Are there any certified TSO/PMA tachs that work like mechanical?

M

maverickps

Filing Flight Plan
Joined
Sep 19, 2020
Messages
8
Display Name
Display name:
maverickps
So my mechanical tach is broken:

https://photos.app.goo.gl/aKQNhcMqj51FUBu28

And the mechanic thinks the cable and the instrument are broken. At any point, if I have to pay 2hrs labor to remove it all for testing, and a new tach cable is nearly $200 and a new instrument is $500+, and the headache of trying to keep track of TTAF with a new reading, it seems best to just get an electrical tach.

As far as options go, there seems to be three:


Right now I am leaning towards the R-1 since it has the mag drop functions and a flight timer built in, but it seems two things are common across all options - there is no battery backup on any of them, and none of them count hours slower when you turn the RPM down.

On the old mechanical tach it was nice to go practice landings and "pay" ourselves 1.1hrs of tach time while getting 1.6hrs of flight time. On all of the above, it seems they don't have this "feature."

Are there any electrically driven certified tachs that do?
 
I flew an Archer that had the EI R-1. An observation I'll make is that it can be hard to read at a glance in bright sunlight. If you're paying for it in your own plane, that isn't being rented by the clock hour, having a tach that adjusts for "tach time" is very important, or your service intervals could be substantially more frequent than they need to be.

With a new mechanical tach, they can clock it ahead to whatever you need it to be to match the TTAF.
 
maverickps said:
and none of them count hours slower when you turn the RPM down.
Click to expand...

Well, yes and no. The R1 you posted above states that it does not record any time until the engine is above 1300 rpm. That means that your startup, taxi out, some of your runup, taxi back, and shutdown are NOT recorded at all. Yes, if you're airborne it's recording 1:1, but once you touch the ground it isn't recording anything (unlike a mechanical tach which will still record about 1/2 rate or a little less) - as long as you stay below 1300 rpm, which is a good target anyway.

I couldn't find similar information for the other two you posted in the links, but they're probably similar.
 
maverickps said:
On all of the above, it seems they don't have this "feature."
Click to expand...
And they wont for the simple reason how they count time. The electronic versions counts electrical impulses vs the mechanical counts direct cable rotations and is the reason it must be calibrated to a specific RPM to be accurate. So the electronic will never have the lead or lag in the accuracy department which is the reason for your "feature".
 
Just connect the power through a "push pull" circuit breaker and leave it turned off any time you don't want to accumulate hours.
 
If anyone is curious what happened, I decided on the EI R-1 tach, but wanted to give an alternate mechanical tach a test to see if it was the tach or the cable. The new mechanical tach worked just fine. Switched the old tach back and it was still bouncy. Went for a test flight, tightened everything up, and now the original tach seems to be steady.

Will monitor it for health, but hopefully all the messing with it knocked whatever was interfering loose.
 
Bell206 said:
And they wont for the simple reason how they count time. The electronic versions counts electrical impulses vs the mechanical counts direct cable rotations and is the reason it must be calibrated to a specific RPM to be accurate. So the electronic will never have the lead or lag in the accuracy department which is the reason for your "feature".
Click to expand...
I don't understand. Electrical impulses are directly related to cable/crankshaft/camshaft rotations.
Seems like you could monitor one cylinder, divide by 2 over the integration interval, and calculate rpm across all values. I do not see an electronic or mechanical reason for the 1300 rpm limit.
 
Domenick said:
I don't understand. Electrical impulses are directly related to cable/crankshaft/camshaft rotations.
Seems like you could monitor one cylinder, divide by 2 over the integration interval, and calculate rpm across all values. I do not see an electronic or mechanical reason for the 1300 rpm limit.
Click to expand...


I called EI and they said the guidance from Continental and Lycoming changed. They say 1hr at 2150rpm should count same as an hour at 2450rpm. So that's what the tachs do.
 
Canada has a requirement for an annual tachometer accuracy check. It applies only to the magnetic-drag type of tachs. I frequently found tachs underreading, due to the old magnet in them weakening with age. I once found one overreading, probably being driven by stiffening lubrication in the interfacing shafts inside it.

And underreading tach only reads low on RPM, not hours. It can let the engine be run past redline, hence the requirement for accuracy. Overrevving the engine isn't good; overrevving the prop is dangerous. Props are already highly stressed, and if you run one 10% over design redline, you are increasing the centrifugal forces in it by 21%.

The electronic tachs are far superior and never go out of calibration. And so they never need an accuracy test. They never bounce around, either, if installed properly according to the manuals.
 
Domenick said:
I don't understand. Electrical impulses are directly related to cable/crankshaft/camshaft rotations.
Click to expand...
While it does relate to rotations of the cable itself it does not relate to the gear mechanism that drives the counter (hour meter). So for the mechanical tach to be accurate the cable must spin at a constant rpm since its continuously driving the counter. The electronic tach doesn't have that gear mechanism so its accurate throughout its range. As to the 1300 notation that would be only a vendor requirement for their product. All the optical/magnetic/electronic tach pickups on my test equipment have useful ranges from 100-10,000 RPM so EI must have a reason for the 1300.
 
Dan Thomas said:
... I frequently found tachs underreading, due to the old magnet in them weakening with age. ...
Click to expand...
The tach in my 1979 Warrior II reads about 100 rpm low at my typical cruise rpm. With an optical tach, I created a correction grid for "indicated" to "actual" rpm. It's posted on the blank spot's cover next to the tach.
 
Domenick said:
The tach in my 1979 Warrior II reads about 100 rpm low at my typical cruise rpm. With an optical tach, I created a correction grid for "indicated" to "actual" rpm. It's posted on the blank spot's cover next to the tach.
Click to expand...
That tach would be at the accuracy tolerance limit here: 4% error, measured at the middle of the cruise range. 100 RPM low at 2500 engine RPM is 4%.
 
A tach cable is easy to troubleshoot. Disconnect both ends, pull the inner cable out, clean, inspect, lube, and reinstall. 15 minute job if access is not a challenge. It would be unlikely for a tach cable to be "broken" without some severe mechanical event.

Mechanical tachs do wear out. Bouncing is the primary symptom. But an overhaul is not expensive, and neither is a new instrument. Very simple to replace.

Mechanical and electronic tachs use a different connection point to the engine, so there is no reason you cannot have both if you have the panel space. The mechanical tach makes a nice backup to the more accurate electronic tach.

These days many folks are going with an engine monitor, most of which have primary tach.
 
Last edited:
You must log in or register to reply here.
Back
Top