New Tachometer

[JOhnH]

While my plane is in the avionics shop having other work done, they found that my tach reads about 150 rpm low. They suggested a new digital tach for about $900 installed. (no wonder I was going so slowly and getting around 6 or 7 gph).

TTAF is 3139
Tach time is 1374
Total time on engine is 88.3

My question is:
What do you suggest that I have them set the new tach to? Does it make any difference to anyone?

 

[jesse]

What do you suggest that I have them set the new tach to? Does it make any difference to anyone?

The speed of the engine/prop. I guess I'm confused what you're asking?

 

[EdFred]

Set the new tach to the current tach setting, or set it to the new engine time, and have the IA/AP make a note in the logbook on why the tach time changed from 1374 to 88.3

 

[jsstevens]

While my plane is in the avionics shop having other work done, they found that my tach reads about 150 rpm low. They suggested a new digital tach for about $900 installed. (no wonder I was going so slowly and getting around 6 or 7 gph).

TTAF is 3139
Tach time is 1374
Total time on engine is 88.3

My question is:
What do you suggest that I have them set the new tach to? Does it make any difference to anyone?

I, being a lowly private pilot, would think you'd set it to 88.3 to match the engine time, just for ease of tracking.

John

 

[jesse]

Oh. Nevermind, I understand now

 

[EdFred]

I, being a lowly private pilot, would think you'd set it to 88.3 to match the engine time, just for ease of tracking.

John

Except if you only have a tachometer, and no Hobbs (both my planes were like this) all the ADs are done on tach time, and that's a ton of extra work going through and making all those changes.

 

[poadeleted20]

I suggest setting it to total airframe time, in this case 3139. Documentation is just so much easier that way, and remember that the tach will not go with the engine if the engine ever goes.

 

[jsstevens]

Except if you only have a tachometer, and no Hobbs (both my planes were like this) all the ADs are done on tach time, and that's a ton of extra work going through and making all those changes.

So you'd advocate setting it to 1374 like the old tach?

John

 

[ChrisS]

I left mine set to 0.

 

[EdFred]

So you'd advocate setting it to 1374 like the old tach?

John

That's what we did when I had a new one installed. Everything up to that point was documented tach time, and it was easier to just carry those numbers ahead rather than make adjustments for everything. What do all your logbooks have documented time wise?

 

[Bob Noel]

I would set it to the TT of the aircraft. Actually, that's exactly what I did when I replaced the analog tach with a new digital tach back in 2002.

I wanted the aircraft TT time in the tach. I figured I could keep track of SMOH of the engine in the traditional way.

 

[JOhnH]

Well at least I don't have to feel embarrassed about asking a dumb or easy question. So far the count is: TTAF (2) Old tach (.5) Engine (1.5) zero (1).

TTAF is wining but it is close. I am sort of leaning toward setting it to the reading of the old tach so that all the old log book entries will be consistent.

Fortunately, I don't have to decide till tomorrow.

 

[jsstevens]

I'm a renter/club member so I don't know how those log entries are done.

However, the plane has a hobbs as well as a tach. Are airframe AD's still tied to tach hours? Or would they be tied to hobbs since there is one. Also, would all the entries have to be changed or can you simply note a tach time change in the log and go from there?

This is interesting. I think I'm going to learn something here...

John

 

[JOhnH]

Just an update. I called my regular A&P who suggested setting it to the engine time. Someone is going to be confused about something no matter how I do it but it will make following future maintenance intervals easier.

 

[poadeleted20]

TTAF is wining but it is close. I am sort of leaning toward setting it to the reading of the old tach so that all the old log book entries will be consistent.

Since all the old logbook entries (including the ones from before the current tach was installed) should have TTAF in them, not just tach time, and the older ones from before 1374 hours ago can't have a tach time consistent with the current tach but should have TTAF, it seems to me that if you want consistency, you should use TTAF for the new tach.

 

[Tom-D]

While my plane is in the avionics shop having other work done, they found that my tach reads about 150 rpm low. They suggested a new digital tach for about $900 installed. (no wonder I was going so slowly and getting around 6 or 7 gph).

TTAF is 3139
Tach time is 1374
Total time on engine is 88.3

My question is:
What do you suggest that I have them set the new tach to? Does it make any difference to anyone?

Have the tach set at TT and be sure that is noted in the AF log.

Removed old tach reading 1374, TT at this date is equal to 3139, new tach set to 3139.00 A/F Total Time prior to installation on 3/12/11

 

[Dan Thomas]

While my plane is in the avionics shop having other work done, they found that my tach reads about 150 rpm low. They suggested a new digital tach for about $900 installed. (no wonder I was going so slowly and getting around 6 or 7 gph).

That low-reading tach should have fooled you into running at a higher true RPM, not lower, so cruise and fuel burns should have been high.

