GolfOscarDelta
Topic Author
Posts: 102
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Hobbs, Tach Time And Real Flying Hours

Sun Feb 21, 2010 8:37 pm

Question to GA pilots:

So what counts as real flying hours :- The stuff that you log in your logbooks?

From 14 CFR § 1.1

Quote:
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.


Until now i've been logging whatever the Hobbs meter showed me. However, I've noticed that on the C172's that i fly, the tach does not run when the engine is idling and that the Hobbs doesn't run when the RPM's are below 1000. Now when taxing the engine RPM's are below 1000 and so the Hobbs doesn't run but the Tach is running and thus counting airframe/engine hours.

I'm glad sometimes that i've to pay for the rental by the Hobbs since it doesn't count the startup checklist and taxi up and down time (sometimes taking up to 20+ min), but also that those hours will not count for my logs since the Hobbs doesn't log the time when engine is running lower than 1000 rpm


So how do you guys really count time? Hobbs? Tach? or Stop watch from start to end of flight?

Regarding the same topic i only found this:
Hobbs Vs. Tach? (by Shaun3000 Aug 6 2002 in Tech Ops)
discussing differences between tach time and hobbs time but no info on time you actually log.



[Edited 2010-02-21 12:39:21]
 
Goldenshield
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RE: Hobbs, Tach Time And Real Flying Hours

Sun Feb 21, 2010 9:13 pm

All the planes I've flown had it rigged so that the Hobbs would turn as long as the master switch is on. What kind of plane is this you are flying?
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Mir
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RE: Hobbs, Tach Time And Real Flying Hours

Sun Feb 21, 2010 9:21 pm

The Hobbs should be running whenever the engine is running, so I might look into that (I recall hearing that it works off of oil pressure, so the fact that it's not ticking could be a sign of trouble).

But regardless, there is no requirement to use the Hobbs or tach time as what you put in your log. You could start a stopwatch when you first move under your own power and stop it when you come to rest after the flight, and then log whatever time it shows you. You could also write down the time you start the engine, and the time you shut it down, and evaluate how much time elapsed between those two, which is what would go in your logbook.

-Mir
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GolfOscarDelta
Topic Author
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RE: Hobbs, Tach Time And Real Flying Hours

Sun Feb 21, 2010 9:35 pm

Quoting Goldenshield (Reply 1):
rigged so that the Hobbs would turn as long as the master switch is on
Quoting Goldenshield (Reply 1):
What kind of plane is this you are flying?

Yep thats what i've been told too, that the Hobbs is rigged to the master, and actually was surprised that my observation was different, so i checked the POH and it shows the Hobbs rigged via an Oil Pressure switch. So i'm assuming that the Hobbs doesn't run until some specific oil pressure is attained, which in the C172P that i fly is i think about 1000 Rpm.
 
bassbonebobo
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RE: Hobbs, Tach Time And Real Flying Hours

Sun Feb 21, 2010 10:20 pm

The Pipers that I normally fly have 3 different meters running. There is the tach which is pretty self explanatory. Then there is a Hobbs meter that runs whenever the oil psi reaches a certain threshold. I don't remember the exact number but it is fairly low. Lastly there is a maintenance Hobbs. This is connected to a switch in the pitot/static system and runs any time the airspeed is greater than 30kias. We use the tach for engine times and maintenance. Rentals are billed based on the Hobbs and that is also the time that we log. The maintenance Hobbs is used for 50 and 100 hour inspections.
Rule #176. Any device that can crawl across the table on medium, does not need to be brought into the office.
 
Fly2HMO
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RE: Hobbs, Tach Time And Real Flying Hours

Sun Feb 21, 2010 10:55 pm

Quoting GolfOscarDelta (Thread starter):
However, I've noticed that on the C172's that i fly, the tach does not run when the engine is idling and that the Hobbs doesn't run when the RPM's are below 1000.

If the prop turns, the tach moves. The tach meter is a function of RPM not time. So it will tick slower at idle than when at full power and vice versa.

Quoting GolfOscarDelta (Reply 3):
Yep thats what i've been told too, that the Hobbs is rigged to the master, and actually was surprised that my observation was different,

Not quite. The hobbs meter is always on, like the clock, unless of course the engine is off.

Quoting Mir (Reply 2):
(I recall hearing that it works off of oil pressure, so the fact that it's not ticking could be a sign of trouble).

IIRC for the C172S the limit is 20PSI.

