Is 2.9 HR Hobbs time normal for 1 hr & 36 min of real-time flight time?

[PaulS]

Some people take a *very* long time in the runup area though. I've sat right seat a few times and legit hung out in the run up area for a good 0.3. Methodically reading checklists, getting nervous, listening to ATIS again, then doing a slow run up again.. oops, let me recheck that mag.. hmm. Does that oil pressure look right? Let me text a pic to the owner quick just to be sure.. it's at the top of the green arc during run up...

Next thing you know we've been sitting in the airplane for 20 minutes and have yet to ask for a takeoff clearance

I wouldn't find it unreasonably to spend a long time on the ground, especially if a passenger was having a headset issue, this may be one of your first cross countries, so you're rechecking the flight plan, the app, weather, etc. Time seems to move real fast in the airplane

Yeah, I'm a slow poke, take my time on pre flights and run ups although I don't do the nervous recheck thing, unless it's warranted. I'm flying rentals so I figure I never know what the last few guys did to it, even though they are fairly new planes. Plus I fly out of a towered airport, which adds to the time on the ground. Don't care what anyone thinks about it though, it ain't a race.

 

[Tantalum]

Yeah, I'm a slow poke, take my time on pre flights and run ups although I don't do the nervous recheck thing, unless it's warranted. I'm flying rentals so I figure I never know what the last few guys did to it, even though they are fairly new planes. Plus I fly out of a towered airport, which adds to the time on the ground. Don't care what anyone thinks about it thought, it ain't a race.

The whole measure five times cut once sort of thing..

but I have found a surprising amount of time passes between turning on the airplane and actually flying.. setting up the radios, calling the tower getting taxi clearance copying down a route clearance etc

I would honestly be surprised to be able to do that and less than 0.2 Hobbes, at least at my home airport.. probably closer to 0.3

If the OP tach was 2 and his Hobbs was 2.9 and he did a full stop at his cross-country airport then the time, well certainly on the higher-end, does not seem unplausible

for what it's worth my foreflight seems to tack an additional 0.2 to 0.3 on to the Hobbes time, but I don't log that in the book

 

[Kenny Phillips]

Hobbs meters are typically tied to an oil pressure switch, so that it only records time when the engine is running. Leaving the master on should not increase the Hobbs time. Sounds like you have a broken Hobbs meter.

I think our flight school hooked at least some of them directly to the master, 'cuz money.

 

[Tantalum]

although if it is in fact tied to the oil pressure switch then this gives new meaning to the threads we see on here about how long somebody can fly without oil

 

[dmspilot]

The whole measure five times cut once sort of thing..

but I have found a surprising amount of time passes between turning on the airplane and actually flying.. setting up the radios, calling the tower getting taxi clearance copying down a route clearance etc

I would honestly be surprised to be able to do that and less than 0.2 Hobbes, at least at my home airport.. probably closer to 0.3

If the OP tach was 2 and his Hobbs was 2.9 and he did a full stop at his cross-country airport then the time, well certainly on the higher-end, does not seem unplausible

As I said before, Hobbs being 0.9 more than Tach does not equate to 0.9 idle spent on the ground.

I log Hobbs and Tach times on every flight after dealing with too many broken and inaccurate Hobbs meters. That differential is not normal, even for a pilot who taxies and does checklists EXTREMELY slow.

 

[smv]

but I have found a surprising amount of time passes between turning on the airplane and actually flying.. setting up the radios, calling the tower getting taxi clearance copying down a route clearance etc

Gets even longer when it is an IFR flight. Make it an IFR Training flight and sometimes you gotta taxi back to refuel....

 

[smv]

I think our flight school hooked at least some of them directly to the master, 'cuz money.

If you turn off the master with the engine running does the Hobbs stop?

 

[PaulS]

I just looked at the hobbs versus flight time on three of the aircraft I fly. The difference between the meters is between 30 to 40 percent for the three aircraft. The one that is used mostly for training is 41%, it is an SR 20, the 22s, which are used less for training are 31% and 35% respectively. This again is flight time versus hobbs, these are factory installed meters and I don't know what triggers them. The more I think of it using flight radar versus the hobbs is not a valid comparison for this type of angst. If you are really worried about it, set a timer from engine start to engine shut down.

Note: Not sure I did the math correctly, but I divided the hobbs by the flight meter for each.

 

[Dana]

I don't have a Hobbs in my plane. My GPS logging app logs whenever the plane is moving more than 5 knots, stopping when it's been under 5 knots for two minutes, which agrees with the clock when I remember to check it. Tach time is about 80% (125% worked the other way) of that on a typical local flight of 45 minutes to an hour. 30-40% seems excessive, but I know you Cirrus guys spend a lot longer running up and getting ready than I do (mags, carb heat, mixture rich, altimeter set, oil pressure and temp OK, let's go!)

 

[benyflyguy]

 

[flyingron]

Most recording tachs are simple revolution counters. All they do is count the number of revs and divide by the nominal continuous power setting.
Electronic tachs typically start an elapsed timer after the RPMs exceed a certain value.

None of these times are what legal time in service is for airframes. That's time the aircraft is off the ground. When I had the Hobbs on the gear switch, that was about as close as you'll come.

 

[Domenick]

Your tachometer timer is a revolution counter NOT a clock. It simply counts revolutions of the engine. At some rpm, say 2700 rpm, tach time equals elapsed time (Hobbs time). If your tach reference is 2700 rpm and we assume tach time = 1.3 Hobbs time, then during your flight (from engine start to engine shutdown) you averaged 2700/1.3 = 2077 rpm.