Hobbs or Tach?


#1

So I know that Hobbs time measures hours and so doesnt the Tach for maintenance purposes.
Suppose I rent a plane with no or inop hour meter. Would I get a better deal if rentals went off Tach time instead of Hobbs?
Whats the main difference between the two?
Does RPM have anything to do with Tach time?


#2

Yes you are correct. The Hobbs meter is running when the engine is running whereas the tach is only operating above a set RPM. There’s usually not too much difference between the two times but you would see lower times if you were going off of the Tach.


#3

Most of the airplanes I know with Hobbs meters are activated by the Master Switch.

Master ON, Hobbs starts running…Engine running, tach starts working…rather go with Tach time.


#4

I’m sure some are wired differently. On the two aircraft I’m most familiar with, the Hobbs starts when the engine is started. The tach time is activated by a certain RPM…(I can’t remember the exact but I beleive it is 1000 on the aircraft I’m thinking of). I’m taking one of them out later today I’ll see if I can figure out where it starts.


#5

Actually the tach is connected to the engine via a cable that turns with the engine and thus clocks at engine rotation speed. If you sit and idle long enough you will see that it is moving ever so slowly. It is designed to reflect a true hour at typical cruise RPM. The Hobbs on the other hand is usually connected to an oil pressure switch that makes when the engine starts and records ,supposedly, a true hour. In the case of turbine powered aircraft, the hobbs is on a squat switch. For flight training purposes you would prefer to pay by the tach, especially at a busy airport with takeoff delays. If you have a loss of oil pressure indication the hobbs would probably reflect it. However, by then the non-rotating propeller probably would too. Happy trails and I think the flight levels are over rated.


#6

But pay much better!! :smiley:

I do miss low and slow


#7

Thanks Monty. I guess my end result was there (almost) just the way I got to it was all wrong.


#8

Yes thanks alot guys for your feedback. You answered my questions, thanks again…


#9

Renting an aircraft of course has its own set of rules - set by the FBO, flight school, or whatever. Where I rent, they record both Tach and Hobbs but the rental is based on the Hobbs.

One person earlier got it right that the Hobbs is usually connected to both the master switch ***and ***an oil pressure switch - thus the engine has to be running before the Hobbs starts clocking time.

Generally speaking, a recording Tach is calibrated to run as 1 Hour = 60 minutes at about 70% power. Therefore, if you “cruise” below 70% power, the Tach will run slower and rack up less than an hour in 60 minutes of actual time. If you “cruise” above 70% it will run faster and rack up an hour in less than 60 minutes of actual time. In a sense, it works out well for keeping track of how hard the engine is being run and it averages out over time, but cross countries are obviously different than prolonged pattern work during which very little time is spent at full power and most of the time is spent at much reduced power or even an idle. Flight schools and FBO’s however want to guarantee a consistent return on their rentals, so they rent the airplane based on the Hobbs and track their maintenance using the Tach.

However, while that is an industry standard practice, the FAA definitions of Flight Time (for pilots) and Time in Service (for maintenance on the aircraft) still differ considerably - and the Hobbs running off the master and an oil pressure switch works out well for recording Flight Time but almost nobody in General Aviation really properly tracks Time in Service for maintenance - which is the time from when the airplane takes-off (wheels leave the ground) until it lands again. TIS does not include start-up, warm-up, run-up, or taxi time. In order to properly track TIS, a Hobbs meter would have to be wired through a squat switch - which kinda’ assumes that the aircraft has retractable landing gear (and isn’t a seaplane or an amphibian!) It would actually be possible to mount a squat switch even on a fixed landing gear aircraft - so long as it is equipped with torque link scissors on at least one oleo strut (but it wouldn’t work on a spring-steel landing gear on a taildragger for example), but nobody (or manufacturer) is going to go to that much trouble or expense when Tach time is accepted across the industry as a valid alternative.

(*I always hated that older Piper Seminole and Seneca twins were equipped with separate recording tachs. You end up with 3 different numbers to track - a tach time for each engine and a Hobbs for the airframe time! It’s also easier to synch the engines using a single tach with dual pointers/needles than it is with two separate tachs a couple of inches apart. And it gets worse when one tach has been replaced and the new one does not exactly match the old one.)


#10

It is my understanding that flight time and time in service should be the same. I use a stop watch, from throttle forward for take off to end of landing roll. That’s how I keep my time/distance for XC. But I sometimes forget or don’t bother for local(sometimes 50+ miles)…

So…, I end up using tach time because it’s always there… :neutral_face:


#11

If that is what you think, then you don’t “understand” the subject at all. Try re-reading FAR Part 1 - both FT and TIS are defined there, the definitions are essentially as I already mentioned above.

The formal definition of FT says that it starts when the aircraft starts to move under its own power and goes until its parks or shuts down (“comes to rest”) again. The odd thing about FT is that you don’t have to actually fly in order to log FT; you just have to taxi out with the intention of flying; if you happen to discover something during your run-up that you don’t like and decide to taxi back and shut down, you can still log all of that time as Flight Time.

Time In Service (TIS) on the other hand is only the time that you are actually flying (go figure!) as the formal definition is the time from when the aircraft leaves the surface of the earth until it touches down again at the next point of landing. In other words, there is no TIS until you actually do fly - and only while you are flying.

Theoretically, at a large airport for example and per my “bad run-up” example above, you could easily log 0.5 hour of Flight Time for the pilot for the time he spent taxiing out to the run-up area, actually running up the aircraft and finding something wrong, and then taxiing back to the ramp again - but during all of that the aircraft itself would have accumulated 0 hours of Time in Service.

They are completely different things, recorded for completely different reasons, and there is nothing about either of them that implies in any way that they should be the same - 'cuz they ain’t!