Tach and hobbs

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Popcorn1120

Filing Flight Plan
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Popcorn1120
I need to get a better understanding of tach and hobbs and when each is applied to aviation maintenance and how to determine the calculation.
 
My understanding is tach hours is what ya use for maintenance and Hobbs is what FBOs use for rentals- not all of us even have a Hobbs… I don’t..
 
Tach "time" is not really time, it is counting the number of revolutions of the engine but displaying it as "hours" for convenience. At some high rpm e.g. 2500 the tach "time" is fairly accurate. Especially during training, the average rpm is quite a bit lower than 2500 and so the tach under reports the time. Flight schools figured this out so they installed the Hobbs meters which generally indicate actual clock time whenever the engine is running. This way they can bill the student more! There is also a slight benefit to the student/pilot as they can log more hours than the tach would indicate.
 
Popcorn1120 said:
I need to get a better understanding of tach and hobbs and when each is applied to aviation maintenance and how to determine the calculation.
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To add, the most accurate method to track maintenance time is by time in service which is defined as the moment the aircraft leaves the ground till the moment it lands. A recording tach and hobbs are only approximations with a tach recording less time than a hobbs. So most use a tach for mx time and a hobbs for flight time.

FYI: a hobbs can be configured to record actual time in service if one chooses.
 
Some planes start the Hobbs meter when the electrical power is applied, others start it at engine start, so there’s some variation. But for maintenance purposes, go by tach time.
 
flyingbrit said:
Tach "time" is not really time, it is counting the number of revolutions of the engine but displaying it as "hours" for convenience. At some high rpm e.g. 2500 the tach "time" is fairly accurate.
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Not all tach timers work that way. Some start counting real time when a threshold is exceeded (e.g. 1300rpm) and stop counting below that threshold.
 
There are also Hobbs activated by the gear (gear up to gear down, so close to actual flying time) and some by airspeed (counts time above say 40 knots). Most trainers run the Hobbs off electrical (there is a thread here on PoA about a guy that when solo, turned off the Master to pay for less flying time), or oil pressure.

Most people use the Hobbs time for logging, but technically, if you start the engine and sit with it running while messing with the avionics, that time does not count. You have to start taxiing. :D

And you cannot tell by looking at it. My Mooney has what looks like a Hobbs meter, but it actually is Tach time.
 
Pinecone said:
...but technically, if you start the engine and sit with it running while messing with the avionics, that time does not count.
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True. Per the regs, flight time is

...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.​

Of course, a brake test would get that started, as long as its purpose is making sure the plane is in condition for the pending flight, and you do indeed go fly after messing with the avionics for half an hour. ;)
 
Half Fast said:
True. Per the regs, flight time is

...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.​

Of course, a brake test would get that started, as long as its purpose is making sure the plane is in condition for the pending flight, and you do indeed go fly after messing with the avionics for half an hour. ;)
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Even without a brake check the airplane is moving. The propeller is moving through the air and the whole plane is making tiny movements called vibrations from its own power. Sure this is probably not in line with the intent of the regs, but as soon as the engine starts part of the plane is moving.
 
For some EFIS systems, like my Garmin G3X, in the setup you can user select what drives HOBBs Total Time between none, oil pressure (>5psi), flight time (airborne from GPS data and indicated airspeed), or RPM (>100). As an aside, my EFIS also records a separate Tach Time and that's driven solely by RPM (>100). You can also set a Standard RPM that records time 1:1. IOW at that standard RPM 1 hour of operation records as 1 hour on the tach. Faster than standard RPM records more time, slower RPM records less time.
 
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Improper usage can cause a lot of problems with Aircraft Records.

Is “TT” Total Time ( Time in Service) or Tach Time?

Bell nailed it with his comment that all mx should include the Time in Service

for Log Entries.

Specifically the AIRCRAFT TIS should be used.

The current Tach Reading being included makes tracking easier.

Engine or Prop Total Time or Time Since Overhaul may be included but not in place

of Aircraft Time in Service.

Due to the effect incorrect determination has regarding things like

Engine, Prop, Magneto O/H times and AD Compliance this is not to be taken

lightly. A mistake here could result in the aircraft being Un- Airworthy with the

Insurance Policy invalid.

The little bit of ink saved by not being specific can cause issues.
 
My Hobbs is set up to start counting at 80 knots, which is basically rotate speed, so I guess it’s marking TIS.

I was a little concerned that it was under-counting time, but after reading @Bell206 ’s post, I feel better that it’s probably a more accurate way to track time.
 
