Old Mr. Hobbs

[Rick6a]

I've occasionally heard many flight schools charge for flight time based upon the Hobbs meter. If the Hobbs unit installed in "Darla" is any indication....small wonder. Is Mr. Hobbs inherently inaccurate? I've got my example wired to activate when the oil pressure rises, still....it seems that for every 60 minutes logged on the Hobbs, the actual time the engine has run seems to be in the area of 50 minutes or so. I really didn't notice this discrepancy until I started comparing the flight hours automatically logged in my handheld GPS against what Mr. Hobbs was indicating. The most glaring problem I have with this variance is although the Hobbs indicates almost 100 hours of operation have elapsed to date, in reality, I suspect the true hours the engine has actually run and the flight hours I have really accumulated is significantly short of that. Given the price of oil/filter changes every 25 hours (among many other issues based upon elapsed time), this is no small potatoes.

Rick Galati RV-6A "Darla" 98 hours..... on the Hobbs

[TShort]

My 172 has a tach and a hobbs (oil pressure switch) .... the ratio of Hobbs to tach is pretty similar to what you describe. I just crossed 900 on the Hobbs and have 750 some on the tach. The tach time in mine is run by the tachometer cable itself and reflects how much the engine is run (RPM) - since I don't run the engine hard it lags behind the Hobbs.
I use the Hobbs to log time, and the tach for maintenance (works great for me! ). Will also use the tach as the TTAE when I sell the plane (this is how it was advertised when I bought it)

Thomas

[Dgamble]

I'm looking at an old tach that I had to replace in my store-bought plane, and it has a calibration decal on the back that says:

Hourmeter recording rate is correct at 2292 RPM.

I'm not sure if this means it reads too low below 2292 and too high above 2292, or if it means it reads everything correctly at 2292 and higher.

Either way, if a lot of your time is spent on the ground or at reduced power levels, it seems normal for Hobbs to read higher than tach. This is, of course, why I log Hobbs instead of tach in my logbook, but use tach to log things I'd prefer to delay.

[N916K]

Originally Posted by Rick6a

I really didn't notice this discrepancy until I started comparing the flight hours automatically logged in my handheld GPS against what Mr. Hobbs was indicating.

Check your GPS setttings. Generaly the GPS won't start recording a flight until you are flying. That leaves all the time between startup and takeoff as Hobbs time but not GPS time. If you have an engine monitor you can change the settings to closer reflect Hobbs time or Tach time.

Cam

[Tandem46]

Rick,
Pardon me if I'm off here, in general, your GPS flight time is always going to be less, substantially less over time, than your oil press operated hobbs. Your GPS should only start the clock above a certain programmed or default airspeed. So, your start-up, taxi, engine run-up, and post-flight taxi/shutdown will never show on the GPS. At least that's how mine works. I have three timing devices on my -7, a hobbs, a digital tach from EI that keeps track of total tach time and flight time based upon programmed rpm, and then I have a 396. I all but ignore the hobbs, mine is ticking away anytime battery is on. I use the EI for maintenance, gps for flight time.

Tobin

[RV7Factory]

Originally Posted by Dgamble

I'm not sure if this means it reads too low below 2292 and too high above 2292, or if it means it reads everything correctly at 2292 and higher.

You got it... it means an hour at 2292 rpm will be an hour on both the tach and hobbs, an hour (real time) above 2292 rpm will be over an hour on the tach, and an hour below 2292 will be less than an hour on the tach.

Like you, my citabria had both. If I stayed in the pattern to practice landings my tach was typically 80% of my hobbs. On longer flights, when the rpms were higher for longer periods of time, they often matched, or if I was in a hurry and ran the rpms high (above the 2500 rpm hour rating of the tach), the tach would exceed the hobbs.

[N941WR]

On a club plane I used to rent they would charge you hobbs time if your flight was less than 2 hours, Tach time if over 2 hours.

The idea was that if you flew two hours or less you were doing touch and goes, which is harder on the plane and engine.

For maintenance they would use the tach.

