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Total time....Hobbs or Tach?

MacCool

Well Known Member
Here's a bonehead question. When stating total time on airframe/engine, does one quote Hobbs, or Tach? For me, the difference is considerable.

Hobbs.....442.22
Tach.......364.22
 
Hobbs is 100%. Tach time is a percentage based on rpm.

Most all rentals are based on hobbs time.
 
Yup..this I know.

If you were selling your airplane, to the prospective buyer would you quote Hobbs, or Tach?

To put it another way, if you were buying an airplane, would you be more interested in Hobbs time, or Tach time?
 
Here's a bonehead question. When stating total time on airframe/engine, does one quote Hobbs, or Tach?

I’ve always assumed tach time is the more accurate indicator of useful life. Not sure what the “official answer” is, or if there is one. I’d use tach.
 
Here's a bonehead question. When stating total time on airframe/engine, does one quote Hobbs, or Tach? For me, the difference is considerable.

Hobbs.....442.22
Tach.......364.22

According to the FAA it is up to the owner to determine and record aircraft time. You could use a stop watch for that matter, but the normal for non commercial use is tach time.
george
 
Times

Hobbs time for the airframe. For the engine and prop, tach time and time remaining to overhaul. Also interested in calendar age of the airframe and engine to calculate average hours flown per year.

I'm accustomed to Lycoming maintenance e.g. oil change and 100 hourly per tach time. But Rotax appears to be different and needs to be serviced based on Hobbs time.
 
According to the FAA it is up to the owner to determine and record aircraft time. You could use a stop watch for that matter, but the normal for non commercial use is tach time.
george

Tach time for maintenance and use records. Hobbs time for your log book and rental use.
 
Someone will correct me if I'm wrong, but here's how I remember it:

Time In Service (TIS) for maintenance that is related to hours, (i.e. a 100-hour inspection) is calculated or recorded as time from takeoff to landing. That includes engine hours TIS.

Logging pilot time, or Flight Time, is from start of taxi, to end of taxi after landing. Or near enough, engine start to engine shutdown.

Tach time approximates TIS. Hobbs time approximates Flight Time.

Most GPS's will give you a time based on liftoff and landing. That should be close to tach time.

Or, you can look at your watch 4 times during the flight......... Start, Roll, End of rollout, and stopping before shutdown.

Nothing is ever easy! :rolleyes:

So, when advertising a plane for sale, the buyer should be interested in the Time In Service (Tach), rather than the pilot flight time.
 
Or, you can look at your watch 4 times during the flight......... Start, Roll, End of rollout, and stopping before shutdown.

ACARS used to (and maybe still does) call this OOOI...Out, Off, On, In. Out of the gate, off the ground, on the ground, in the gate.

I use the most conservative approach for maintenance items, that is, Hobbs time. I figure the engine is turning, plugs are firing, etc. So I change oil a little bit more often, or clean/rotate the plugs a little sooner, etc. In the grand scheme, it's just easier to keep one set of numbers for everything, and I treat my engine a little nicer. :)
 
I like to be simple. Hobbs time for everything. Although some may think it is conservative for maintenance, I don’t, as if engine is turning things are wearing out in the engine. As for pilot time, Hobbs is also correct as butt in seat with propeller turning is pilot time as you are operating the plane.

A bit of drift
In buying I would want to know cycles as well for both engine and airframe. 500 hours with 1000 engine starts and take-offs and landings is much different than 500 hours with 250 engine starts and take-offs and landings. I don’t know if anybody really ever advertises cycles for GA aircraft. I don’t track mine, per say, but I keep a flight log of each flight with fuel stops, oil adds, and type of flying (altitude and general activity including number of TOs & Landings) that I can go through that flight log and count cycles if asked.
 
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For the purpose of selling TIS would/should be quoted, time from engine start to engine stop, tach time is for service intervals.
 
I like to be simple. Hobbs time for everything. Although some may think it is conservative for maintenance, I don’t, as if engine is turning things are wearing out in the engine. As for pilot time, Hobbs is also correct as butt in seat with propeller turning is pilot time as you are operating the plane.

A bit of drift
In buying I would want to know cycles as well for both engine and airframe. 500 hours with 1000 engine starts and take-offs and landings is much different than 500 hours with 250 engine starts and take-offs and landings. I don’t know if anybody really ever advertises cycles for GA aircraft. I don’t track mine, per say, but I keep a flight log of each flight with fuel stops, oil adds, and type of flying (altitude and general activity including number of TOs & Landings) that I can go through that flight log and count cycles if asked.

