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Rocket Tach time runs up even when at idle


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Hey everyone... We have noticed that our tach time rolls even when we are at low RPM...  This results in a 1 hour flight recording 1.2 or 1.3 for an entire flight.  As I understand it, the tach should not be recording time in this manner, but much slower when at lower RPM.  We mostly use this to put money in the kitty for maintenance/overhaul, however we have realized that we may be wracking up engine hours which devalues the aircraft.

I cannot see any brand name/ model number on this tach so no help there....  I am wondering if any other mooney/rocket owners can let me know what this situation might be.

 

Thanks

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Tach time runs if the engine is running.

Old mechanical tachs will run slower than real time at low RPM.  At some point the RPMs will be at or close to real time (some number the manufacturer decides upon).  And at full power the tach time is faster than real time.

My Electronic Tach runs in real time all the time the engine is running. The newer JPI units someone told me runs like the old mechanical tach.  Slower at lower RPMs.

Slower at lower RPM is actually better if you base your TBO on tach time. 

Question is if the meter is really off or the tach is really showing higher RPM than the engine is turning.

 

 

Edited by PeteMc
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The way you describe it is expected.

On the side of the tach will be an RPM where 1 hour = 1 hour. Anything below that RPM is scaled in slower than actual time and anything above it is faster than 1:1 time.

An hour at idle will show less than an hour on the tach and an hour at climb RPM will show more than 1 hour on the tach.

You might be thinking of oil pressure activated hobbs meters, but I think most are tied to the master now.

Edited by smwash02
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IIRC usually, tach time is typically set to 1:1 at a "cruise" RPM, but I've never seen that defined anywhere, so I suspect there is some leeway.

IIRC also, you are allowed to use either Hobbs or tach time for determining required maintenance intervals.  Since we have very few as part 91 operators, it's not a huge difference.  For devaluation, I suppose it does make a difference, though.

That's funny, I think I have a Hobbs meter in my aircraft, but I'm not sure.  If I do, I never look at it.  I THINK it's in the passenger side of the center pedestal where it's impossible to read unless you're standing on the wing looking in with a flashlight.

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I have one there too.  Easiest way to read it is to slip in between the wing and the open door and peer in, but haven't done that in ages. :lol:

Edited by PeteMc
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Tach time is an interesting measurement...

It is exactly right at one rpm... off a small amount going faster or slower...

This is all driven mechanically inside the tach... rolling over numbers on its face... based on a set gear ratio...

So... at idle where the rpm is about half the cruise speed... sure... you are using engine hours while sitting and waiting for a clearance...

Strong memories from IR training, sitting and waiting to T/O, fifth or sixth in a long line... that was not moving while waiting for planes to land... KMMU. Morristown, NJ...  the engine was barely working, FF was small, and the CFII was sitting waiting for me to ask more questions...   :)

More modern machines use a timer that actually counts real hours while the engine is running... the timer comes on and off based on a switch... sometimes an oil pressure switch...

There isn’t a very good method of counting or using engine hours... fortunately we have learned quite a bit about what it takes to operate beyond TBO... successfully...

Best regards,

-a-

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20 hours ago, PeteMc said:

Tach time runs if the engine is running.

Old mechanical tachs will run slower than real time at low RPM.  At some point the RPMs will be at or close to real time (some number the manufacturer decides upon).  And at full power the tach time is faster than real time.

My Electronic Tach runs in real time all the time the engine is running. The newer JPI units someone told me runs like the old mechanical tach.  Slower at lower RPMs.

Slower at lower RPM is actually better if you base your TBO on tach time. 

Question is if the meter is really off or the tach is really showing higher RPM than the engine is turning.

 

 

Wow that should be a big disclaimer. That pretty much would kill any plans I may have had of even going to a digital tach. You’ve basically increased your maintenance  cost by about 20%!!
 

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18 hours ago, PT20J said:

I believe Mooney used 2300 rpm mechanical recording tachs. Higher rpm will read faster than actual time.

Indeed about 2300rpm when I time mine in the M20J against the clock in few cruise legs

Edited by Ibra
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Mechanical tach's count engine revolutions. Electronic tach's count time.

If you are operating part 91, your tach time doesn't matter for your maintenance costs. You can do oil changes and engine overhauls whenever you like. Heck, you can never change your oil, we will tease you about it, but nobody will ever give you crap about it. Well, it is kind of required at annual, but you could drain out your used oil and put it back in again. 

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5 minutes ago, N201MKTurbo said:

Mechanical tach's count engine revolutions. Electronic tach's count time.

If you are operating part 91, your tach time doesn't matter for your maintenance costs. You can do oil changes and engine overhauls whenever you like. Heck, you can never change your oil, we will tease you about it, but nobody will ever give you crap about it. Well, it is kind of required at annual, but you could drain out your used oil and put it back in again. 

This ^^^^^^

Really, for Part 91 time only pertains to ADs and sales negotiations.

The official definition from Part 1 is

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.

If you are really concerned about it, the solution is to install an hour meter activated by an air switch as some commercial operators do.

Skip

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25 minutes ago, N201MKTurbo said:

Mechanical tach's count engine revolutions. Electronic tach's count time.

If you are operating part 91, your tach time doesn't matter for your maintenance costs. You can do oil changes and engine overhauls whenever you like. Heck, you can never change your oil, we will tease you about it, but nobody will ever give you crap about it. Well, it is kind of required at annual, but you could drain out your used oil and put it back in again. 

