RPM gauge / tach reading high

[mccdeuce]

As I understand it the hours on the tach are based on a cruise rpm.  For my last flight from Master On to Master Off - was 50 minutes.  Tach read 1+06.  Cruise RPM was 2500.   

RPM is a digital Horizon Tach (that I don't like and have an EI awaiting install).  

Is the gauge wrong or is cruise RPM for a J model 2400? (not trying to discuss ideal cruise rpm)

[cnoe]

Are you saying it read 66 minutes or it read 1.06 hours?

My understanding is that the electronic tachs are simply timers without regard for RPMs (I cannot verify this though).

But if your tach is showing 66 minutes when it should read 50 (or less) that's costing you big $ in depreciation/market-value. Your tach will show the engine at TBO (2,000) when it's actually only run for 1,515 hours! At typical engine-reserves that's over $7,000! I'd replace that thing yesterday.


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[jetdriven]

From the chief engineer at Mooney, the correct recording speed of a M20J mechanical tach is 2566 rpm. Skip the crappy Mitchell brand and get an SLI unit. The Mitchell broke 3 times in just a few hours and I gave up, had a hassle getting my money back.  

Edited December 16, 2016 by jetdriven

[mccdeuce]

3 hours ago, cnoe said:

But if your tach is showing 66 minutes when it should read 50 (or less) that's costing you big $ in depreciation/market-value. Your tach will show the engine at TBO (2,000) when it's actually only run for 1,515 hours! At typical engine-reserves that's over $7,000! I'd replace that thing yesterday.
 

yup.  showing 66 minutes when it should be showing less.  esp if recording speed should be 2566.  will investigate timer aspect.

the depreciation was exactly my fear.  or that the rpm is higher than shown.

[mccdeuce]

From the Electronics International install manual:   The tach timer keeps a running total of time the engine is above 1300 RPM. The time is stored in memory for life.

From the Horizon install manual:  Engine hours are recorded in real time beginning when the engine is assumed to be able to sustain combustion, at or above 800 RPM. A high accuracy engine clock yields true TBO times (engines are rated in real hours, not RPM-hours) and more accurate fuel consumption calculations. 

 

So to me this says we all need to use analog gauges?!?

[mooniac15u]

1 minute ago, mccdeuce said:

From the Electronics International install manual:   The tach timer keeps a running total of time the engine is above 1300 RPM. The time is stored in memory for life.

From the Horizon install manual:  Engine hours are recorded in real time beginning when the engine is assumed to be able to sustain combustion, at or above 800 RPM. A high accuracy engine clock yields true TBO times (engines are rated in real hours, not RPM-hours) and more accurate fuel consumption calculations. 

 

So to me this says we all need to use analog gauges?!?

Your engine doesn't care how many hours your tachometer thinks it has.  TBO is magic number that doesn't directly apply to part 91 operations.  Your engine will need to be replaced when it needs to be replaced, not when your tachometer reaches 2000.

[mccdeuce]

true - but resale value in 10 years?  currently have 200 hours on the engine. If I fly for 100 hours a year - should have around 1200 on the engine.  Tach would show 1500.  realistically it prob doesn't matter.

[Guest]

What about AD's which are based on hours in service? You're doing those early as well.

in Canada we use a journey log book, in which we record our flying, start to stop for pilot in command time, wheels up to wheels down for airframe time.

Clarence

[mccdeuce]

38 minutes ago, M20Doc said:

What about AD's which are based on hours in service? You're doing those early as well.

in Canada we use a journey log book, in which we record our flying, start to stop for pilot in command time, wheels up to wheels down for airframe time.

Clarence

so whats interesting is that this would mirror these electronic rpm gauges working total airframe time for all maintenance.  

Interestingly enough thats what we do with the helicopter.  Time on airframe and time on engines is based 100% on wheel up to wheel down time.

[mooniac15u]

The FAA defines time in service as follows:

"Time in service, with respect to maintenance time records, means the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing. "

So, when we are using tach time it is just an approximation anyway.

[jetdriven]

Yes the FAA published an AC which said basically exactly that, tach time was an approximation. But it's a better approximation than something that runs like a Hobbs meter, on the ground and everything.  

[cnoe]

Still, as was mentioned earlier as a Part 91 operator I'm looking for an engine TBO of ~3,000+ hours. Fly frequently, keep the CHTs low, change oil regularly, borescope during those (oil changes), and keep the prop out of the dirt. That's my basic plan.

I know it could all go to hell mañana but if well maintained I see no need for arbitrary overhauls.


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[DonMuncy]

There are two scenarios at play here. If we are talking about keeping our plane for a long time, TBO is pretty meaningless. But if we decide to sell, TBO becomes a pricing factor. Maybe we need to keep two sets of logbooks. One for our use and one to sell with.

[carusoam]

Also consider using an oil pressure switch to initiate the time being collected.  Real time associated with the actual running of the real engine.

Then share that with the new buyer...

Best regards,

-a-