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Here's something interesting, and others can help out on this. tach time and total time probably will NOT agree. Because tachs are setup to read correctly at a given tach RPM reading, if you don't run it at say 2550 for a rpm setting the tach will not read actual hours. It's not noticeable over a few hours of flight, but if you say run it at a higher rpm say, the actual hours in the air will creap up towards the actaul tach time. This is what I learnt from doing this in a 172 I used to own. Try it over a few hundred hours, now imagine several thousand hours

True - Tach time and Real time don't coincide. Tach time runs faster or slower as the engine goes faster or slower. At 2550 RPM (or thereabouts), it's running in real-time.

Thus, Tach time is a good indication of engine usage - the more (and faster) you use the engine, the more (and faster) the components and fluids age.

Some planes have a Hobbs meter that is separate from the tach. It is a real-time clock that runs whenever the master-switch is on or the oil-pressure is up (ie - engine running). A Hobbs meter will run at the same speed regardless of flight performance. Since a Tach runs slower at idle, Tach time typically lags Hobbs time on a plane.

If the plane only has a Tach, then yes, the sum of all the Tach times should equal the airframe time, since that is the only measure available.

If the plane has a Hobbs, then that is typically used to measure airframe time, and will likely not match the tach times.

My plane has only a Tach. Even if I had a Hobbs, I would still use the Tach for all maintenance intervals. It's a truer measure of the workload on the engine and airframe. A Hobbs is more useful for rental billing purposes since it represents the time the plane is 'in use' and unavailable to others.

Agreed, and thanks for the info. Both planes have Hobbs meters. Here's a thought...what if the replacement Tach was used? Could there have been time on it?
This would explain the weird math needed on the one plane.

Don't you mean tach time and Hobbs time (or clock time)?

All certified aicraft have at least a tachometer. Not all have a Hobbs meter, so total times should always be figured using tach time, although you are correct about it's possible inaccuracies. In theory, it all averages out about right over time.

Weird Math?

Your car has 15,000 miles on it when the odometer fails. You have it replaced, and the new odometer starts at 0.00! How many total miles are on your car? It's not weird math! You add 15,000 miles to whatever your odometer now reads!

But, If you replaced the failed odometer with a used one that already had 10,000 miles on it, the difference between the failed odometer and the used replacement is not 15,000 miles, but only 5,000 miles, which is what you now add to whatever the odometer reads!

Ahhh! A "real world" use for algebra! (remember word problems? )

log time @ tach failure: A
elapsed flying time after tach R&R: X
Total time: T

T = A + X .....(eqn 1)

time on used tach at installation: B
time on used tach after flying: C

C = B + X ......(eqn 2)

rearrange eqn 2 to solve for elapsed time:

X = C - B .....(eqn 3, "the difference")

subsitute eqn3 for X in eqn 1:

T = A + X = A + (C - B)

Which is exactly what Carl was saying, but without those pesky confusing numbers.

Hello, Mr. Herb Rose.

The slick magnetos are not required to be overhauled at 500 hours. There is a 500 hour inspection that is listed in the component maintenance manual for the slicks.

The slick mag overhaul is prohibitively expensive to be accomplished in the field, or even at a mag shop. Slick has priced their replacement parts prices so high that by the time you comply with the list of replacement parts on the overhaul, plus the labor required, you could have exchanged your old mags for new at a lesser cost.

I do the 100 hour inspections myself. When it's 500 hour inspection time, I remove the mags and send them to the shop where they're completely disassembled, inspected IAW the listed 500 hour inspection program as recommended by Slick, then reassembled. They're bench checked and the e-gap is adjusted to provide for the optimum spark reliability and output from 0-3000 rpm.

Lots of folks run their mags either to overhaul of the engine to which they're attached, or to mag failure. A&P's should be more adamant with customers about the 500 hour inspection requirements.

There is neither a requirement nor a recommendation to overhaul the mags at 500 hours. Compliance with Slick's maintenance program will suffice.

Weird math because he said to subtract the new tach time from the old, and then add the difference to the new.

If I have absorbed the algebra and help correctly, I should have noted:

Tach time listed at replacement (say 5400)
Tach time on the replaced unit (I think it was 4760)
and the Tach time now (I think it was 5300)

5400 + (5300 - 4760) = 5940

Appreciate the correction.

Almost! You said it right in the first line, but transposed the numbers in the formula. It should read:

5300 + (5400 - 4760) = 5940

You did it right! You just didn't type it right!

Gary:

I stand corrected in the terms used. As noted, the darn thing needs to see the shop at 500 hours. I knew of this only because, I had reviewed the log books with Scott when we did the top-overhaul this past October at Columbia. Wrong term, but the right idea.

Thanks, Herb

See what I mean! Dem pesky numbers!