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Discussion in 'Flight Following' started by bflynn, May 31, 2020.
No, right? The Hobbs should always be bigger than the tach?
Hobbs should always be higher. I guess you could defeat it by flying around with the master off, assuming it is wired through the master.
As Bob Dole said when asked whether he wears boxers or briefs...depends.
My tach was calibrated for 2300 rpm. If I got into the air quickly and ran 2500 rpm for a few hours, tach would exceed flight time.
Depends what RPM the tach is calibrated too, and how hard you are running the engine. This is really quite simple. If the reference RPM is 2400 and you run the engine for 5 hours at 2700RPM, then of course the tach is going to show more than 5.0 hours.
Also depends how the Hobbs is wired
As they say, your mileage may vary. In my former plane (1983 C172P), the Hobbs came directly off the battery (skipped the master switch). It was protected by an in-line glass fuse.
I haven't checked in my current plane (pre or post master switch in my 1979 C172N), but anytime there is oil pressure, the Hobbs is running. As others have said, the tach hour meter starts counting after a certain RPM. Not sure of the threshold (above what RPM will tach start counting).
So, I guess the only way the tach will exceed the Hobbs is if you're flying along and your oil pump goes kaput (extremely rare, according to Mike Busch) or you run low on oil and starve the pump.
Of course, such a condition would be short in nature because pretty soon, RPM will drop off and you'll get flight time free of Hobbs OR tach
The Hobbs is usually a smaller instrument. Have never seen one larger than about 2" in diameter. The Tachometer, on the other hand, has always been bigger than that. Especially the mechanical ones. Usually at least 3" in diameter.
Then again, the tach display on an MFD can be pretty small. Sometimes smaller than a Hobbs, actually... So I guess it depends on what kind of tach you mean.
If you are using an EI primary engine monitor, the tach hour meter runs at anything over 1300RPM. By that measure, tach hours should always be equal to or greater than Hobbs hours. But as mentioned above, this is different than how most other tachs measure hours.
Yup, I flew an airplane where the Hobbs was an air switch. I've also flown one that only ran when the gear was retracted.
Don't you have that backwards?
Historically, my hobbs runs 1.1 - 1.5x what I see on my tach.
Tach could be higher if you put a used engine into a new aircraft.
Good point. The original post is vague and leads to assumptions. I assumed the OP is referring to putting more Hobbs hours than tach hours in a given flight (or over the course of ownership ... or something in between, etc). Could be wrong.
@bflynn , what say you?
No, the tach always counts, proportional to engine rpm. It's the Hobbs that starts at a certain rpm (actually a certain oil pressure).
yes, same flight, tach is higher than Hobbs - 3.0 vs 2.6.
Maybe I recorded numbers wrong. I didn’t think I was pushing it that hard.
Or the Hobbs is failing(?)
I didn't think the Tach could record more than "100%" - I.O.W - at full throttle, it can't record more than a tenth every 6 mins. I could be wrong. It's happened a time or two before
My hobbs is failing. Sometimes a flight gets more tach than hobbs. I have ignored my hobbs for a long time.
Best response ever!
No, not always. See post #7.
Does a Hobbs vary it’s time based on voltage? So one plane with a voltage regulator that’s different than another would have differing Hobbs times? If so, then maybe there is an electrical problem orthe Hobbs is going bad. Easy enough to check next time you fly, use a timer to verify the Hobbs.
Tach on the other hand may be wrong too, not as basic to check, but at least you can verify with an optical tach if you have one.
If the tach is calibrated for 2300 rpm and you’re running 2500 rpm, that’s over 108%.
Probably not. It's nothing more than a geared motor and will run at the same rate within a specified voltage range depending on internals. It should run at the same rate regardless of aircraft system with a serviceable electrical system. If there were low voltage issues you'd have additional problems.
FWIW: There are a number of issues that can cause a hobbs or tach to not count correctly. However, have seen more tachs in error than hobbs and usually caught the problem when balancing a prop. If in doubt with either one, record the time from start up to shutdown with a watch or clock and compare.
The Hobbs is an electric clock. They will run at a constant speed when fed with anywhere between 10 and 80 volts dc, so unless your electrical system has real problems (and dropping below 10 volts), the Hobbs will continue to count fairly accurately. The Hobbs knows nothing other than someone applied power to it. I've seen Hobbs connected to the master, an oil pressure switch, the gear squat switch, the power to the gas-fired heater, etc...
Mechanical recording tachs are just revolution counters. They assume some nominal cruise setting and divide that to get "hours."
I always pray that if I have an instrument failure, it is the Hobbs. I tell folks that it is the most expensive instrument in the panel. It's the one that costs me money. Anything else is the club's problem.
Finally. Give that man a cookie.
The tach hours are proportional to the engine RPM. There's no magic "start" RPM, it's a straight mechanical ratio.
If the tach is calibrated to show 1.0 tach hour for 1.0 clock hour when the engine is operating at 2500 rpm (a typical 65% power value for a fixed pitch O-320), then what it's really doing is moving the drums 1 "hour" for every 150,000 engine revolutions (2500x60=150,000). At a constant 2500 rpm it'll show 1.0 tach hour for every 1.0 Hobbs hour.
If you are instead cruising at a constant 2100 rpm, then your engine is only turning over 126,000 revolutions every Hobbs hour and the tach will only move the drums .84 tach hours (which will show as .8 hours on the tach). 126,000/150,000 = .84.
If your tach is calibrated for the above mentioned 2500 rpm/65% cruise rpm, but you instead cruise at 2600 rpm (74% power in my aircraft), then the engine will turn 156,000 revolutions in 1 Hobbs hour and the tach time will be 1.04 tach hours, which still shows as "1.0" on the tach. You won't see a difference until you are 3 hours into the flight when the tach will reflect the 468,000 engine revolutions and 3.12 tach hours (3.1 hours on the tach) compared to 3.0 hours on the Hobbs.