And at my school we just logged whatever the hobbs showed.

[Edited 2010-02-21 14:57:13]
 
dw747400
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RE: Hobbs, Tach Time And Real Flying Hours

Mon Feb 22, 2010 12:59 am

Quoting bassbonebobo (Reply 4):
The Pipers that I normally fly have 3 different meters running.

What type of Piper is that? The ones I fly most definitely use the traditional Hobbs/Tach arrangement. They are relatively new, but they don't have glass/digital air data systems.
CFI--Certfied Freakin Idiot
 
nws2002
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RE: Hobbs, Tach Time And Real Flying Hours

Mon Feb 22, 2010 2:01 am

You should log the start time when the airplane begins moving under its own power, basically whenever you begin taxiing away from the FBO. Log the end time when the aircraft stops moving under its own power, basically whenever you apply the parking brakes at the FBO at the end of the flight. Write them down and figure out how much time it is.

That's the correct way to log flight time, at least in the United States. Flight time does not usually equal hobbs time or tach time. Hobbs time and tach time are generally used for aircraft maintenance and rental payments.

[Edited 2010-02-21 18:03:20]
 
ThirtyEcho
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RE: Hobbs, Tach Time And Real Flying Hours

Mon Feb 22, 2010 2:18 am

It has been common knowledge for a long time that tach time reads lower than Hobbs time if you cruise a bit lower than the POH numbers. This especially true with the C172. I was first tipped to this about 50 years ago; this is not really anything new and it was even better before most airplanes had the Hobbs meter.
 
Fly2HMO
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RE: Hobbs, Tach Time And Real Flying Hours

Mon Feb 22, 2010 6:36 pm

Quoting ThirtyEcho (Reply 8):
It has been common knowledge for a long time that tach time reads lower than Hobbs time if you cruise a bit lower than the POH numbers.

It will read lower all of the time regardless of power settings, unless you have the plane sitting on the ramp for hours on end at idle. All the tach meter is actually doing is counting the number of RPMs. As a rule of thumb Tach time is usually 3/4 of what you get in your hobbs times. I dispatched thousands of flights while working at my school and not once was tach time greater than hobbs (aside from when the hobbs was broken) the most you got was nearly equal hobbs and tach times on very short flights, and that only happened once about every 100 flights.

But 99.99% of the time for a 1hr flight the tach was .7, for 2hrs 1.7, 3hrs 1.7 etc. It was really predictable. It was in the shorter flights where it was harder to guess the tach times.
 
KELPkid
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RE: Hobbs, Tach Time And Real Flying Hours

Mon Feb 22, 2010 8:08 pm

Quoting Goldenshield (Reply 1):
All the planes I've flown had it rigged so that the Hobbs would turn as long as the master switch is on. What kind of plane is this you are flying?

I've flown quite a few that are rigged so the Hobbs turns no matter what 

Seriously, a buddy of mine and I tried, one day, in class G airspace, a long ways away from any nearby controlled airspace, in the rather expensive to rent Guttless Cutlass, while practicing Commercial Pilot checkride maneuvers, shutting the master off. The Hobbs kept turning, and our scheme to eke out a few minutes of aircraft rental for free failed    I later talked to a mechanic buddy after that, and he said that it is usually activated by an oil pressure switch which is hard wired to the battery...  
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bassbonebobo
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Joined: Sun Apr 06, 2008 3:09 am

RE: Hobbs, Tach Time And Real Flying Hours

Mon Feb 22, 2010 9:36 pm

Quoting dw747400 (Reply 6):
What type of Piper is that? The ones I fly most definitely use the traditional Hobbs/Tach arrangement. They are relatively new, but they don't have glass/digital air data systems.

They are 2003/2004 vintage Warriors and Arrows. None have any of that fancy glass crap. Now I'm doing a lot of flying in a good old Cutlass. It's fast, it's cheap, and it's always available (no one else wants to fly it because it is 28 years "old"). That plane just has a tach and maintenance hobbs.
Rule #176. Any device that can crawl across the table on medium, does not need to be brought into the office.
 
Fly2HMO
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RE: Hobbs, Tach Time And Real Flying Hours

Mon Feb 22, 2010 10:11 pm

Quoting KELPkid (Reply 10):
The Hobbs kept turning, and our scheme to eke out a few minutes of aircraft rental for free failed I later talked to a mechanic buddy after that, and he said that it is usually activated by an oil pressure switch which is hard wired to the battery...

