getting the live fuel lines out of my cockpit were an early priority. The Shadin fixed that pronto, and the lines are empty, capped, and inert now.

Getting the live oilP lines out requires a very expensive primary instrument that I can't fathom where to even place in my panel. So I stare at them angrily. Luckily they're on the passenger side, and there is a restrictor orifice in the line so in theory, it wouldn't empty the crankcase too quickly. Likely not a great treat for whoever my passenger would be if those decide to give up.

I consider the oil far less of a potential hazard than the fuel lines. The lack of a good, inexpensive primary replacement for my factory tri-gauges annoys me though.
 
The lack of a good, inexpensive primary replacement for my factory tri-gauges annoys me though.
The lack of a good, inexpensive airplane annoys me and a lot of other people. There is nothing inexpensive about aviation.
 
Getting the live oilP lines out requires a very expensive primary instrument
Go the field approval route using Bendix transducers or something similar to replace the Oil press system. Its straight forward with a number of off-shelf components that keeps the cost manageable. Approaching each system individually helps keep the approval method within most FSDOs vs the ACO. Perhaps research a bit more, discuss with IA or who ever you use to sign annuals/337, then discuss with local ASI.
 
Getting the live oilP lines out requires a very expensive primary instrument that I can't fathom where to even place in my panel

I found the opposite. Replacing my mechanical oilP is dirt cheap by aircraft part standards. I can get a custom marked electric gauge and sender for $200, or a generic gauge for even less:

https://www.aircraftspruce.com/catalog/inpages/mitchell5056.php

I wonder if I could replace my fuel pressure gauge with a fuel flow computer? That would meet the part 23 requirement, which says I need a fuel pressure or flow gauge if I have a pump. However, my AFM lists a fuel pressure gauge as required equipment.

I keep going back to the EI FP-5 as best bang for the buck. It comes with both flow and pressure transducers.
 
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I've seen the old mechanical stuff last a LOT longer with no hassles. Decades longer. I'm not anti-technology, and I think that when a mechanical tach is giving problems it should be replaced with an STC'd electronic tach that is always accurate and doesn't suffer all the wear and drive cable problems of the mechanical instrument.

Before I replaced my Oil Temp gauge, all my gauges were mechanical. No electrical power needed to operate and monitor the engine. My AFM procedure for electrical system failure is "land as soon as practicable if electrical power needed for continued safe flight."

When my tach went out last year, I looked hard at electrical tachs, but decided on a mechanical tach and cable from the aircraft factory. I like the old school look in my old school plane, and the new tach is rock solid stable. It's cheap enough at a few hundred bucks that I don't mind swapping out every 5 or 10 years, assuming Mitchell keeps producing them. I like the idea of MP and RPM not depending on electrical power. One less stress point if you do have an electrical issue. If I ever get a fancy display-all engine monitor, I'd probably keep those two gauges as backup.

tach small.jpg
 
The first two pics show a soldered ball-end fitting similar to those used on primer nozzles. The last one appears to be a simple compression union fitting. You might want to get parts catalog for your airplane and see what's supposed to be there.

Here is what the parts catalog shows. Google did not return anything on the parts numbers. I'll ask the next time I call the aircraft factory parts guy. Huge benefit to have an aircraft still in production.

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The only requirement is to have a Time in Service record as stated in 91.417. Hobbs meters and recording tachs are convenient methods to maintain that record however the device its self is not a requirement. Most people stick with a hobbs/tachs because they always forget to write down the additional time in the record.

If I have a recording tach, is there any advantage to also having a Hobbs?
 

Maybe its just the angle of the picture but this one disturbs me. I hope the fuel line isn't being chaffed by the cable. Could be easily remedied with a couple of Adel clamps.

pxl_20211017_221238216-jpg.101047
 
Maybe its just the angle of the picture but this one disturbs me. I hope the fuel line isn't being chaffed by the cable. Could be easily remedied with a couple of Adel clamps.

pxl_20211017_221238216-jpg.101047
Good catch, that. Also, the fuel line should be clamped to the airbox to prevent flexing at the servo connection. That flex, due to engine movement and vibration, wil fatigue the tube and it will crack and break and fuel will be squirting out. Awesome fire hazard.

I once had the 1/8" copper oil pressure line break at the engine fitting. In flight. When I landed, 3/4 of the oil had already gone overboard. I know of another airplane that suffered the same thing, but all the oil was lost and the engine seized.

Details matter.
 
I recently replaced all the engine instruments in my Twin Bonanza to, in part, get the live fuel and oil lines out of the cockpit. While I had one oil pressure gauge leak a couple years ago (and take out an audio panel), my biggest concern was the lines. Even with inspection, there are places you simply cannot see/inspect. When I removed all of the lines as part of the upgrade, some were in fact chaffed pretty badly and a couple looked pretty close to cracking in a few spots. I was glad to be rid of them. The wallet did hurt, though.
 
Maybe its just the angle of the picture but this one disturbs me. I hope the fuel line isn't being chaffed by the cable. Could be easily remedied with a couple of Adel clamps.

It's just the angle. Here is another view. There is a 2 inch gap between the line and the cable.

PXL_20211017_221442903.jpg
 
That little window, under the instrument panel, next to my intercom box? That's my fuel gauge. Try reading that while you are flying.instrument.jpg instrument.jpg
I'd love to have a "real" fuel gauge. sigh....
 
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