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

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LastXdeth

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LastXdeth
Yesterday, I rented out a Piper Archer and did a quick XC (83 NM [166 NM round trip]) with my friends. When I returned back to home base, I was surprised to see that the Hobbs meter indicated that I was out for 2.9 hr! The flight did not feel like 2.9 hours. After referring to FlightRadar history, I noticed that my total real-time/flight time round-trip was only 1 hour and 26 minute (1.4 hr if converted to Hobbs format).

Now, I don't think I left the battery on. Two of my friends are also pilots and my extra set of eyes, and I vividly remembering myself turning the battery from OFF to ON when executing the before-start checklist as we were getting ready to go back home. Even if I did leave the battery on while were out in town, it still wouldn't make sense because we were out exploring for more than 2 hours; the hobbs would've surely been more than 2.9 hr if that was the case.

I do understand that FlightRadar24 doesn't measure time running on the ground, but this still doesn't justify the excess 1.5 Hobbs hr. I'm not the fastest checklist runner on the ground, but 1.5 on the ground is ridiculous; that's like the same time as a XC leg! My home airport is a Class D and the destination was a class C; both weren't busy at all and I didn't have to wait to taxi and takeoff.

I don't like pointing fingers, and the flight club that I rent from seems to be reputable, so I was wondering if y'all can give me some insight into this before I bring it up to them.

Thanks!
 
I have seen this happen when someone mistakenly installed a 12VDC Hobbs in a 24VDC system.
 
I’ve had Hobbs issues before. Sometimes the transducer that converts oil pressure to an electric current can go wacky. If it’s the oil pressure type. I wouldn’t hesitate to send them the ADS-B data and dispute it. Seems logical.
 
Is it possible that the master switch was on for 2.9, considering its state during preflight et al?
 
I once had 2.3 for 3 touch n goes. The accident on the parallel runway slowed us up some (all were fine).

You paid for it, you log it.
 
I would comment to the FBO. You did check the hobbs before the flight right? Might be a paperwork error Might be calibration. Might be a call to the state bureau of weights and measure as that does seem like an extreme difference.

But it’s another reason to own. Just saying ;)
 
A Hobbs is an electric clock. It knows nothing other than how long it had power applied. You need to understand WHAT is powering the Hobbs on your particular aircraft. It could be connected to the master switch, it could be connected to an oil pressure switch, it could be connected to the gear switch (many Bonanazas and my Navion were set up this way), I've even seen one connected to the heater.

So, did you spend a lot of time on the ground with the engine running or the switches on? Hobbs are pretty simple device. Never seen one fail.
 
Do the plane keys come with a "piece of paper with the in/out times on it"?Maybe the engine was last run by a mechanic, or someone else who was not a renter, who didn't add their flight to the list, and you didn't notice?
 
Is it possible that you wrote down the starting number on the Hobbs incorrectly?

Most of the time I use my phone to take a snapshot of the Hobbs before startup and after shutdown to avoid errors.
 
Did you note the tachometer start and end? Tach time is usually < Hobbs time, but they should be close over such a short time period.
 
flyingron said:
Hobbs are pretty simple device. Never seen one fail.
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I've seen numerous fail, as in stop working, run too fast, or run too slow.

One even alternated between too fast and too slow on every flight.
 
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Initial Fix said:
I would comment to the FBO. You did check the hobbs before the flight right? Might be a paperwork error Might be calibration. Might be a call to the state bureau of weights and measure as that does seem like an extreme difference.

But it’s another reason to own. Just saying ;)
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Thanks for all y'all's insights, gents!

Yes, I did check the Hobbs before taking off. Our Hobbs time are stored on paper and electronically. Before we check the plane out, we have to verify that the starting Hobbs and tach on paper matches the record stored in the computer. I also verified it when I went out to the plane.

As I mentioned in my first post, if I did leave the battery on, then my Hobbs would surely have to be way more than 2.9 hr as we were out in town for about 2 hours.

Unfortunately, I don't remember the tach, but I did record it both on paper and in the computer. I have a subtle memory of it maybe being 1.6? If so, I don't know why it didn't appear as a red flag to me...
 
