Apparent tachometer cable issue, what is possible/legal in the meantime before it is fixed ?

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CarlinNH

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CarlinNH
During my last flight, the tachometer started jumping wildly around the dial, then went to zero. No change in the engine sound, so possibly just a broken cable, but I put the plane down as soon as I could.

I am trying to get it looked at by A&P asap, but currently I am not really sure what the problem is and how soon (or if) it can be fixed. So, my question is: what can I do in the meantime to legally fly ?

The plane is a Cessna 182 (Continental O-470) = constant speed prop.

I understand that people like optical tachometers, so that seems to be a real option, but, what do I do about engine time recording ? What other related considerations am I overlooking ?

Thanks for advice -- especially if you have faced a similar problem.

EDIT: Inspection of the cable by my A&P (accessed from the right side engine hatch, removed the cabin heater hose to get at the cable housing behind it), revealed that the tach cable was broken in two, at the end that went into the engine. Attached is a photo of the broken end. New cable on order.
 

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It has to be fixed before you can fly. Tachometer is required by 91.205 (b).
 
Disconnect one end of the tach cable, the engine end is usually easier, Pull the core out and clean it with avgas. Then Lube it very lightly with a thin oil such as LPS 1. Wipe with a cloth and repeat the Lube/wipe a couple times. Reinstall, and it’ll likely solve you problem for years to come.
 
Right, I get that about needing tach in order to legally fly.

Part 91.205 (b) states, in the list of minimum required instruments, "Tachometer for each engine." That sounds like putting an optical tach on the top of the panel is all I need to do in order to comply and legally fly while the original is getting looked at and (hopefully) issue resolved. But I am wondering about the engine time issue, as well as what other issues I might be missing.
 
Had this same thing happen to me back in January. Couldn't do anything to keep flying it so plane (and I) was grounded for a few weeks. I ended up upgrading to an electronic tachometer at the occasion. No more dealing with those fragile tach cables.

Only reason why it was taking a few weeks is because of slow delivery. If you can get an electronic tachometer delivered quickly, installing it is no big deal so you wouldn't be grounded very long.
 
I should have mentioned that the cable maintenance is a job for your A&P but it’ll probably be a quick fix regardless. If not you’ll need to look at replacing the cable, and or the tachometer. There’s a few places that repair tachometers but I haven’t had much luck with them.
The digital ones can be programmed with your existing tach time. You’ll have a logbook entry to keep up the time anyway.
 
Excellent, I appreciate the input. Would you be able to confirm for me: does the electric tach need to be installed by the appropriately licensed avionics shop, due to the need to set the engine time ? I know there is a line between what my A&P is allowed to do, and what I need a licensed avionics expert to do, but not sure whether "setting the engine time" on a tach counts.
 
You’re probably going to find that the electric tach will need STC paperwork completed. If your A&P is also an IA he can take care of that. If not, he’ll probably have an IA who can help with that.
 
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Jdm said:
You’re probably going to find that the electric tach will need STC paperwork completed. If your A&P is also an IA he can take care of that. If not, he probably have an IA who can help with that.
Click to expand...

Very good, I appreciate the clarifying details, thanks !
 
CarlinNH said:
Excellent, I appreciate the input. Would you be able to confirm for me: does the electric tach need to be installed by the appropriately licensed avionics shop, due to the need to set the engine time ? I know there is a line between what my A&P is allowed to do, and what I need a licensed avionics expert to do, but not sure whether "setting the engine time" on a tach counts.
Click to expand...

The tach I installed was installed by my mechanic, not an avionics shop. Took him about two hours. It's this one: https://www.aircraftspruce.ca/catalog/inpages/tso_tachometer.php
 
CarlinNH said:
I know there is a line between what my A&P is allowed to do, and what I need a licensed avionics expert to do,
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FYI: there is no line. Anyone working this needs an FAA Airframe certificate to do the work... or work for an FAA repair station. However, any change to the type of tachometer is considered a major alteration with additional requirements. Basically theres no shortcut to keep flying in between.
 