Dan

 

[Tom-D]

Since all the old logbook entries (including the ones from before the current tach was installed) should have TTAF in them, not just tach time, and the older ones from before 1374 hours ago can't have a tach time consistent with the current tach but should have TTAF, it seems to me that if you want consistency, you should use TTAF for the new tach.

True....... all ADs are Airframe Total Time. Engine time is tracked from date installed at THAT Airframe Total time. Setting the new tach at the Total A/F time Is a lot easier and a lot less prone to math mistakes. And a simple math mistake could be the difference between flying a unairworthy aircraft, and not.

 

[Trapper John]

Isn't it a little strange for an avionics shop to take the initiative to check the calibration of a mechanical tach, especially without being asked?

 

[Tom-D]

Isn't it a little strange for an avionics shop to take the initiative to check the calibration of a mechanical tach, especially without being asked?

Not really, They use a hand held electronic tach,


I changed a tach last month and the person on the phone when we ordered, asked what we would like to have the hours set to.

Pretty common these days.

 

[Trapper John]

Not really, I changed a tach last month and the person on the phone when we ordered, asked what we would like to have the hours set to.

Pretty common these days.

I understand how you check to see if the existing tach is accurate.

Did you just happen to check the tach in the course of doing other work or did the customer ask you to replace the tach in the first place?

 

[Tom-D]

Did you just happen to check the tach in the course of doing other work or did the customer ask you to replace the tach in the first place?

I check them as a matter of routine, on the out of annual turn up.

 

[Trapper John]

I check them as a matter of routine, on the out of annual turn up.

Sure, I'd expect you to check the tach calibration as part of an annual.

But an avionics shop? Just seems odd that they'd do that without the customer asking.

 

[Tom-D]

Sure, I'd expect you to check the tach calibration as part of an annual.

But an avionics shop? Just seems odd that they'd do that without the customer asking.

Do they sell electronic tachs?

 

[Trapper John]

Do they sell electronic tachs?

I don't suppose they'd want to sell the mechanical one at half the price, would they?

 

[JOhnH]

Isn't it a little strange for an avionics shop to take the initiative to check the calibration of a mechanical tach, especially without being asked?

Actually, I did ask them to have their A&P check s few things out for me. Mainly, my newly reman engine seems to run higher CHTs that I would expect, especially on climb-out. My regular A&P doesn't seem all that concerned and says it takes about 100 hours for the nickel cylinders to break in. But I wanted a second opinion. I also like my plane to be "perfect" so I had a small list of other things for them to go over so I am happy they checked the Tach. They are also putting a JPI fuel flow monitor next to my JPI edm700.

I know I will never be able to sell this plane for what I have in it, but It is beautiful inside and out, I like it, and more importantly, my wife loves it. It will have everything except glass MFD/PFD and auto pilot. Maybe next year.

 

[Trapper John]

Actually, I did ask them to have their A&P check s few things out for me. Mainly, my newly reman engine seems to run higher CHTs that I would expect, especially on climb-out. My regular A&P doesn't seem all that concerned and says it takes about 100 hours for the nickel cylinders to break in.

Aren't you the guy that had a cylinder come apart that ended in an off-airport landing not too long ago? I think you were posting about high CHTs before that first engine let go if I remember correctly.

I have a hard time believing that any cylinder could take 100 hrs. to break-in regardless of what type of surface treatment it has.

What kind of CHTs are you seeing? How about oil temps? Baffles and baffle seals in good shape?

I think you said in another thread that you have a 172N. If so, check and see if there has been a lip added to the bottom of the cowling where the cooling air exits - that lip comes from a service letter/service kit specific to the 172N, and if yours doesn't have it, you'll probably want to get it installed.

As long as you're getting a second mechanic to check things out, if the oil cooler is old, check the vernatherm to make sure it opens all the way and make sure the cooler isn't gunked up, and that the carb is set up so it's delivering full-rich at full throttle and check to see the mags' timing is set correctly.

 

[Tom-D]

Mainly, my newly reman engine seems to run higher CHTs that I would expect, especially on climb-out. My regular A&P doesn't seem all that concerned and says it takes about 100 hours for the nickel cylinders to break in.

That is a common misconception actually break in was done on the test cell at the factory or overhaul facility.

Which do you believe will give a higher CHT and EGT ?

your old engine going chitty chitty pop pop bang bang.

or you new engine going bang bang bang bang?

You engine uses heat to make power, the new one is making better power so it will run hotter

 

[denverpilot]

So if these digital tachs can be (relatively easily) set to arbitrary hour numbers, what's to stop an unscrupulous owner from constantly fiddling with it? Kinda odd.

We replaced a Hobbs, a normal ol' mechanical Hobbs last year. It was at "0". That was the only option.

 

[Steve]

As long as you're getting a second mechanic to check things out, if the oil cooler is old, check the vernatherm to make sure it opens all the way and make sure the cooler isn't gunked up, and that the carb is set up so it's delivering full-rich at full throttle and check to see the mags' timing is set correctly.