Are you also keeping tract of the calendar for your engine? Your engine may already be past the recommended time of the OEM for overhaul regardless of hours on it.
george
 
Hobbs or tach?

With neither, I log lift off to touch down... (Air time)

For my personal log book, I add point 2 (.2) (Flight time)

J
 
I record Hobbs and Air time (wheels up to wheels down) in my Journey log. Hobbs is also used for my pilot log. My Engine log gets Tach time, Airframe and Prop logs get Hobbs. Easy to do with the Skyview set to display all three times each flight.
 
So just to play devil's advocate...I did a quick search in both the engine and prop manuals, and nowhere could I find any reference to anything called "tach time" for maintenance intervals (might be there and I missed it). What they both specify are "hours"...e.g., "Accomplish a detailed inspection at 100 hour intervals not to exceed twelve (12) calendar months..." (Hartzell prop manual).

If I were to take a lawyerly approach to this, I'd say that means hours of operation, not hours of operation scaled by some arbitrary factor (arbitrary in that everyone can pick their own "preferred" RPM for computing tach time).

I don't think it makes a hill of beans worth of difference if you're taking good care of your engine and prop in general, but we have an awful lot of discussions here that usually involve some aspect of "what does it say in the manufacturer's manual?". Well, Lycoming and Hartzell don't say a thing about computing something called "tach time" and using that for maintenance intervals. They just specify "hours", which to me means the common English use of the term...the number of hours part X has been operated. Period.

As Mike Myers used to say on SNL in that sketch..."Discuss!" :)
 
So just to play devil's advocate...I did a quick search in both the engine and prop manuals, and nowhere could I find any reference to anything called "tach time" for maintenance intervals (might be there and I missed it). What they both specify are "hours"...e.g., "Accomplish a detailed inspection at 100 hour intervals not to exceed twelve (12) calendar months..." (Hartzell prop manual).

FAA Part 91.205 says that all general aviation aircraft must have an operational airspeed indicator, altimeter, fuel gauge and a magnetic direction indicator. Each engine on the aircraft must also have a corresponding oil pressure gauge, temperature gauge and tachometer as well as other basic equipment for the safe operation of the aircraft.

So, while a hobbs meter is not required, a tachometer is. One could quite easily interpret this to mean.. If the FAA wanted us to track hours on parts, they'd require a hobbs meter if that's how they wanted us to measure hours. Instead, they've chosen to use a tachometer to measure time on said parts..
 
FAA Part 91.205 says that all general aviation aircraft must have an operational airspeed indicator, altimeter, fuel gauge and a magnetic direction indicator. Each engine on the aircraft must also have a corresponding oil pressure gauge, temperature gauge and tachometer as well as other basic equipment for the safe operation of the aircraft.

So, while a hobbs meter is not required, a tachometer is. One could quite easily interpret this to mean.. If the FAA wanted us to track hours on parts, they'd require a hobbs meter if that's how they wanted us to measure hours. Instead, they've chosen to use a tachometer to measure time on said parts..

But how does the *manufacturer* interpret the word "hours"? ETA: because the FARs also state, in Part 43:

Each person performing maintenance, alteration, or preventive maintenance on an aircraft, engine, propeller, or appliance shall use the methods, techniques, and practices prescribed in the current manufacturer's maintenance manual or Instructions for Continued Airworthiness prepared by its manufacturer
 
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Amateur lawyer

FAA Part 91.205 says that all general aviation aircraft must have an operational airspeed indicator, altimeter, fuel gauge and a magnetic direction indicator. Each engine on the aircraft must also have a corresponding oil pressure gauge, temperature gauge and tachometer as well as other basic equipment for the safe operation of the aircraft.

So, while a hobbs meter is not required, a tachometer is. One could quite easily interpret this to mean.. If the FAA wanted us to track hours on parts, they'd require a hobbs meter if that's how they wanted us to measure hours. Instead, they've chosen to use a tachometer to measure time on said parts..

1. FAR 91.205 does not say ‘all’ aircraft. EAB are excluded.
2. A tachometer measures RPM. Some, but not all, may have a ‘tach-hour’ function.
 
1. FAR 91.205 does not say ‘all’ aircraft. EAB are excluded.
2. A tachometer measures RPM. Some, but not all, may have a ‘tach-hour’ function.