Somedudeintn said:
Even without a brake check the airplane is moving. The propeller is moving through the air and the whole plane is making tiny movements called vibrations from its own power. Sure this is probably not in line with the intent of the regs, but as soon as the engine starts part of the plane is moving.
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Queue the pedantry!
 
flyingbrit said:
Tach "time" is not really time, it is counting the number of revolutions of the engine but displaying it as "hours" for convenience. At some high rpm e.g. 2500 the tach "time" is fairly accurate. Especially during training, the average rpm is quite a bit lower than 2500 and so the tach under reports the time. Flight schools figured this out so they installed the Hobbs meters which generally indicate actual clock time whenever the engine is running. This way they can bill the student more! There is also a slight benefit to the student/pilot as they can log more hours than the tach would indicate.
Click to expand...
Excellent points flyingbrit, makes sense to me.

When I got my 172 the fuse was out that powered the clock and hobbs meter probably for 20 years to save the battery when it was not being flown.
When I replaced the fuse both started working again but both were intermittent. I guess from not being used?
I fly it a lot more than the last owner so I leave the fuse in.
When I got a new panel installed in 2018 I got a new hobbs meter and now I know at a glance how many hours I have on my panel.
I log TT and hobbs time every flight so I could tell if it is skipping. I replaced the oil pressure switch for preventative maintenance before the new hobbs so I was pretty sure the hobbs was intermittent.

And my original clock finally loosened up after about 3 years , it now works for months at a time until it gets behind. I always compare it to my I pad and it matches 99% of the time. I almost bought a new one.
 
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Half Fast said:
Some planes start the Hobbs meter when the electrical power is applied, others start it at engine start, so there’s some variation. But for maintenance purposes, go by tach time.
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I worked for a part 135 operator that flew missions for the forest service here in AK. The forest service would only pay for flight hours, so they configured the hobbs meter to start working at 50mph. I think there was a pressure switch on the wing strut that closed and started the hobbs
 
The military only counts take off to touchdown. But the times are logged by someone, no Hobbs.
 
W
Pinecone said:
The military only counts take off to touchdown. But the times are logged by someone, no Hobbs.
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With some folks I flew with there were several touchdowns and one approach!
 
Magman said:
W


With some folks I flew with there were several touchdowns and one approach!
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Lol my late friend said when he was doing his solo long X country many years ago, when he went inside to get a signature at one of his airports the manager said he was going to sign 3 times because that’s how many landings he had. lol
 
My Hobbs starts when there is power to the ship - either master or external. So when I’m messing around with avionics with external power my Hobbs adds a lot of time.I log by Hobbs so I have to be careful to recheck it prior to each flight.
 
I count time by tach hours for maintenance purposes. Not trying to build time.
 
Tach time generally averages around 80% of "real" or Hobbs time for a flight My plane has no Hobbs, so if I forget to wind or pay attention to the clock for pilot time, I take the tach time and divide it by 0.8.
 
That depends on your cruise RPM.

There is an RPM that Tach and Hobbs are the same time.
 
Maybe if years ago folks used “Recorder Reading” ( RR) it would not have been

confused with either of the TT designations.

Of course, then it’s which Recorder?
 
Calvin is the kid, Hobbes is the tiger.

Maybe I misunderstood the title.
 

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Half Fast said:
True. Per the regs, flight time is

...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.​

Of course, a brake test would get that started, as long as its purpose is making sure the plane is in condition for the pending flight, and you do indeed go fly after messing with the avionics for half an hour. ;)
Click to expand...

I would argue that the intent of the rule is to count the time when the pilot is in control of the power for the purpose of flight. So the pilot using the brakes, elevator, rudder etc to maintain control during engine start does count as pilot time in my opinion.
 
How "MOVES under its own power" not clear?

That said, most people log engine run time as flight time.
 
sarangan said:
I would argue that the intent of the rule is to count the time when the pilot is in control of the power for the purpose of flight. So the pilot using the brakes, elevator, rudder etc to maintain control during engine start does count as pilot time in my opinion.
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You log your way, and I’ll log mine.
 
Flight Time FYI: the key to this definition is in the qualifiers and the reason there is a different definition for Time-in-Service. "For the purpose of flight" means the taxi time it takes to get to the runway is included but the taxi time to the shop on the other ramp is not. "Aircraft comes to rest" means once the flight is completed and the pilot is no longer required to be on board. There are several LOIs that show flight time can still be logged even after the aircraft has landed and parked but still requires the pilot to remain onboard which further defines "to rest." Fun stuff to listen to in certain discussions.
 
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