Just a nit, but the vernatherm closes to direct oil to the cooler. Seems intuitively non-sensical to me, too. And, if the oil pressure is too high it won't close completely even in a high temperature condition.

This article is just plain wrong in describing the vernatherm's operation when tested.

http://www.avweb.com/news/maint/182617-1.html?type=pf

Oil can also bypass the oil cooler if the vernatherm valve is defective. The vernatherm is just like the thermostat on your car's radiator. It's a temperature-sensitive valve that regulates oil temperature by controlling oil flow through the cooler. As a rule, it strives to keep oil temp at around 170 degrees F.

You can check the vernatherm's basic operation the same way you would a car's thermostat. Just drop it in boiling water and it should open wide.


I'm surprised AvWeb hasn't redacted it by now (or maybe I'm not).


A much better explanation (with pictures!) can be found in

http://ellis-assoc.com/uploads/Anatomy_of_a_Vernatherm.pdf


.

 

[Tom-D]

So if these digital tachs can be (relatively easily) set to arbitrary hour numbers, what's to stop an unscrupulous owner from constantly fiddling with it? Kinda odd.

The case must be open to adjust the readings.

We replaced a Hobbs, a normal ol' mechanical Hobbs last year. It was at "0". That was the only option.

pretty normal, old tech. even the new mechanical tachs can be set to any reading you want but it must be done before the case is closed.

 

[Trapper John]

Just a nit, but the vernatherm closes to direct oil to the cooler. Seems intuitively non-sensical to me, too. And, if the oil pressure is too high it won't close completely even in a high temperature condition.

<Johnny Carson> I did not know that. </Johnny Carson>

I assumed it opened when hot, like an automotive thermostat. Thanks for the link, very interesting.

 

[JOhnH]

Aren't you the guy that had a cylinder come apart that ended in an off-airport landing not too long ago? I think you were posting about high CHTs before that first engine let go if I remember correctly.

I have a hard time believing that any cylinder could take 100 hrs. to break-in regardless of what type of surface treatment it has.

What kind of CHTs are you seeing? How about oil temps? Baffles and baffle seals in good shape?

I think you said in another thread that you have a 172N. If so, check and see if there has been a lip added to the bottom of the cowling where the cooling air exits - that lip comes from a service letter/service kit specific to the 172N, and if yours doesn't have it, you'll probably want to get it installed.

As long as you're getting a second mechanic to check things out, if the oil cooler is old, check the vernatherm to make sure it opens all the way and make sure the cooler isn't gunked up, and that the carb is set up so it's delivering full-rich at full throttle and check to see the mags' timing is set correctly.

You remember correctly. I was having high CHT values before the off landing event too although my regular A&P, the FAA and the Rebuild shop all say that had nothing to do with the failure. But they can't say what did. It is a 172N with a PennYan 180 upgrade. The replacement was also a 180. I am on my fourth A&P in my search to discover the cause of the high CHTs, although they are coming down somewhat as I approach the 100 hour mark. At first I always exceeded 450 (cyl #3 &4) on climbout and those two ran around 410 during level flight. Now, at 80 hrs, #3 & 4 max out at about 440 momentarily during climb out and about 390 at level flight. Funny thing (yeah right) is that #2 has run cooler since the begiinning (after engine swap). #1 runs somewhat hotter. #s 3 % 4 are the ones I worry about. yeah,I know, the back cylinders always run hotter. But that doesn't make it right or safe.

I did ask this new A&P to double check the baffling the throttle mixture and anything else he thought of (hence the tachometer). The baffling is new (about 3 years). He richened up the mixture, which could have been causing the high temps on climb-out. He is still working on it and I get it back in about 2 weeks. I will know more then.

Some A&Ps tell me that since so few 172s have multi cylinder digital engine analyzers that most people have the same temps but never know it and don't worry about it.

 

[Trapper John]

I am on my fourth A&P in my search to discover the cause of the high CHTs, although they are coming down somewhat as I approach the 100 hour mark. At first I always exceeded 450 (cyl #3 &4) on climbout and those two ran around 410 during level flight. Now, at 80 hrs, #3 & 4 max out at about 440 momentarily during climb out and about 390 at level flight.

Those CHTs sound awfully high - are you sure the probes are reading correctly?

Some A&Ps tell me that since so few 172s have multi cylinder digital engine analyzers that most people have the same temps but never know it and don't worry about it.

All the ones I've flown have the standard single probe instrument. I should know, but I don't, which cylinder it measures - I would assume the one that runs hottest. But I've never seen anything over 400 F that I can remember...

 

[JOhnH]

Those CHTs sound awfully high - are you sure the probes are reading correctly?

I am not sure of anything regarding this problem. That is why I am on my fourth A&P. But I will ask this latest one if he can test them.