CarlinNH said:
Right, I get that about needing tach in order to legally fly.

Part 91.205 (b) states, in the list of minimum required instruments, "Tachometer for each engine." That sounds like putting an optical tach on the top of the panel is all I need to do in order to comply and legally fly while the original is getting looked at and (hopefully) issue resolved. But I am wondering about the engine time issue, as well as what other issues I might be missing.
Click to expand...
Just to be clear, dropping one of these on your dash isn’t complying with 91.205

https://www.sportys.com/optical-tac...V6xfUAR2yVgRAEAQYAiABEgIj6fD_BwE&gclsrc=aw.ds
 
Salty said:
Just to be clear, dropping one of these on your dash isn’t complying with 91.205

https://www.sportys.com/optical-tachometer.html?mrkgadid=&mrkgen=27&utm_source=google&utm_medium=cpc&utm_campaign=Smart_Shopping&creative={AdId}&device=c&matchtype=&mrkgcl=596&acctid=700000001999345&dskeywordid=92700067107255390&lid=92700067107255390&ds_s_kwgid=58700007109433166&ds_s_inventory_feed_id=97700000007407630&dsproductgroupid=296303633664&product_id=2245A&merchid=2857566&prodctry=US&prodlang=en&channel=online&storeid=&device=c&network=u&matchtype=&locationid=1015089&creative=524093874389&targetid=pla-296303633664&campaignid=13261768744&adgroupid=122815601677&gclid=EAIaIQobChMIgoT1oves-QIV6xfUAR2yVgRAEAQYAiABEgIj6fD_BwE&gclsrc=aw.ds
Click to expand...

Ok thanks. Do you happen to know the details regarding why ?
 
CarlinNH said:
Ok thanks. Do you happen to know the details regarding why ?
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A reasonable question.

The simple answer is that you'd still need an A&P to sign off on it in order to make it legal, and you're not going to convince one to do so. The reason why they wouldn't is more complicated. I'll let one of the A&P's that participate here answer that one.
 
I put an EI in mine a few months ago. Nice upgrade over analog, and not expensive
 
The description of the failure sounds like the tach itself failed. Not uncommon. I’ve replaced mine in my 180 twice. Easy swap. A trained monkey could do it. New tachs can be ordered with appropriate markings and tach hours pre-set to match the original. Swap it yourself, have your mechanic sign it off. I have no interest in an electronic tach in that plane. The last time it failed I made a couple of flights with an inoperative tach. I know the plane well. I wasn’t concerned. It needed to get home to fix it so not much choice.
 
Stewartb said:
The description of the failure sounds like the tach itself failed. Not uncommon.
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Yup. Old tachs can seize up, then they get erratic, then the cable breaks. Replace the cable and it will soon break again.

The tach input shaft runs in a bronze bearing. The oil in that bearing gets old and sticky and tough and starts loading the cable. Many tachs had a tiny oil hole in the top of the input shaft bearing boss, but almost none get any lube. I fixed a few erratic tachs just by lubing them. Fixed a few more by lubing the cable. Cessna has a spec for the grease; it's very similar to a light Lubriplate.
 
Failure sounded similar to mine, which turned out to be a broken tach drive on the back of the engine. But otherwise, it sounds similar to a cable failure...tach reading dropping as the cable slows down and starts twisting, jumps when the stored energy uncoils, etc. On the plus side, it's the cheapest repair.

As several folks have mentioned, the tachometer is a required item per 91.205. However, the time-recording function is *not*. You could, legally, just record times based on noting what your watch says. However, since the tach needs to be fixed, anyway.....

Since I wasn't inclined to remove the engine accessory case to fix the tach drive, I switched to an electronic tach. The one I picked didn't have the capability to set it to the previous engine time (whatdaya expect for $50?), so the logbooks record both the tach time and the computed total time.

