Hobbs vs. Tach

[Tug Driver]

I was talking to a mechanic today about Hobbs time vs. Tach time. He said that your not suppose to use the Tach time in your logbook. I was in a flying club and we used Tach time to keep track of who flew. I have over 300 hours of "Tach" time in my logbook.

The mechanic says that I should add on .2 per Tach hour that I have logged. this is a difference of around 60 hours. Does anyone know what the correct way of adding this time would be? I don't want to just make up trips, like the mechanic suggested that I do.

Ideas??

Thanks in advance

 

[Alchemy]

If you've already logged 300hrs as tach time, I wouldn't go back and change it retroactively. It would make a huge mess of your logbook.

From here on out, use the hobbs meter. The mechanic is right about adding .2 hours for every hour of tach time, but way off on suggesting that you "make up" trips to account for the difference.

 

[EatSleepFly]

Yeah, I wouldn't go correcting anything either. Leave it as is.

In my opinion, 60 hrs. isn't enough in the grand scheme of things, to worry about explaining it to the feds, or in an interview.

Just my $.02.

 

[vclean]

It really depends on the type of flying that you do. In cruise tach and hobbs will be about equal. If you are doing airwork, touch and go's etc, then it is normally acceptable to multiply your tach time by 1.2.

If your A/C is rated at 2500 rpm, and you operate at that setting, tach and hobbs (true time) will be equal. Anything below 2500 (or whatever the max rpm is) will 'slow' the elapsed time recorded on the tachometer.

If the A/C you are flying/renting doesn't have a hobbs, use your watch and note the time from engine start to shutdown.

 

[avbug]

Everybody is kind of in the ballpark.

The hour representation included on your tach (the odometer) is based strictly on engine speed. The faster the tach turns, the faster the instrument records "time."

The electronic hour meter is dependent on voltage...operate it at a lesser voltage, it runs slower. A higher voltage, it runs higher. it should be reasonably accurate, but not necessarily. I flew a light turboprop last night for 41 minutes and saw .3 register on the electronic hour meter installed in that aircraft. (gotta have a look at it tomorrow).

The electronic hour meter only represents time when it's been activated, and a varietyof means are employed to do this. The switch to activate the hour meter may be on the same circuit as the battery master switch; turn on the electricity, and it starts registering time. It may be wired through an engine oil pressure switch; the meter only runs when the engine is developing a given oil pressure. It may be attached to a landing gear squat switch, or sometimes to a uplock circuit; only when the weight is on the gear, or when the gear is up will the meter register time. Some meters use an air pressure vane or switch to activate the hour meter.

Each of these installations changes the amount of time that the meter will register during any given flight, by changing the time it starts and stops.

The hour meter on your tachometer, the odometer, shows time from engine start, but the rate at which the time is scheduled varies with the engine or propeller speed. At idle, you'll need to sit a lot longer than an hour to register an hour.

The rate at which one may convert "tach hours" to the hours in your logbook will vary with the tach. Some add .2 per hour, others .2 per flight. Some instruments will be 1.4 hours per indicated hour, others 1.2. I've seen wide variations in indications.

Additionally, very few tachs actually read correctly. I sampled a light piston single yesterday and noted that the actual prop RPM was 300 rpm less than indicated. This isn't at all uncommon. The instrument may be measuring at the wrong rate in conjunction with indicating the wrong value.

If you're short a certain value of time based on having referenced the tach vs. the clock, you can certainly correct it. You needn't correct every entry. Simply make one entry for the value you desire...if you need to add 60 hours, make a single entry as though it were a flight, with 60 hours as the value. Then in the remarks column note that it's a time correction for taxi and conversion factor. It's not messy, it's discreet, and nobody is going to fault you.

If you're looking at an airline application, some will disallow for taxi time; some will reduce your total flying, others will add to it. The policies vary. You can't account for them all; simply log your time as professionally as you can, and make any necessary adjustments on an application as the individual employer may require. The employer will let you know.