[zilik]

Originally Posted by Rick6a

I've occasionally heard many flight schools charge for flight time based upon the Hobbs meter. If the Hobbs unit installed in "Darla" is any indication....small wonder. Is Mr. Hobbs inherently inaccurate? I've got my example wired to activate when the oil pressure rises, still....it seems that for every 60 minutes logged on the Hobbs, the actual time the engine has run seems to be in the area of 50 minutes or so.

From the other posts I think you now know that Mr. Hobbs is accuratly measuring time the engine has run and the GPS is shorting you on hours logged.

In my RV-6A I only have a mechanical tach that logs correct hours at 2566 rpm and since I cruise at 2300-2400 I would short myself hours logged if I went by the tach. So I jot down the time at engine start/shutdown and log my flying time that way. A hobbs would be esier but I did not like the fact that a hobbs usually has more time than a recording tach would log.

Since I have kept accurate records of actual logged time (startup to shutdown) vs tach time I find that the actuall (hobbs) on my aircraft would be about 1.2 times more than tach. So if I had a hobbs, my TTAE recorded by the tach would read 1000 hours and the hobbs would read 1200. I like to keep my airplane young, so no hobbs for me.

Gary

[gmcjetpilot]

I don't have a Hobbs, because I have too many clocks and timers already. Do we need a Hobbs meter at all?

Thinking about it, I have: 3 clocks, 3 flight timers and 3 elapse timers in my RV, with out even trying. Every electronic goodie seems to throw in at least a clock or timer or two:

(2) EFIS: Clock & count-up/down timer
(2) EIS (engine monitor): Engine time (RPM limited), Flight timer based on RPM
(3) 3-in-1 panel clock: Clock, count-up elapse timer & flight timer (air or gear activated)
(2) GPS: Clock time & time in motion

The above does not include the watch on my wrist, which is what I really use when I write my block times down for the purpose of logging pilot time, which is all I would use a Hobbs meter for if I had one.

For pilot flight time I use block to block, as most do. I just look at the clock and write the times down, after engine start and shut down. Some times I use one of many elapse timers. If I forget to write the time down or start a timer, I just use flight time that the EIS automatically records and retains till after yhe flight and up to next flight power up for 3 minutes. Other wise the total engine/hobbs time is always retained in no-volatile memory.

Flight Time (actual airborne) and Engine-time are one in the same. I use the electronic engine monitor. It accumulates flight time in memory for maintenance and airframe time. It is based on engine RPM's above a min RPM I set. Below this RPM it records no time, e.g.: taxi, run-up and short approach (low RPM). The EIS also uses this time flight time for the purpose fuel burn. It clears the flight time every flight. The flight time is used with the fuel flow totalizer. If you remember to reset the fuel quantity after fill-up with know quantity, it gives you fuel endurance time based on Fuel Flow. This is most accurate and accounts for actual fuel used.

I guess the Hobbs would be a nice back up, but could not justify adding a 10th clock.

HOBBS AND TACH
Could a particular brand or model of Hobbs meter be more inaccurate or just off calibration more than others? Is the pressure switch set to activate at low oil pressure? Hobbs is just an electric clock turned on and off with a pressure switch, right. It would seem well with in the area of technology to make it accurate, if you wanted to.

Agreed, the traditional Hobbs is too conservative for maintenance. You don 't need to measure time unless in flight, i.e. high RPM/high power. The Tach hour-meter always is going to be less over time from the Hobbs, because it's rate of time accumulation is less at lower RPMs, like taxi, low or idle power, as was stated above. I think that is what the FBO's do. Charge on Hobbs and do maintenance on Tach. George

[Rick6a]

Originally Posted by zilik

From the other posts I think you now know that Mr. Hobbs is accuratly measuring time the engine has run and the GPS is shorting you on hours logged. Gary

Gary

This RPM, that RPM, tach time, smack time............................................
Now I don't trust either the GPS or Mr. Hobbs! From now on, I'll take a page from the Gary Zilik playbook and WRITE DOWN actual start/stop times.

Rick