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Logging Tach Time

Old 09-13-2006, 08:36 AM
  #1  
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Default Logging Tach Time

I have searched around a bit and seen this touched on before, but I am still not entirely clear on the best way for me to log my flight time in the aircraft I am currently flying.

The aircraft is a Mooney M20C (Mark21) which does not have a Hobbs meter (the way I always used to track time) and only Tach time. I realize I could probably note time at startup/shutdown and log time off a wrist watch that way, however I do tend to forget those types of things and would greatly prefer to go off an installed device.


Nearly everything I have seen says that Tach Time will nearly always be *less* than what a Hobbs meter would read...since ground operations count slow on the Tach, and times at high RPM (climbout) aren't often enough to offset the lost ground time. I have timed several flights with a watch (startup - shutdown) to compare to the logged Tach time and have found that my Tach time has been *higher* than what an installed Hobbs would have read.

A couple examples...

Tach: 2.87 Time: 2.56
Tach: 2.56 Time: 2.43
Tach: 1.60 Time: 1.53
Tach: 2.92 Time: 2.66
Tach: 3.38 Time: 3.13
Tach: 1.02 Time: 1.00

I have a bunch of other flights I am waiting to log, but those are the only ones I have accurate wristwatch comparisons.

I do operate at somewhat high power settings as the increased speed is worth the added fuel costs for the flights being taken. The aircraft is set to 25"/2500RPM for climbout and 25/2500 for cruise flight if able (generally flown at altitudes where 25" can not be maintained, so Full Throttle/2500RPM is used)

So...what would be the recommended times to enter in a log book?
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Old 09-13-2006, 10:09 AM
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I would log wrist watch time. Tach is only used for engine mx times and is useless for logging time unless the engine always runs at the specified RPM. A timer merely helps you log it accurately. How do people log actual IMC time? Surely not with the Hobbs.

I've heard of "rules of thumb" like multiply by a factor or something for a conversion but as you noted it depends on power setting...

Also, are the times you noted for "wristwatch" in H:MM or straight minutes converted to hours/decimals.... That could affect your interpretation. Reason i ask is:
Tach: 2.92 Time: 2.66
Hard to get 66 minutes...

just my .02
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Old 09-13-2006, 10:22 AM
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The wristwatch times listed were converted to decimals to reflect how a Hobbs would display the times (in my experience they were all oil pressure activated and ran very accurately at .1Hobbs=6mins)

Actual IFR and other entries I have always logged as good 'guesses'...I can't go hit a timer every time I enter a cloud.


Generally there is enough going on that I end up forgetting to start/stop a timer simultaniously with engine start/shutdown, which is probably why they have Hobbs timers to begin with.
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Old 09-13-2006, 07:23 PM
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Tach time is usually less than hobbs time. The tach time is usually only recorded when the RPMs are in a certain range. Usually the green arc or higher. Hobbs time is measured whenever the engine is running.
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Old 09-13-2006, 08:21 PM
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I explain it like an electrical meter on a house. With everything turned on it turns fast. with all the lights and AC off the meter turns slow. Same with the engine. With the RPM's fast the hobbs will turn faster. with them slow it will turn slower. I personally say around .8tach=1.0hobbs if in the pattern. If on a flight X-C it's usually the same or sometimes faster based on RPM.
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Old 09-14-2006, 05:22 AM
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I understand how a tach runs, or atleast how my tach runs. In this case (and I believe all cases?) the tach is always running reguardless of engine speed, however the SPEED it runs out varies with RPM (household electrical meter good analogy) ranging from very slow operation at idle/taxi RPMs increasing as RPMs are increased. Hobbs (generally) run on oil pressure, which is also always present when an engine is running, however the rate it runs is calibrated to TIME rather than RPM like the tach.

I always thought that Tach would be lower than hobbs time, so I was surprised to find my tach time consistantly higher than what a hobbs, if installed, would be reading. All my flying is cross country, usualy in the 2-4hr range with cruise rpm of 2500or2550RPM, which means that the RPM is running close to (or even faster than) true time. I suppose the "Tach<Hobbs" I see/hear all the time comes from flight instructors who operate much more commonly at low or even idle power settings.

I'm really just looking for an easy way to track my logbook time. Unfortunately in the airplane I'm already keeping track of flight time (wheels up to wheels down) for CC planning purposes, time duration on each fuel tank, IFR Approach/Hold Timing, and I would prefer not to start a timer for engine start/shutdown as well.
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Old 09-14-2006, 06:03 AM
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If its really that hard for you, take your cross country as timed then add .2 on the front end and .1 on the tail. Thats a 6 minute taxi in and 12 minute startup/runup/taxi out. If you take your time, maybe .3 on the front.

Its not rocket science here. The Feds arent gonna hunt you down over .1, but its how exact do YOU want to be.

Even using a hobbs can be innacurate. Since hobbs only ticks every 6 minutes, if someone shut down after 5, then 1 minute into your flight you already have .1. Same can happen on the tail end if it ticks, then 1 minute later you shut down. You take credit for the full .1. Can help you by 5 minutes or short change you by 5 minutes. A watch is probably the most accurate of all when it comes down to it.

Last edited by U-I pilot; 09-14-2006 at 06:05 AM.
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Old 09-14-2006, 09:52 AM
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You'd be legal if you used an egg timer to clock your flights, and wrote it down on a scrap of paper. There's no reg that says how you have to time your flights or what defines a logbook.
As a side note, running at that high a power setting for extended periods of time can't be good for the engine.
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Old 09-14-2006, 11:00 AM
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The aircraft red lines at 2700RPM and the POH specifies cruise power settings anywhere between 2300-2600RPM, so I don't see any problems with a standard 2500RPM power setting.
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Old 09-14-2006, 11:19 AM
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My first 100hrs were in an aircraft that did not have a hobbs meter (Cessna 172N). Here's what I know, and what I'd recommend.

The tach is calibrated for a certain engine RPM, very close to full power. I was told it was something like 2300RPM's. Anyway, if you are taxing at saying 1150RPM's, you could taxi for .2 hours before .1 would turn over on the tach.

If you were doing a training flight (takeoffs, landings, stalls, other maneuvers), the tach was much less than the actual time. Therefore I would recommend looking at what time you start up the engine (recording it), and then recording the time at which you shut down your engine for those flights. I have gotten 1.1 hours of flying for .6 hours of tach!

For cross country flights, I found that .8 hours of tach generally equaled 1.0 hours of flight time. This seemed pretty accurate for me. If you are running your engine at full power, your's would probably be about .9 hours tach for 1.0 hours of flight.

The most accurate way is to obviously wear a watch. But if you are doing a cross country at the power settings you say you fly at, it is between .9 and 1.0 hours of tach for 1.0 hours of flight. .9 looks pretty consistent with your results. Hope this helps.
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