You're right that 91.205 excludes EAB aircraft, but your ops limits probably reference 91.205 anyhow.

Regardless, this discussion isn't specifically limited to EAB... As a matter of fact, one could actually argue that TBO etc applies even less to EAB, since the engine itself may be experimental :D
 
You're right that 91.205 excludes EAB aircraft, but your ops limits probably reference 91.205 anyhow.

Regardless, this discussion isn't specifically limited to EAB... As a matter of fact, one could actually argue that TBO etc applies even less to EAB, since the engine itself may be experimental :D

Your Ops Lims will probably only reference 91.205 when talking about night or IFR flight - not Day VFR, for which nothing in 91.205 is required.
 
Your Ops Lims will probably only reference 91.205 when talking about night or IFR flight - not Day VFR, for which nothing in 91.205 is required.

Day VFR flight operations are authorized provided aircraft instruments and equipment installed under § 91.205 are inspected and maintained in accordance with the requirements of Part 91.

I am not sure whether that means that all of the equipment required by 91.205 is required, or if it means that only the equipment installed that is required by 91.205 are inspected and maintained in accordance with the requirements of Part 91.
 
Mechanical tachometer reads low at low RPM... So it is really a close approximation of flight time. Since most maintenance is around engine this makes sense.

Hobbs records actual clock time whenever there is oil pressure even at idle in the tie down.

Tach is all that was used in the OLD days... HOBBS were found on rental aircraft for charging you rental fee.

For air-frame and maintenance Tach is OK and no one should argue... but it is not actual hours engine is running. However who has mechanical Tachs now?

Most Tach's in our RV's are part of EFIS, basically a Hobbs meters reading clock time from start to shutdown.

For logging civilian pilot time it is BLOCK time... Block Out to Block In... although practically speaking it is engine start to engine shutdown.

The method of using take off to touch down and adding 0.2 for pilot time is a military thing, not an approved civilian thing. If you tried to explain that getting a flying job or to FAA it might raise an eyebrow... It is expected of military pilots. Most of my taxi times per flight are less than 0.2 minutes total.

Tracking Takeoff and Landing times is a pain in my private plane... I have other things to do at that point... I have an air switch on a chronograph that measures air time (if I remember to reset it).

Up to you. Be consistent and accurate...
 
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So just to play devil's advocate...I did a quick search in both the engine and prop manuals, and nowhere could I find any reference to anything called "tach time" for maintenance intervals (might be there and I missed it). What they both specify are "hours"...e.g., "Accomplish a detailed inspection at 100 hour intervals not to exceed twelve (12) calendar months..." (Hartzell prop manual).

If I were to take a lawyerly approach to this, I'd say that means hours of operation, not hours of operation scaled by some arbitrary factor (arbitrary in that everyone can pick their own "preferred" RPM for computing tach time).

I don't think it makes a hill of beans worth of difference if you're taking good care of your engine and prop in general, but we have an awful lot of discussions here that usually involve some aspect of "what does it say in the manufacturer's manual?". Well, Lycoming and Hartzell don't say a thing about computing something called "tach time" and using that for maintenance intervals. They just specify "hours", which to me means the common English use of the term...the number of hours part X has been operated. Period.

As Mike Myers used to say on SNL in that sketch..."Discuss!" :)

In Canada the only requirement in our CAR's for time recording that I have found is Air Time. This is specified in SCHEDULE I
(Paragraph 604.127(i), subsection 605.94(1) and Item 3 of Schedule II) for both the Journey and Tech logs. No mention of Hobbs or Tach times.

I failed to mention in my previous post that while I record Hobbs, Air and Tach times in my logs I do all my maintenance based on Hobbs time. Never liked the idea of using numbers that didn't reflect the total time the engine was turning or the plane moving.

Hopefully the lawyers don't go after me for doing my mtc before it is due.:rolleyes:
 
It's your airplane so do what you want.

FWIW; I will continue to use industry standard methods (tach for aircraft/hobbs for aircrew) which has been proven over many millions of "hours" and many decades of use. YMMV

 
Did early aircraft use Hobbs?

Earliest Aircraft were built without electrical systems. So I was wondering what was used for military record keeping before Hobbs Meters were invented then I learned not all Hobbs meters are activated the same way. Then I remembered a Flight School aircraft Hobbs that would run with just the Master switch turned on even while pre-flighting. then I read this: https://en.wikipedia.org/wiki/Hobbs_meter
 
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