Shame on you... you failed the task "Operation of Systems" in the FAA Practical Test Standards.  
 
dw747400
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RE: Hobbs, Tach Time And Real Flying Hours

Tue Feb 23, 2010 3:49 am

Quoting bassbonebobo (Reply 11):
They are 2003/2004 vintage Warriors and Arrows.

That is really interesting, as the 03 Arrow I fly only has standard hobbs and tach. An option aimed at flight schools trying to pinch pennies perhaps?
CFI--Certfied Freakin Idiot
 
KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

RE: Hobbs, Tach Time And Real Flying Hours

Tue Feb 23, 2010 5:46 am

Quoting FLY2HMO (Reply 12):
Shame on you... you failed the task "Operation of Systems" in the FAA Practical Test Standards.

Heh, I'd bet many FAA check airmen would be hard pressed to know how the Hobbs meter is actually hooked up   
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
Fly2HMO
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RE: Hobbs, Tach Time And Real Flying Hours

Tue Feb 23, 2010 6:17 am

Quoting KELPkid (Reply 14):
Heh, I'd bet many FAA check airmen would be hard pressed to know how the Hobbs meter is actually hooked up

HA! Good point. touche sir.

You know you're having a good check ride when you teach the examiner things 
 
GolfOscarDelta
Topic Author
Posts: 102
Joined: Fri Feb 01, 2008 5:17 pm

RE: Hobbs, Tach Time And Real Flying Hours

Tue Feb 23, 2010 2:45 pm

Quoting FLY2HMO (Reply 5):
So it will tick slower at idle than when at full power and vice versa.

So does that mean that when an aircraft goes in for a 100 hour, it has actually run for more than 100 hours (or may be less than 100 depending on what avg RPMs the engine was running in the 100 "hour" interval).

Quoting FLY2HMO (Reply 9):
All the tach meter is actually doing is counting the number of RPMs

Ok i need to understand this better, from the definition of tach i know that Tachs technically only measure the number of RPM's. So in say a 172 how does the tach reading actually show the time on the engine does it show "time = total revolutions/avg. revolutions per hour"? Because i don't really see any other way that an RPM/ No of revolutions can be turned into a time unit of hours without dividing it by the avg revs/hour. If the answer to the above question is yes, then how is the average Revolutions/hour of the engine determined? (i.e. at what power setting is the engine running at "avg. revs/hour"?)

Thanks guys for the replies.
 
Fly2HMO
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RE: Hobbs, Tach Time And Real Flying Hours

Tue Feb 23, 2010 4:23 pm

Quoting GolfOscarDelta (Reply 16):
So does that mean that when an aircraft goes in for a 100 hour, it has actually run for more than 100 hours (or may be less than 100 depending on what avg RPMs the engine was running in the 100 "hour" interval).

No. Remember the tach meter is not measuring time, it is only counting revolutions. The tach time is only relevant for MX personnel because of that. But if the engine had been on for 1hr, then the hobbs meter will show 1hr. The tach will probably show .7 or so.

Quoting GolfOscarDelta (Reply 16):
So in say a 172 how does the tach reading actually show the time on the engine does it show "time = total revolutions/avg. revolutions per hour"?

It doesn't. The tach meter is in essence no different to the clicker F/As use to count pax.
 
GolfOscarDelta
Topic Author
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RE: Hobbs, Tach Time And Real Flying Hours

Tue Feb 23, 2010 7:09 pm

Quoting FLY2HMO (Reply 17):
Quoting GolfOscarDelta (Reply 16):So in say a 172 how does the tach reading actually show the time on the engine does it show "time = total revolutions/avg. revolutions per hour"?

It doesn't.


Sorry but after i asked that question here i did some more searching and the internets said that it does (but i'd rather defer to the experts here than the murky info online). Apparently from wikipedia (again not that great of a source with no citations) the tach time is indeed = total revolutions/avg. revolutions per hour with the avg. revolutions/hour being 2400 x 60
 
Fly2HMO
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RE: Hobbs, Tach Time And Real Flying Hours

Tue Feb 23, 2010 9:12 pm

Quoting GolfOscarDelta (Reply 18):
Sorry but after i asked that question here i did some more searching and the internets said that it does

Did you skip the first sentence in the wiki article?  
Quote:
The Tach(ometer) Timer is an instrument used in aviation to accumulate the total number of revolutions performed by the engine

So, it doesn't.

