tach vs. hobbs

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Matt C

Pre-takeoff checklist
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Matt C
I did my first oil change on my plane (172G) Saturday. I went over my logs and total times from the tach and hobbs since last oil change. The tach indicates 24 hours and the hobbs indicates 33 hours. I'm sure that I didn't spend 9 hours sitting still with the master on and the engine not running. Does this seem normal to you? Does the tach not register until a certain RPM threshold? Side note, I got a lesson in safety wire, I like that....
 
the tach runs slower at lower RPMs.....something around 2,200 gives you 100% time. Anything less is less. So, it's very possible if this plane spent lots of time in the pattern or slow flighting.....or whatever else at reduced power it could see that.
 
FastEddieB said:
I thought 1:1 on the tach was normally at cruise power, more like 2,400 rpm.

But I will easily yield to someone who actually knows!
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ya....I wouldn't actually know. lol :D

it really depends....cause they can be slightly different. THe hobbs can be wired differently too....either off the oil pressure (switching on at sensing oil pressure) or using the mag switch....or some other mechanism (squat switch), which will yield slightly different results.

IMHO all are close enough.....this isn't ain't an exact science.
 
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1:1 is normally at 100% power

Hence tach is going to be lower than Hobbs.
 
Matt C said:
I did my first oil change on my plane (172G) Saturday. I went over my logs and total times from the tach and hobbs since last oil change. The tach indicates 24 hours and the hobbs indicates 33 hours. I'm sure that I didn't spend 9 hours sitting still with the master on and the engine not running. Does this seem normal to you? Does the tach not register until a certain RPM threshold? Side note, I got a lesson in safety wire, I like that....
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That's about what I see when my flying is mostly local/pattern work. When I'm mostly cross country the Hobbs and tach are much closer to agreeing with each other.
 
I'm convinced that rental planes have super fast moving Hobbs meters. Seems like it clicks 3 times every time u blink.
 
1.375 sounds ok to me. The club plane I fly usually is around 1.3. I take off from a class C airport so I taxi a lot at times, usually for free.
 
The tach time is number of hours of running at a certain, specific reference speed of rotation. If the reference speed of rotation is 2400 RPM then the timer runs in real time when the engine is running at 2400 RPM, half speed while the engine is run at 1200 RPM or at 5/6ths real time at 2000 RPM.

The displayed number is incremented by one if the engine is run at its reference speed for one hour. If the reference rotation speed is 2400 RPM then the tach timer records: total revolutions / ( 2400 ∗ 60 ).
 
citizen5000 said:
The tach time is number of hours of running at a certain, specific reference speed of rotation. If the reference speed of rotation is 2400 RPM then the timer runs in real time when the engine is running at 2400 RPM, half speed while the engine is run at 1200 RPM or at 5/6ths real time at 2000 RPM.

The displayed number is incremented by one if the engine is run at its reference speed for one hour. If the reference rotation speed is 2400 RPM then the tach timer records: total revolutions / ( 2400 ∗ 60 ).
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Does your Cirrus have a meter that measures tach time?

Mine just had a Hobbs. I know some Cirrus owners installed a second meter controlled by an air switch, to cut down on maintenance intervals. Did you have one installed?
 
Interesting discussion, so do ivguys change oil per Hobbs or tach? Last time mechanic changed oil he put tach number in the log book, I didn't note the Hobbs, but may be should from next oil change . I always thought Hobbs was only for billing and kept wondering why the heck does my plane have Hobbs when it was never a rental...

Sent from my SM-G935V using Tapatalk
 
WannFly said:
Interesting discussion, so do ivguys change oil per Hobbs or tach?
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As mentioned above, lots of modern planes forego any sort of tach time meter, and only have a Hobbs. Some of those are controlled by an oil pressure switch, others by some combination of master/alternator switch positions, and some just run whenever the master is on.
 
What gets me is some pilots COMPLAIN when they have to pay by tach time (because the cost per hour of tach time is more) and wont rent the plane because "it doesnt have a Hobbs".
 
Time in service (which is what the FAA bases maintenance times on) is defined by the time the aircraft is in the air. My plane has a hobbs on the gear switch which is pretty darn close. The FAA doesn't really care if you use a recording tach or a hobbs (connected to whatever) as long as you're consistent and it's reasonably reliable.
 
Years ago, an airplane that I rented, had it's Hobbs quit. So we just used tach time untill it was replaced. I found that I could get a "bigger bang for my buck" by throttling back a bit. Too bad they fixed the Hobbs in a couple weeks.
And for mx, Tach time is what is usually used.
 
On both planes that I fly and at my doing nothing but flying around cruise setting, Hobbs = 1.2 x Tach.
 
tinerj said:
On both planes that I fly and at my doing nothing but flying around cruise setting, Hobbs = 1.2 x Tach.
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This would yield an incorrect Hobbs count. Hobbs is supposed to be the amount of time the aircraft is in use. They are activied via oil pressure or when the electricial system is activated or engine alternators are on (as is the case with Cirrus) or an airpressure sensor attached to the wing or landing gear. If a Hobbs were to be tied to a ratio of Tach time then Hobbs would record fractional engine RPM and not be recording the total time the aircraft is in use.
 
btw...I don't believe either the hobbs or the tach (not the tachometer -RPMs) are required equipment.
 
citizen5000 said:
This would yield an incorrect Hobbs count.
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Accurate or not, 1.2 X Tach is the general rule for logging pilot time where no Hobbs is available.