If you cruise at 2700 rpm and 81% power, the tach time will be 1.08 hours, so you'll see 1.1 on the tach shortly after the Hobbs shows 1.0.
In my Citabria, doing a mix of stop and goes, full power climbs to 1000 ft-3500 ft, and airwork at 2100-2300 rpm, my tach time averages about .73 hours for every hour on the Hobbs.
On a cross country with a full power climb to 3500-4500 ft and a 2200 rpm cruise, it averages .92 tach hours for every hobbs hour on a 2.5 hour flight. (As opposed to a straight .88 tach hours for every Hobbs hour at a constant 2200 rpm).
That tendency for the tach time to be less than the actual clock time, nearly (but not always) all the time is why Hobbs meters became a thing. Renting aircraft where a pilot could fly for 1 hour and only show 45 minutes on the tach cost FBOs revenue. To be fair, the engine sees less wear at low rpm, engine TBOs are based on tach hours, and fuel burn is lower at the lower rpm settings, so the actual costs are at the the 45 minute figure (not counting instructor time), but it's still lost revenue.
Once Hobbs meters became a thing, it didn't take renters long to figure out they could turn off the master switch for awhile and reduce the bill. Some FBOs started tracking both Hobbs and Tach and could quickly figure out who was cheating. But it also became common to tie the Hobbs to an oil pressure sensor, so that the Hobbs ran anytime the oil pressure was over a certain minimum value. I suppose if you were renting something like a DA 20 with an 11-1 glide ratio, you could climb to 10,000 ft, shut down the engine, slow enough to stop the prop, glide 19-20 miles or so, restart the engine, and then repeat the process.
Fun Fact: The tach on a Beech Sierra counts 10ths and 100ths of an hour...
The tach and the Hobbs are two different instruments that are used to record two different things. The Tachometer is used to count the number of revolutions that an engine has turn, converted to an assumed time. Revolutions per minute. The Hobbs is used to record flight time for the airframe and engine. The two times are always different. The tach is recording from engine start until engine shut down. By the FAA definition flight time for the airplane is defined as the time from when the airplane leaves the ground (in flight) until is lands (stops flying) It does not include taxi time, engine warm up time, or preflight check time. Tach time will always be greater than Hobbs time. Hobbs time is the time used in required maintenance intervals, not tach time.
@Computerjim , these statements are most definitely not true in many, many cases.
In most single-engine pistons, the Hobbs is wired to an oil pressure switch. It records actual engine running time. The tach, on the other hand, varies with engine speed. Therefore tach time will almost always be less than Hobbs time.
In many twins (such as in your profile pic), the Hobbs is wired to a weight-on-wheels switch, therefore only recording when the airplane is airborne. In these cases, tach time and Hobbs may be roughly equal or tach time may be greater than Hobbs, it depends on the rpm you run at and what the 1:1 rpm is on the tach.
Aircraft time-in-service is indeed officially the time the aircraft is in the air. However, since most singles do not have a clock wired to the weight-on-wheels switch, the tach time is most often used as a close-enough approximation. In general the FAA doesn't care how its measured as long as it's consistent, and tach time is better than using Hobbs time (since it's lower) for this purpose. In most single-engine aircraft, the TTAF, SMOH times and others are tach time.
Also, I will add to the existing discussion that most of the talk about tach time recording lower at low rpms and higher at high rpms is only for mechanical tachs. Many (most?) electronic tachs just start recording normal 1:1 time at anything over a certain rpm, say 1300, and do not record anything under that. So the variance between Hobbs and tach in these cases only adds up to the amount of time spent under 1300 rpm.
FWIW: that is actually the definition of "Time-in-Sevice" and is used for mx times. Flight Time, or "pilot time" is when aircraft under own power for purpose of flight. Both are in Part 1 and there is guidance for same use. Regardless, it's up to the operator/owner which method is used but most use tach time for mx records per the existing guidance.
Apparently you are not of the "fly it to the chocks" school of thought, eh?
Yes, flight time/pilot time is for pilot log books. Time in service is for maintenance issues.
We used to have a plane in the club that did not have the Hobbs run through a pressure switch on the oil. The Hobbs ran any time the master was on. We got that fixed very quickly as a number of us did not like paying for Hobbs time while doing a pre-flight. It didn't add much to the bill, but it did add.
As with most things aviation, the answer is, "it depends."
Not necessarily. Generally, if the airplane still has the original Hobbs meter and the original tach and they're both the traditional variety of instruments (analog) then it's *almost* certain that the number on the Hobbs is higher than the number on the tach.
But... The Hobbs may be wired to the master, an airspeed switch, a squat switch, an oil pressure switch, or some other things. I once saw an airplane that had four of them wired to different things!
And a tach may be calibrated for different RPMs and run proportional to that number, or you may have an electronic tach that is just "on" above a certain RPM and "off" below. For example, the aforementioned EI comes on at 1300 RPM. The Horizon comes on at 700 RPM so is pretty much on any time the engine is turning.
Plus, any of the above can be replaced at any time and may thus reset to zero, requiring that you do some looking at the logbooks and some math to determine the actual TTAF and engine times.
There are darned few planes with an "original" Hobbs meter. My plane came originally with a recording mechanical tach (rev counter) though it had been replaced long before I bought it. When I did the first renovation in 2004 I put in a Hobbs connected to the gear switch. When the current renovation was done, someone connected the Hobbs direct to the master. However, I now use the EI MVP-50 (which like the electronic tach mentioned above, uses some nominal threshold of RPMs but then does clock time from there); I've set the thing to read the actual time in service since the airframe beginning of time (not the former adjusted tach or Hobbs values).