Calculating Total Airframe Time

Fearless Tower

Fearless Tower

Touchdown! Greaser!
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Fearless Tower
For the aircraft logs -what do you use to calculate the airframe time? Do you use the Tach time?

I was updating my aircraft logs and noticed that the previous owner/A&P was logging airframe time two decimal places while the Tach only records one decimal place. Not sure why.
 
Fearless Tower said:
For the aircraft logs -what do you use to calculate the airframe time? Do you use the Tach time?

I was updating my aircraft logs and noticed that the previous owner/A&P was logging airframe time two decimal places while the Tach only records one decimal place. Not sure why.
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I use Tach, it is a prorated time to the engine operation, unless you have a squat switch you really can't tract flight time.

Years ago we use the journey log to tract flight time as the Canadians do now.

Did you change tachs? some have two places new ones don't.
 
The generally accepted practice for maintenance purposes is to use tach time when there is a recording tachometer. That's also the best estimator of engine wear since it actually counts engine revolutions, not real time. The technical definition in the FAR's for maintenance purposes is takeoff to landing time, but few people do it that way unless they don't have a recording tachometer, and the FAA is perfectly happy with tach time.

And, as Tom said, if the tach is ever changed out, you'll need to carry the old tach time forward in the log books so you know what the actual TTAF is based on the ending time of the old tach plus the current time of the new tach.
 
Ok, that is probably what happened - they changed the tach out when they did the engine changeout. I'm guessing that the previous A&P IA just kept adding the new tach time to the previous two decimal places.

So, do I need to continue that, or just round up (not like it makes that much difference, but don't want to have problems down the road with some log nazi)?
 
Fearless Tower said:
Ok, that is probably what happened - they changed the tach out when they did the engine changeout. I'm guessing that the previous A&P IA just kept adding the new tach time to the previous two decimal places.

So, do I need to continue that, or just round up (not like it makes that much difference, but don't want to have problems down the road with some log nazi)?
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the entry that shows the tach change must be very explicit, it must show the tach reading of the tach being removed and the tach reading of the replacement. and this is where the installer should enter the Aircraft TOTAL TIME so going forward anyone can simply add the accumulated time to that entry.

as far as using the tach reading, simply use what is shown. The old tach reading may have had two numbers to the right of the decimal, and the new one may only have one. thats not a bigey, simply write what you see.
 
Tom-D said:
the entry that shows the tach change must be very explicit, it must show the tach reading of the tach being removed and the tach reading of the replacement. and this is where the installer should enter the Aircraft TOTAL TIME so going forward anyone can simply add the accumulated time to that entry.

as far as using the tach reading, simply use what is shown. The old tach reading may have had two numbers to the right of the decimal, and the new one may only have one. thats not a bigey, simply write what you see.
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Ok, found the entry - they actually replaced it when the previous one failed and before they changed the engine. Log entry has all the times before and after. Looks like he just kept adding the new tach time to the previous two decimal places. All making sense now.
 
Fearless Tower said:
Ok, found the entry - they actually replaced it when the previous one failed and before they changed the engine. Log entry has all the times before and after. Looks like he just kept adding the new tach time to the previous two decimal places. All making sense now.
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OK, so now, the old reading was 1234.56, and the new tach reading is 224.4 you add the two numbers you still get 1458.96. as total time.

Don't forget, to insure the engine log reflects the tach time the engine was removed and replaced after the overhaul, because that becomes the major number to be used to calculate TT and TSMO in the engine logs.
 
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Since the Hobbs is linear, why wouldn't that be used for airframe instead? Engines are throw-away and unrelated to useful life of the airframe.
 
Jaybird180 said:
Since the Hobbs is linear, why wouldn't that be used for airframe instead? Engines are throw-away and unrelated to useful life of the airframe.
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Because the hobbs starts counting as soon as you turn the power on/start the engine. Not much wear and tear on the airframe sitting on the ground waiting to takeoff and depending on where you are, that could be a significant amount of time.
 
Fearless Tower said:
Because the hobbs starts counting as soon as you turn the power on/start the engine. Not much wear and tear on the airframe sitting on the ground waiting to takeoff and depending on where you are, that could be a significant amount of time.
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Plus the Hobbs requires electrical power the tach doesn't.
 
What is the hobbs really for in a personally owned aircraft?
 
Technically, pilot time in airplanes is chock-to-chock, not start-to-shutdown (which is what most Hobbs meters read via an oil pressure sensor), but the FAA doesn't quibble about this point.
 
Ron Levy said:
Technically, pilot time in airplanes is chock-to-chock, not start-to-shutdown (which is what most Hobbs meters read via an oil pressure sensor), but the FAA doesn't quibble about this point.
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Actually pilot time is what ever he put in his log.
 
Jaybird180 said:
What is the hobbs really for in a personally owned aircraft?
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It's for determining time in service. My plane doesn't (and most late model Beeches don't either) have a recording tach. The hobbs is on the landing gear switch, which makes it very close to the time in service number definition.
The Tach's a reasonable interpretation as it runs very slowly at idle.

I however take issue with "revs" being a major factor in engine wear. Engine wear in a well cooled and lubricated engine is negligible. It's the time when the lubrication is not up to snuff (cold starts, etc...) or there is excessive heat, or corrosion sets in from disuse that causes the wear issues. Our "billed by the hobbs" (i.e., no incentive not to run them hard) trainers still going strong way past TBO.
 
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Tom-D said:
Actually pilot time is what ever he put in his log.
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Ha, ha. Pilot time is defined by the FAA as follows.
14 CFR 61.1(b)(15) Pilot time means that time in which a person--

  • (i) Serves as a required pilot flight crewmember;
    (ii) Receives training from an authorized instructor in an aircraft, flight simulator, or flight training device; or (iii) Gives training as an authorized instructor in an aircraft, flight simulator, or flight training device.

14 CFR 1.1 Flight time means:

  • (1) Pilot time that commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing; or (2) For a glider without self-launch capability, pilot time that commences when the glider is towed for the purpose of flight and ends when the glider comes to rest after landing.
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The only remaining question is whether the entries in the pilot's log are being done to the FAA's satisfaction or not.
 
Ron Levy said:
Ha, ha. Pilot time is defined by the FAA as follows.

The only remaining question is whether the entries in the pilot's log are being done to the FAA's satisfaction or not.
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And if they fly 100 hours and only enter 5 ? There are pilots like me that don't need to show anything except currency.
 
Ron Levy said:
Technically, pilot time in airplanes is chock-to-chock, not start-to-shutdown (which is what most Hobbs meters read via an oil pressure sensor), but the FAA doesn't quibble about this point.
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My plane moves forward a tiny bit when I start the engine and never fully comes to rest until it is shutdown.
 
Unless you wanted to quibble about the definition of a full stop landing needed for night or tail wheel currency. :)
 
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