Hobbs / Tacho

[Laichtown]

What is the difference between Hobbs / Tacho? How does 1 unit relate to 1 hour?

[BackPacker]

A hobbs always runs true, ie. one hour = one hour. Hobbs can be wired to some sort of electric switch, usually the engine master, or some sort of pressure switch, eg. oil pressure or pitot pressure.

The tacho is related to engine RPM and will only run true at a certain RPM, typically cruise. So when the engine is idling, you pay less.

[Laichtown]

Cheers,

But how does 1 tacho unit relate to time? ie what to put in the logbook?

[Mark 1]

Your log-book times are chock-to-chock, so more related to Hobbs (less warm-up and shut-down times).
Tacho time will depend on type of use, amount of taxying and the "standard RPM" of the tacho. My records show 20% more Hobbs hours than tacho hours, but this is only a sample of one aeroplane.

[BackPacker]

It does not. Unless you run the engine on cruise power RPM exclusively during taxi, take-off, climb, descent, landing and taxiback. But if you start doing that, let me know and I'll come and watch.

Technically, what you put in your logbook is the time "when the aircraft first moves under its own power" to "when the aircraft comes to a final rest after landing". There is no accurate instrument which counts that time for you automatically, at least not in small aircraft. Hobbs time, as long as it is connected to the engine master (in a FADEC equipped aircraft) or the oil pressure sensor comes pretty close and is used a lot.

But if your hobbs meter is wired differently, or if you don't have a hobbs then what you need is to write down your off-blocks and on-blocks time based on a clock (or your own watch) and calculate your logbook time from that.

[TheOddOne]

We pay for tacho time in our PA28 group. Our tacho (and I thoght they all did) reads 1 unit for an hour spent at FULL power, e.g. red-line RPM. The advantage with charging per tacho time is that it encourages pilots to come back to an economical cruise, saving fuel and wear 'n' tear.

There are 2 ways of working out your log-book time, neither of which work for either tacho or hobbs.

1. Actually record the time you start moving from the parked position until you park up again.

2. Record your take-off and landing time. Add either 5 mins or 6 mins either side of this time (depending upon whether you choose to keep a 'minutes' or 'decimal' log-book). The advantage of this method is that it ties in with the requirement to record the airborne elapsed time in the aeroplane's technical log.

Both methods are approved, but require manual noting of the times. I've never seen an automatic method for either.

Our flying school, in common with many others, charges for the airborne time plus 12 mins. This has the adavantage for the early student that the 15+ mins often spent explaining the details of how to do taxi checks, power checks etc is free after the 1st 6 mins!

Cheers,
TheOddOne

[BackPacker]

Paying based on tacho time makes a lot of sense since it reflects pretty accurately the biggest expense in running an aircraft: fuel. My club uses tacho time for all aircraft that have a tacho.

For our aircraft without a tacho (DR200-135CDI and DA40-TDI) we use hobbs time and it's a major source of discussion: paying the full price regardless of whether you're sitting at the hold or flying flat-out at 100% towards your destination.

[Mike Cross]

The legal requirement for airframe and engine log books is take-off to landing time.

The legal requirement for your personal flying log book is the time during which you act as a member of the flight crew. As stated above it's from when you first start to move to when the a/c comes to a final stop. It's common to make it t/o to landing plus 10 mins to allow for taxying.

How the costs are calculated is entirely a matter between you and the person you hire it from. Tacho and Hobbs both have the advantage from the owner's point of view that he can check what you should be charged rather than relying on what you tell him.

[rustyflyer]

The G1000 in the 172 I'm lucky enough to play with has an automatic timer. Looking in the manual it says it can either be set to start when the system turns on or ground speed exceeds 30kts. Neither are quite right for log-book purposes... It also records departure time with the same choice of criteria. Must have a look next time I go up.
Too many menus too little brain!
R

[bjornhall]

In addition, most COM/NAV and ADF radios and all GPS have a flight time counter, perfect for recording block! With dual COM/NAV, ADF and a clock on the panel you will have about a eight independent timers available, plus whatever is in your GPS and your watch; shouldn't be a problem to log both block and trip then... ;)

[SNS3Guppy]

Time tracking on an engine is done by the period of time which the engine has been running. The tachometer with an odometer function (tach recording speed, odo recording miles or in this case, hours) fills this function, and actually does very well. When the engine isn't working as hard, it records the hours more slowly.

A "hobbs" or "datacon" is a particular brand of hour meter. It's just an electrical recording device. Hour meter installations may be activated by weight on wheels (squat switch), landing gear position, aerodynamic pressure switches, oil pressure switches, wired in the master switch circuitry, or any number of other means to establish recording of time. The common theme is that unlike the mechanically operated odo on your tach, the hobbs requires electricity.

Recording of airframe time isn't takeoff to landing, and really depends on the airframe. Some want landings, or cycles. Some want total airframe time as recorded by the duration of the flight. Duration of flight is the time the aircraft first moves under it's own power, until it comes to rest after the flight...block to block.

A typical conversion factor for your tach time is about 1.2 to 1.3, depending on how much time you spend at low power during the flight. If you multiply your tach time by 1.2, you'll usually come out close to what the actual flight time has been. Or, you could simply write down the time you fire up and note the time you shut down, and call it good.

You should record the time you start the engine anyway, as it's part of your fuel management; know your takeoff time, your start time, etc. Time your tanks.

Some aircraft installations use separate hour meters for some components in the airplane. While admittedly not a light airplane function, equipment using an auxilliary power source, for example, uses a separate hour meter for that source. How any hour meter is activated, and what it's actually measuring, really depends on the specific installation.