I have ad’s based on time. Also things like insurance and even some IAs desire to sign annuals can be associated with component tach time. 

Edited by RobertGary1
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I have an UMA electronic tach that begins counting at 1500 rpm.  Since I normally cruise at 2350-2400 rpm, I’d say mine is very accurate- and I might come out a little ahead.

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16 minutes ago, Andy95W said:

I have an UMA electronic tach that begins counting at 1500 rpm.  Since I normally cruise at 2350-2400 rpm, I’d say mine is very accurate- and I might come out a little ahead.

I have one of those too. I like it a lot, except reading the tach time.

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Although all mechanical tachs do count time based on the percent of the tach's designated RPM as @EricJ described (I thought Mooney Tach's use 2450 rpm) its not true that all electrical tachs count time 1:1. This is only true of one electronic tach I am aware of, the Horizon which is very popular because it has the same footprint as the legacy factory Tach's - its also one of the best electronic tach options given it features - but it 1:1 recording always killed my desire to install one for me.

@Andy95W just gave an example of the UMA not counting any time till an RPM of 1500. The EI RPM1, works the the same way and doesn't start counting till an RPM of 1300, which is another popular option for Mooneys because it's the same footprint as the modern Mooney's tach's. Not counting till 1300 rpm has given me identical results from my old OEM Mooney the majority of flights.

In contrast, the later JPI engine monitors RPM function are setup to work exactly like your mechanical tachs in that the 1:1 speed is programable with a default of 2400 rpm - which should be essentially identical to the original OEM Mooney tach but allows you to tailor it to exactly what the tach used your replacing in a primary instrument install.

The point is no two electronic tachs count RPM exactly the same and the user should consult their documentation to see how theirs works. 

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24 minutes ago, RobertGary1 said:

I have ad’s based on time. Also things like insurance and even some IAs desire to sign annuals can be associated with component tach time. 

So, magneto and other engine ADs that depend on time, make sense. But things like landing gear ADs that depend on time are  weird. 

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My EI electronic tach displays flight time based on RPM but I'm not sure what RPM it begins to record time.  when I have a longer hold for a clearance i notice a difference between the tach and clock but never more than 15minutes or so.

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Assuming you have an EI R1...

Engine Time is recorded when the engine reaches 1300 RPM. 

Flight Time starts recording when the engine reaches 2000 RPM for 10 seconds.  Stops recording when engine drops below 2000 RPM for 10 seconds

Peak Time is the max. RPM maintained for at least 3 seconds.

 

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This is a little off topic, but has anyone ever compared tach time with the flight time listed in FlightAware?  I haven't but if they are close that would provide an independent data source in the event that the FAA ever questioned logged time in your logbook. 

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Flight aware missed 30 minutes of my last flight. It’s very inconsistent for me so that would not help me in that case. ForeFlight  keeps a good record of my time once I hook up the stratux...

Here it missed my take off and says ‘near Sherman’ when I was closer to Commerce (Sherman is right next to GYI BTW). It ‘guessed’ where I was going back to GYI (gray line). 

wonky....

-Don

86CFA384-D52F-494A-A65A-7D9F347BDD13.png

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Me too. FlightAware varies for me too. But I do see the state using FlightAware to question each flight if you try to claim historical tax exemption. In that case each flight away needs to be for only maintenance or shows. 

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  • 2 weeks later...

 

On 1/14/2021 at 10:40 AM, PT20J said:

This ^^^^^^

Really, for Part 91 time only pertains to ADs and sales negotiations.

The official definition from Part 1 is

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.

If you are really concerned about it, the solution is to install an hour meter activated by an air switch as some commercial operators do.

Skip

I hope this is not too far off-topic but I think it is an interesting tach/Hobbs issue.

The Hobbs meter in my G1000 Ovation 2 is activated by the extra set of contacts in the landing gear airspeed safety switch, which in theory should closely track the time airborne.  This would be great, but because of an apparent design shortcut it shares its power with the cabin lighting, which occasionally blows the 5A fuse in my plane.  (I am guessing the baggage light timer causes this but have not investigated.)  Over time the Hobbs and tach time readings have diverged as a result.  I think the Hobbs is useless as-is and am thinking of wiring a dedicated fused circuit to it from the main bus to get it to work as it should.

I think the G1000 derives tach time by counting engine (magneto) revolutions as it would for the tach itself.  It is always less than elapsed time from engine start to shutdown by a few tenths, but it is very close to flight time shown on FlightAware.

I put some numbers from the mx logs on a spreadsheet to see how the Hobbs and tach have diverged over time, attached below.

Have other G1000 Ovation owners seen this tach/Hobbs problem, or is it just me?

- Dave

Inspection Summary.pdf

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Dave,

It’s pretty common to have hour meters wired hot. That way, the pilot cannot keep the clock from running by turning off the master switch (not that any CB renter pilot paying an hourly hobbs rate would ever do that, of course;)).

I would investigate what’s blowing the fuse. Something isn’t right. 

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+1 for something being amiss...

Blowing a 5A fuse... tanks a handful of amps...

Which isn’t normal for small light bulbs or Hobbs meters...

No matter how long they get left on...

Be on the look out for some damaged wire insulation?

PP thoughts only, not a mechanic...

Best regards,

-a-

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