It uses time to calculate it, but it is not showing you the engines time. Otherwise tach time would always equal hobbs time.

And I don't know for sure but I think the actual reference RPMs mentioned in the wikipedia article are off. I've flown a few several hour long flights at full bore and max RPM and the tach never read more than the hobbs. Which leads me to believe the reference RPM is probably engine red line, 2700rpm in the planes I flew.
 
GolfOscarDelta
Topic Author
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RE: Hobbs, Tach Time And Real Flying Hours

Wed Feb 24, 2010 1:31 am

Quoting FLY2HMO (Reply 19):
I think the actual reference RPMs mentioned in the wikipedia article are off
Quoting FLY2HMO (Reply 19):
2700rpm in the planes I flew

Yeah i agree with you its probably 2700 rpm for the reference. Otherwise the tach would indeed be greater than the hobbs.

Quoting FLY2HMO (Reply 19):
It uses time to calculate it, but it is not showing you the engines time. Otherwise tach time would always equal hobbs time.

Ok the way i understand it: say for example that you are running the engine at 800 rpm during taxi and you taxi for 15 min (lets just assume that there is no run up etc for now), and then you take off and climb at full power (2700 rpm for 10 min) and then cruise at 2400 rpm for 20 min and then you descend, and land at 1500 rpm for 10 min and then taxi back at 1000 rpm for 5 min. Now thats a total of 1 hour and

800 x 15 + 2700 x 10 + 2400 x 20 + 1500 x 10 + 1000 x 5 = 10700 revolutions of the engine prop shaft

now assuming reference RPM is 2700 rpm: 10700/(2700x60) = 0.66049 hours

Thanks Fly2HMO for your patient answers, but one last time. So the display on the tach is

a. Total number of revolutions run? i.e. 10700 / (some convenient factor 1000 or 10000 or so)

or is it

b. the conversion to hours i.e. 0.66 hours? (while the Hobbs shows 1 hour) (and thereby the engine time is 0.66 hours since the engine over 1 hour(of time) has run only 0.66 times of what it would have run, had it run at 2700 rpm for 1 hr)

The only reason i ask is that then why don't the manufacturers put in revs as the units instead of hours?
 
DiamondFlyer
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RE: Hobbs, Tach Time And Real Flying Hours

Wed Feb 24, 2010 3:06 am

Quoting FLY2HMO (Reply 19):

And I don't know for sure but I think the actual reference RPMs mentioned in the wikipedia article are off. I've flown a few several hour long flights at full bore and max RPM and the tach never read more than the hobbs. Which leads me to believe the reference RPM is probably engine red line, 2700rpm in the planes I flew.

Its different for every airplane. Easiest way to check what that specified RPM is, is probably the Type Certification.

-DiamondFlyer
From my cold, dead hands
 
Fly2HMO
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RE: Hobbs, Tach Time And Real Flying Hours

Wed Feb 24, 2010 5:45 am

Quoting GolfOscarDelta (Reply 20):
the conversion to hours i.e. 0.66 hours? (while the Hobbs shows 1 hour) (and thereby the engine time is 0.66 hours since the engine over 1 hour(of time) has run only 0.66 times of what it would have run, had it run at 2700 rpm for 1 hr)

Sounds about right. That would show up as a .7 in the tach and is exactly what I would expect to see after logging a 1.0 on the hobbs.

Quoting GolfOscarDelta (Reply 20):
The only reason i ask is that then why don't the manufacturers put in revs as the units instead of hours?

Just reversing the formula you used to calculate it would give you the actual revs, but MX guys don't really care about that. Besides, you can use smaller digits this way. No need to abbreviate millions of RPMs.

Quoting DiamondFlyer (Reply 21):
Easiest way to check what that specified RPM is, is probably the Type Certification.

Can't find anything on the online type certificate for the C172. If anything the tach manufacturer might know, but OscarDelta's formula seems correct.
 
CosmicCruiser
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RE: Hobbs, Tach Time And Real Flying Hours

Wed Feb 24, 2010 1:53 pm

Gee, I would have never guessed it was such a complicated question. WAAAY back when I was doin my CFI thing the Hobbs meter was used IF it had one or the Tach but really that was just what you paid for. If it were me I'd just look at my watch! I guess you could get really serious like the way the co. does it and separate block time from flight time. I mean really once you start up and you're sitting there doin a chklist you're eating up time that you're paying for but it isn't block time nor flight time is it?

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