For the purpose of this thread - I think Tach is more important for maintenance.
 
MauleSkinner said:
The specific rule for logging pilot time is in Part 61...where is this "general rule" published?
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I think he meant "common practice" rather than "regulation."

Still, 1.2 isn't universally used either. We used to use 1.4 when I ran the flying club out in Colorado.
 
Checkout_my_Six said:
docs are not a requirement....limitations are.
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Pedantic pain. I used docs to include the requirements
on the type certificate plus whatever operating limitations (AFM, placards, limitations document) applies to the aircraft in question.
 
flyingron said:
Pedantic pain. I used docs to include the requirements
on the type certificate plus whatever operating limitations (AFM, placards, limitations document) applies to the aircraft in question.
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section two....to be more specific. :D
 
On my last training flight with lots of takeoff's and taxi-backs we started the tablet (GP) stopwatch right after engine start and stopped it right before I turned off the engine. The time was 1h 39min. The tach read exactly 1.0hrs. That's about 1.67:1. However, this difference is not very typical scenario.
 
flyingron said:
I think he meant "common practice" rather than "regulation."

Still, 1.2 isn't universally used either. We used to use 1.4 when I ran the flying club out in Colorado.
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I would say that the " common practice" could easily be considered in violation of the regulation, but it's not the only common practice that is, by any stretch. ;)
 
Thanks for all of the replies - very informative.

Opinion - regarding PIC time, it seems like it would more likely be tied to "Hobbs" time as one is technically in charge of the airplane as soon as the master goes on (perhaps even before)....
 
Matt C said:
Opinion - regarding PIC time, it seems like it would more likely be tied to "Hobbs" time as one is technically in charge of the airplane as soon as the master goes on (perhaps even before)....
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It can be, but it depends on how the Hobbs is activated and how you operate the airplane.

If it is an electrical Hobbs and you turn on the master while doing your preflight or leave the master on without the engine running, it is going to yield an inaccurate number from a logging pilot time aspect.

Same if you start the engine and sit there in thie down spot while warning up or messing with radios.

When I was renting, I used Hobbs time for logging out of convenience, but nowadays, I just use my watch.
 
Matt C said:
Thanks for all of the replies - very informative.

Opinion - regarding PIC time, it seems like it would more likely be tied to "Hobbs" time as one is technically in charge of the airplane as soon as the master goes on (perhaps even before)....
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"Being in charge of the plane" has squat to do with logging PIC time.

You log PIC time for the flight time that you were the sole manipulator of the controls of an aircraft for which you are rated. Flight time is the time from which the aircraft first moves under its own power for the purpose of flight until it comes to rest at the destination. Do not insert other imagined requirements into the regs.
 
Matt C said:
Opinion - regarding PIC time, it seems like it would more likely be tied to "Hobbs" time as one is technically in charge of the airplane as soon as the master goes on (perhaps even before)....
Click to expand...
What Ron said...

Don't confuse "logging" with "acting".
 
tinerj said:
On both planes that I fly and at my doing nothing but flying around cruise setting, Hobbs = 1.2 x Tach
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I'm merely pointing that when I finish a flight and divide metered Hobbs time by tach time the answer averaged 1.2.

One of the planes I fly has no Hobbs meter. Instead I note the time of engine start and engine shutdown after the flight and use that as Hobbs. That method, too, averages about Hobbs = 1.2 x Tach.

I would not expect that ratio to be the same for an hour of real slow flight or firewall forward high speed flight.
 
citizen5000 said:
This would yield an incorrect Hobbs count. Hobbs is supposed to be the amount of time the aircraft is in use. They are activied via oil pressure or when the electricial system is activated or engine alternators are on (as is the case with Cirrus) or an airpressure sensor attached to the wing or landing gear. If a Hobbs were to be tied to a ratio of Tach time then Hobbs would record fractional engine RPM and not be recording the total time the aircraft is in use.
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You totally misunderstood what he wrote.
 
tinerj said:
On both planes that I fly and at my doing nothing but flying around cruise setting, Hobbs = 1.2 x Tach.
Click to expand...

citizen5000 said:
This would yield an incorrect Hobbs count. Hobbs is supposed to be the amount of time the aircraft is in use. They are activied via oil pressure or when the electricial system is activated or engine alternators are on (as is the case with Cirrus) or an airpressure sensor attached to the wing or landing gear. If a Hobbs were to be tied to a ratio of Tach time then Hobbs would record fractional engine RPM and not be recording the total time the aircraft is in use.
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Read his post again and then your reply and tell me that you really understand. I don't think you do.
 
I'm logging the time to read this thread as dual, who wants to sign off on it?
 
I'll sign it off as Duel, but not Dual. <- see that?
 
SkyDog58 said:
Read his post again and then your reply and tell me that you really understand. I don't think you do.
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Hobbs = 1.2 x tach will yeild an incorrect Hobbs time. Why? Because Hobbs is "time the aircraft is in use." Taking a ratio of tach time will NOT yield the total time the aircraft is in use. No simpler way to state it.
 
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