Tach vs. Hobbs

[spinfire]

Until now I've been a renter, but if all goes well I will be an owner soon. As a renter of course I mostly paid attention to the Hobbs time since that is what I paid. A post on the Red Board about oil consumption got me thinking.

Under what circumstances is tach time used vs. Hobbs? Oil change intervals - by tach time? Tracking oil consumption? Other maintenance intervals? Engine and Airframe time?

 

[BillTIZ]

Engine time, oil changes, etc are normally done according to tach hours.
Tach hours are close to clock time at a set rpm, normally about 2300-2400.
Tach time accumulates slower during ground operations or descents when the engine is operating at reduced power, less stress.

Some inspection or maintenance intervals are based on time, and many use tach time as that measuring device. Other inspections may be based on calendar time, AD inspections due "at annual", etc.

My Pawnee does not have a Hobbs, so tach time it is!

 

[Capt. Geoffrey Thorpe]

Engine time, oil changes, etc are normally done according to tach hours.
Tach hours are close to clock time at a set rpm, normally about 2300-2400.
Tach time accumulates slower during ground operations or descents when the engine is operating at reduced power, less stress.

Well, that depends on the tach.

If my tach matched the clock at 2400 rpm, it would be running at double the clock speed during a normal cruise.

But, it's an electronic tach and matches the clock speed all the time.

 

[Mattl]

My club bases our rates on tach time. In my pp training I did around 52-55 hobbs hours (dont remember when I added it up exactly) and if I remember correctly I had paid for 40 tach hours. Since ground ops, decents, etc get billed at a percentage of time based on RPM.

A great example was this past Tuesday night, I got .6 Hobbs hours and .4 tach hours when doing three full stop TO/L for night currency saving me .2 hours of billed flight time if billing was based on Hobbs.

 

[flyingron]

Maintenance is done on time in service which is defined as the time the aircraft is off the ground. Pilot experience is done by the time between when the airplane first moves until it comes to rest.

The FAA will accept any reasonable and consistent measurement for either one.
Most people use the tach time for maintenance. I don't have a recording tach.
I've got a hobbs in my plane which is wired to the gear that I use for maintenance reasons. I usually just use my watch for the pilot logs.

 

[drizzt76]

I was in a club that billed off the Tach and not the Hobbs. I also benefited from the savings of this system. I think it is closer to the actual maint. cost and fuel burn. You didn't feel like you were paying as much to let the engine warm up or taxi to the far runway. I think of this now that I am out of the club and renting again .

 

[BiffJ]

I think that Hobbs time is mainly for billing. The typical Hobbs meter either starts when the master is switched on or is tied to any oil pressure switch, squat switch or wind switch so that they only bill for time you're running. The tach on the other hand registers engine time and as an owner I don't care much about Hobbs time for maintainence, I use the tach. It reads revolutions and that is how the manufacturer of the engine sets TBO and other time related maintainence.

Frank

 

[flyingron]

It reads revolutions and that is how the manufacturer of the engine sets TBO and other time related maintainence.

Nope. The correct answer is the one I gave earlier. The tach is just as much of an approximation for maintenance times as it is for pilot time.
It's a fallacy that the airplane wear is measured purely in the number of accumulated revolutions.

 

[LearDriver]

Renter: Rent on tach time - Log on Hobbs time

Owner: Rent on hobbs time - maintain on tach time

 

[W. Stewart]

The maintenance is done off tach time. Most people log pilot time off the Hobbs if they have one, or off their watch.

For simplicity, I log even my time off the tach. I keep an old hospital type aluminum clipboard in the plane and enter all flights on it, along with oil changes, VOR checks, etc. I also keep W&B info in it, list of squawks, glide chart, etc. About once every month or two I take the clipboard home and transfer the info into my pilot logbook. This might not be the best option for someone wanting to pad their hours, but I have no need for any more hours.

Wells

 

[flyingron]

The maintenance is done off tach time.

Again that is not correct. Maintenance is done off time in service which is the time the aircraft is in the air. As I stated before, the FAA doesn't get worked up over what time approximation you use (tach time, hobbs, time, watch time, hour glass) as long as you're reasonably consistent.

 

[BiffJ]

Nope. The correct answer is the one I gave earlier. The tach is just as much of an approximation for maintenance times as it is for pilot time.
It's a fallacy that the airplane wear is measured purely in the number of accumulated revolutions.

Having read in both Lycoming and Continental literature that revolutions were how they figured wear and times I'd say its quite true. Not sure if they still say that but it was a basis for the original figures. If you look at how the Tach works you'll see that it is as the Limey's note, a "rev counter". The tach is driven by a cable from the engine through a magnet and gearing system that runs the hourmeter. It runs slower at low rpm and faster at high. At one rpm it actually reads clock time so for figuring flying time its only accurate at one particular rpm.

Frank

 

[flyingron]

I don't argue that a tach counts revolutions.

I argue that your assertion that the number of revolutions is some indication of either actual engine wear or the FAA and manufacturer requirements for maintenance is incorrect.

By your argument, a plane that never gets run over 1000 RPM should go much longer between maintenance than one that is flown hard. Lubrication, corrosion, temperature, etc.. all figure into wear to a greater extent than just revolutions.

In fact, the opposite is true. I guarantee that first one won't make TBO hours. Further, even if your "revolutions" fantasy were correct, it would have no bearing on non-rotating parts of the aircraft that are also subject to time in service rules.