Hobbs Time or Tach Time? Which Do You Use?

[RoundTwo]

I’m trying to get the hours “cleaned up” for the new-to-me J. 
It came from the factory with a mechanical hobbs driven by the electronic tach. The original meter was replaced in 1990. In 2019, the panel was upgraded and a JPI 900 was installed, so now the plane has tach time as well as a new hobbs meter, but the “newish” hobbs is still installed, and since the electric tach is gone, it just clicks away whenever the master switch is on. I’d like to document the appropriate “adder” to use to get current total time and get that clearly visible in the logs to prevent future issues like some that have happened in the past.  
My plan is to convert over to using the JPI numbers going forward and physically remove the mechanical meter. Since the mechanical meter was switched off until the rpm threshold was exceeded, until the avionics upgrade, I have a defensible position to disregard the mechanical meter since then because it has been “running free” whenever the master was on and the CB wasn’t pulled. This CB is also the ignition breaker, so there’s no way to “cheat” on the hours by turning the meter off when desired. 
If you have both tach time and hobbs time, do you differentiate between them for log book purposes, or just use one over the other?

[Pinecone]

Two times for different reasons.  Maintenance is based on Tach Time.   Hours in the maintenance log books are typically Tach Time.

Time you log is basically Hobbs time.

Check the settings on your JPI, tach time should be based on 2500 RPM.  Somewhere is a post where I posted the chart from the factory.   But if you cruise at 2500 RPM, Hobbs = Tach.  If you cruise at lower RPM Tach < Hobbs.

[RoundTwo]

2 minutes ago, Pinecone said:

Two times for different reasons.  Maintenance is based on Tach Time.   Hours in the maintenance log books are typically Tach Time.

Time you log is basically Hobbs time.

Check the settings on your JPI, tach time should be based on 2500 RPM.  Somewhere is a post where I posted the chart from the factory.   But if you cruise at 2500 RPM, Hobbs = Tach.  If you cruise at lower RPM Tach < Hobbs.

My parity is set at 2400. 
Is it correct to say as far as the plane’s logs are concerned, the only time that matters is tach time? This means my plane will age more slowly that one using hobbs time. 

[redbaron1982]

1 hour ago, RoundTwo said:

My parity is set at 2400. 
Is it correct to say as far as the plane’s logs are concerned, the only time that matters is tach time? This means my plane will age more slowly that one using hobbs time. 

My understanding is that there is no clear definition in the FARs regarding what time to use. And as a result, people use the ones that are more convenient, usually tach time for maintenance, because as you mention it grows slower. Also for hobbs I think there are different"variants", one that increases as soon as you turn master on, another that only starts counting after you reach a certain airspeed and I think there is also a variant that uses the sensor in the landing gear that tells whether the aircraft is in the ground or not.

I tried in the past to find through the FARs to see if anything there mentioned a specific time and couldn't, does anyone more familiar with FARs have a better answer?

[Pinecone]

Yes, there are different ways for switching the Hobbs on/off.  The one I have seen the most is with an oil pressure switch, so it runs any time the engine is running.  I have seen the airspeed or gear based ones.

As for the base RPM, there are many standard ones.. Aircraft Spruce sells ones that are 1800, 2050, 2300, 2400, 2566, 3000. 

 

I contacted Mooney and the info they sent was that B&D tachs send 1:1 at 2500 RPM.

 

[redbaron1982]

1 hour ago, Pinecone said:

The one I have seen the most is with an oil pressure switch, so it runs any time the engine is running. 

I forgot that one, as you said is the most common one, except for flight schools that hey want to charge you as much as possible and the hobbs start counting as soon as you turn on master.

[PT20J]

FAR Part 1:

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.

Note that this is not “Hobbs time” or “tach time.” Tach time is frequently used for maintenance records because it is all that is available. Hobbs time is usually used for rentals for billing purposes. The FAA does not specify how you measure time. 

When I had the G3X/EIS installed, I had the B&D meter removed and the time transferred to the G3X which is set to record flight time.

Skip

Edited May 15, 2023 by PT20J

[Pinecone]

2 hours ago, PT20J said:

FAR Part 1:

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.

Since the tach time is counting at 1 to 5 ratio at idle, the difference is small.  But maybe not for those who taxi at 1500 RPM.

But it seems that for a retract, gear up time is pretty close and an airspeed switch to be about perfect.  Of course, with modern avionics, the GPS could track this time exactly

 

[Aerodon]

One of the C172's has 2750 in the logbook at 3000 on the tach.  Canadian way is to count proper flight time, pretty easy to track nowadays with handheld GPS, panel GPS, transponder or flightaware.  Or Hobbs - 0.2.

A future buyer can look at this and say its a 3000 hour plane with 1000 hours or 1100 hours on the engine.  Not enough to really affect the value.  Might make a slight difference for a SB or AD that is due at say 5000 hours.

When I am a buyer, I don't really care one way or another, but do pay attention to how its written up in the logbook. and make a small adjustment in my overall assessment.

Aerodon

 

 

 

 

 

 

 

 

 

 

 

[PT20J]

A lot of mechanical tachs read 1:1 @ 2300 rpm, so at cruise they run fast. 

[kortopates]

9 hours ago, redbaron1982 said:

I tried in the past to find through the FARs to see if anything there mentioned a specific time and couldn't, does anyone more familiar with FARs have a better answer?

What your looking for is the definition of "Time in Service" for maintenance records, Defined in the CFRs  in 1.1 along with all the other definitions - and you're right its from essentially liftoff to touchdown; which few us have an timer for so tach time, or the Mooney hour meter, is our closest approximation