Ron Wanttaja
 
This happened to me last year. It was the tach, not the cable. So I’m replacing both with a $$$$ engine monitor. I haven’t flown since March :(
 
You cannot just pop a tach on the dash. It would need to be STCd to replace the primary tach.

If you install a JPI 830, that would not suffice as the JPI 830 is not STCd to be a primary instrument. You would need to install a JPI 900/930.
 
If you are lubing or replacing the cable it’s time well spent to address lubing the
Tach as if it has the Lube Port Dan referred to. A Tach that is reluctant to turn
WILL take out a new Cable.
 
CarlinNH said:
Do you happen to know the details regarding why ?
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In general, when an aircraft is produced and certified you have 2 options in which to repair or change its original configuration: a part replacement or an aircraft alteration. Each has their own regulatory requirements that are spelled out in the FARs. In your example, placing a 3rd party optical tach on your glareshield does not comply with either option's requirements on a certified aircraft.
 
And much like the required clock can’t be your wristwatch, a required tach needs to be an installed part of the airplane.
 
With all the nice, STC'd electronic tachs out there, I don't know why people would buy another mechanical tach, with its jerky and expensive cable (Cessna calls it a driveshaft), its tendency to start underreading as it ages and its internal magnet weakens, and so on. Canada has a law that requires an annual tach accuracy check that applies to all magnetic-drag-type tachometers, and they must be within 4% when in the middle of the cruise RPM range. That's no more than a 100 RPM error at 2500 RPM, ad I found enough that were well outside the tolerance. Once I found one that was overreading, a result of dried lube in the internal workings that was dragging the needle farther around the dial.
 
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Hopefully the mechanic on the field can clean and lube the cable as mentioned before. It’s a 30 min job and will very likely get you home. If not, he can proceed with simple troubleshooting that will quickly determine the problem. I can’t begin to count how many times I’ve corrected problems such as this with a good clean and lube. Doesn’t always work of course, but it’s definitely worth trying. At least it might get you home so that you can sort through the options that others have mentioned with your normal AP/IA.

Bell206 does a great job of breaking down the regs for ease of understanding. His explanation of legality pertaining this situation is 100% spot on.
 
It seems many Tachs read lower than actual.

So folks push the “ Noise Lever “ up to what seems to be the “ Normal “ reading.

Unfortunately; that could put them right in the “ Realm of Detonation” if leaned.

Cracked jugs?

We are not talking 20 RPM low here.
 
Jdm said:
Hopefully the mechanic on the field can clean and lube the cable as mentioned before. It’s a 30 min job and will very likely get you home. If not, he can proceed with simple troubleshooting that will quickly determine the problem. I can’t begin to count how many times I’ve corrected problems such as this with a good clean and lube. Doesn’t always work of course, but it’s definitely worth trying. At least it might get you home so that you can sort through the options that others have mentioned with your normal AP/IA.

Bell206 does a great job of breaking down the regs for ease of understanding. His explanation of legality pertaining this situation is 100% spot on.
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His tach went to zero and stayed there. That tells me that lube isn't going to fix it. Something's busted.
 
Dan Thomas said:
His tach went to zero and stayed there. That tells me that lube isn't going to fix it. Something's busted.
Click to expand...

You are probably right, but I’ve had them do some strange stuff when a cable starts binding.
I think, and this is pure speculation, that the cable begins binding, which causes the bouncing needle. It then twists internally and shortens enough to pull itself away from the tach just enough to not engage properly, which causes a dead needle.
I learned from a tach OH shop that it’s not uncommon for mechanics to adjust the internal length by trimming the ends. One shop even recommends this for proper fit. If there’s not much cable inserted in the first place it might be possible that it can come out when binding.
I’ve fixed a totally dead RPM gauge by cleaning and lubing, as odd as that sounds:)
 
Jdm said:
I should have mentioned that the cable maintenance is a job for your A&P but it’ll probably be a quick fix regardless. If not you’ll need to look at replacing the cable, and or the tachometer. There’s a few places that repair tachometers but I haven’t had much luck with them.
The digital ones can be programmed with your existing tach time. You’ll have a logbook entry to keep up the time anyway.
Click to expand...
Id like to know what references are used for this remove the tach cable incantation.

