Tach Time vs. Hobbs and how its calc'd

[Vik]

At what RPM would tach time = hobbs time or at what % power?

I'm planning on buying a plane and I have been doing some cost analysis which eventually is getting broken down per cost per hour. Now of course, this is based on TACH time because the largest expense will be engine overhaul and prop overhaul.

To this date, I've been renting so I'm used to thinking in terms of HOBBS time and when the HOBBS is broken, we use 1.3x the TACH to get the HOBBS.

I do plan to use the plane to build multi-engine experience and do not plan to fly it at 75% power all over the plane. So of course if I had a hobbs, it would count hours that would exceed tach time.

So back to my original question .. is there some point at which TACH = HOBBS?

I hope this makes sense.

 

[TZRav8r]

I believe they are equal at redline RPM. However, if your talking multiengine AC, none of the AC I have flown had Tach hour meters. They have all had electric Hobbs that operate off a squat switch or an airspeed switch. If you're concerned about logging time accurately you might start recording your block times, especially if your operating at busier airports where long taxis or ground delays are common.

 

[Vik]

well, my reason for asking is because I am planning on buying a twin and all my costs (I just realized this) are based on tach time. Obviously my real logged time will be much higher than tach time and so my estimate of expense for a Twin Comanche of $135/hr is going to be much lower b/c its $135 per hour of TACH, not hobbs.

The majority of expense being in engine overhaul and prop overhaul which are based on tach time.

So my hourly cost will actually be lower b/c I plan to fly the plane at econ cruise, not 75% power or redline (obviously).

So I'm trying to find out when the two are equal, etc.

 

[LearLove]

I used to do alot of instructing in T-34's that had no hobbs meter. Basicaly it came to .8 to .85 on the engine tach was about one hour on the clock (engine start to stop). This includes start, runup, taxi, takeoff, flight, landing, taxi in and shutdown. The NATOPS had crazy takeoff and landing and approach currancy requirements so mostly this was takeoff and landing stuff, the time would have been better if it was x-country type power settiings as they would be lower than the TO and landing stuff.

As far as i know the tach runs in porportion to engine speed. The faster the engine speed the faster the tach moves. In a Cessna 150 that was charged by Tach time but also had a hobbs meter I got like .59 on the tach for 1.0 on the hobbs. I built alot of time in college this way cheep. The 150 went for like 27 per hour Tach. I'd takeoff and pull the throttle back enough to hold level at best glide and fly somewhere. when it was all said and done I was getting one hour on the clock (and thus in the logbook) for about 17 bucks. Pretty good deal.

My numbers may be alittle off as this was a few years ago but its to the best of my memory.

 

[SDF2BUF2MCO]

Vik, this may not help since you're talking twin, but similar to Lear's comments I'm averaging approximately 82% tach (time I'm paying for) to hobbs time while smashing bugs in a C172. Pretty much flying point to point with most legs around 1.5 hours (hobbs). Once in "cruise", usually keep it around 2200 rpm.
I've asked a similar question before and don't recall receiving a definitive answer. Speculate for the engineer types this would be an elementary question!

 

[A Squared]

I've only ever checked one Tach against real time. for that particular airplane, tach time was almost identical to real time at 2300 rpm. I thinkk that you will find that they are generally going to equal real time at a typical cruise rpm for that engine and airplane. I don't think that you will find any tachs which equal real time at redline.

On a related issue, maintenece times are actually predicated on flight time, which is the real clock time from time the wheels leave the ground till the time they are back on the ground. Tach Time is generally used because it is the best estimate of flight time available, not because it is the correct time.

Most hobbs meters are connected to an oil pressure switch. The time recorded by such a device will be longer than time in service by all the time spent on the ground. I used to work for a company in which the hobbs meteres were connected to an airspeed switch, and the hourmeter was activated above a certain airspeed. A hobbs meter like that is probably the most accurate measure of flight itme for maintenence purposes ... of course it will be somewhat short for pilot flight time logging purposes.

 

[Vik]

But aircraft engine and prop TBOs are all based on tach right?