I don't think an optical tach will cover 91.205 as Velcro to the dash is not an installed gauge.
 
pfarber said:
Id like to know what references are used for this remove the tach cable incantation.
Click to expand...
Well, the parts catalog and the service manuals both deal with it. Like I said, Cessna specifies what lube to use on the cable itself. They tell you to pull it out, clean it lube it, and reinsert it in its housing.

Is that good enough for you?
 
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Dan Thomas said:
With all the nice, STC'd electronic tachs out there, I don't know why people would buy another mechanical tach, with its jerky and expensive cable (Cessna calls it a driveshaft), its tendency to start underreading as it ages and its internal magnet weakens, and so on. Canada has a law that requires an annual tach accuracy check that applies to all magnetic-drag-type tachometers, and they must be within 4% when in the middle of the cruise RPM range. That's no more than a 100 RPM error at 2500 RPM, ad I found enough that were well outside the tolerance. Once I found one that was overreading, a result of dried lube in the internal workings that was dragging the needle farther around the dial.
Click to expand...
dan, just curious, what equipment does TC require you to have and how often must it be calibrated to do that check to those tolerances?
 
My tach started bouncing. Cleaned and lubed cable. No dice. Go to take the tach out to order the correct one and there's a 90 degree adapter and that was a little loose. Problem solved.
 
unsafervguy said:
dan, just curious, what equipment does TC require you to have and how often must it be calibrated to do that check to those tolerances?
Click to expand...
We used an ordinary optical tach. To calibrate it, it is set for the two-blade prop and pointed at a fluorescent light. It should indicate 3600 RPM. The 60 Hz AC grid signal has a frequency tolerance of .0033%, IIRC, which was much more than accurate enough to satisfy TC for calibration. I had the shop's maintenance policy manual written up to use that equipment and calibration method, and it was approved by TC.
 
From the '69-'76 172 service manual, chapter 15, instrument systems:

upload_2022-8-4_20-8-4.png
upload_2022-8-4_20-8-35.png
 
Tru Tach. Worth owning if you play with airplanes.

I know of nobody who’s broken a tach cable. Tachs themselves failing? Oh hell yes. Common.
 
Stewartb said:
Tru Tach. Worth owning if you play with airplanes.

I know of nobody who’s broken a tach cable. Tachs themselves failing? Oh hell yes. Common.
Click to expand...
Last place I worked we had one. They're good but they're $250. Before that I used a model airplane optical tach that cost maybe $40 and was just as accurate.
 
I have the TruTach2. Apparently it’s very accurate but I have no way to know for sure. Every time I use it I find discrepancies between the airplane’s tach and the TruTach2.
Usually about 100 RPM difference at cruise power.
It’d be interesting to check it against a digital tachometer.
 
Jdm said:
I have the TruTach2. Apparently it’s very accurate but I have no way to know for sure. Every time I use it I find discrepancies between the airplane’s tach and the TruTach2.
Usually about 100 RPM difference at cruise power.
It’d be interesting to check it against a digital tachometer.
Click to expand...
Got a fluorescent light? See what the tach says when you get it up close to the light. Set it to two-blade and it should say 3600. Unless you're in an old 50-Hz area. Don't know if there are any left.

No fluorescents? A neon indicator light will work if you're real close. Old deep freezes had them as power-on lights. Or you could by an NE-2H neon lamp from an electronics shop and use, IIRC, a 22K ohm resistor in series with it and apply 110VAC.

Much easier to find a fluorescent light.
 
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