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To add to the confusion, many (all?) Hobbs meters are electrically wound mechanical clocks. The watch spring - the device that actually powers the clock - runs down in a few minutes after the power is removed. Observe your Hobbs as you shut down, and you will see, and perhaps hear, it running after you secure the engine and master.

-Skip
 
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.
 
pburger said:
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.
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that's what I always thought too...but based on other comments I suppose that's not as typical as I thought.

As a renter, I've always intended to do a better job of noting the actual clock time of start, takeoff, landing, and shutdown (especially the start up and shut down because that is $$) but I'm embarrassed to say I rarely do. Seems like I almost always miss one or more of those because I'm focused on more pressing or interesting things at the time. I'll often remember it a few minutes after the fact.... usually good enough of an estimate, but not always...so I end up trusting the hobbs.
 
Of the many rental planes I have flown, only two that I know of ran whenever the master switch was on, as opposed to an oil-pressure switch.
 
I've known some places to have Hobbs meters tied to the master switch so they start running as soon as power is applied. Most, in my experience, are tied to oil pressure or some other signal so they only run when the engine is running. You'll have to find out on your own.

You can probably check the sign out sheet to see if this has been a problem with anyone else. Every place I've rented from requires tach and hobbs be recorded. So you can check back the last however many flights before yours, and since, and see if there was a problem.

It would be interesting to see if the renter after you recorded a lower hobbs number on start (hobbs out vs in). If that's the case then you might have just recorded the wrong number after you shut down.

I can see it being close to 3 hrs, though.

Start up, warm up, taxi, run up, take off -- maybe 20 minutes (0.3)?

83 nm, maybe about 0.7? (estimating 120 ground speed, no wind)

Approach, landing, taxi, shutdown - maybe another 0.1 or 0.2?

So about 1.2 each way, or 2.4 round trip (Hobbs said 2.9)?
 
Are you accumulating hours towards a rating? Sounds like an efficient way to do it. :D
 
How long did you have the plane rented for? Does the time make sense with the hours you flew the plane? Did you do a very long preflighting with power on, followed by a long slow taxi, run up, etc?
 
At what altitude does FlightRadar "see" you?
Tachometer time would be helpful here.
 
If the flight was about 1.4, then it's reasonable the "real" engine running time was about 1.9, not 2.9. If this might be the case, then it's possible that the times were written down wrong, OR even maybe that the times were read wrong.

I've seen cases where, if the engine is shut down as the Hobbs is about to roll over to a new hour (say, from 1528.9 to 1529.0 as a random example), where it's hard to tell which number is right. If in my example, the .0 had started rotating down but the ones digit (the 8 to a 9) hadn't made it very far down yet, the time could have been read and recorded as 1528.0 instead of 1529.0, thereby giving the last pilot a (hopefully inadvertent, although it should have been very obvious) deal and charging you an extra hour.

Just a possibility. I would definitely be checking with the FBO and looking at the previous flight's times to figure this out.
 
One of the part 135 operators I worked for did flights for the forest service. The forest service had them hook up a switch that would not start the hobbs time until airspeed reached 50kts. Very rare to be that picky, the forest service only wanted to pay for flight time.
 
Tantalum said:
How long did you have the plane rented for? Does the time make sense with the hours you flew the plane? Did you do a very long preflighting with power on, followed by a long slow taxi, run up, etc?
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I reserved a 4-hour block, but it was the last flight of the day, so I returned the plane past the end of the block. I did not do a very long preflight with power on. My passengers did have headsets issue before taxi at our home airport, but I think it only added at most an extra 10 minutes.

Domenick said:
At what altitude does FlightRadar "see" you?
Tachometer time would be helpful here.
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FlightRadar24 seemed to pick me up as soon as I lifted off and landed at my home airport, buts cuts off a little bit early during approach and cuts on a little late during takeoff at the destination
  • From HOME (Field Elevation as we were taking off) to DESTINATION (900 FT AGL)
  • From DESTINATION (270 FT AGL) to HOME (Field Elevation)

Anyways, I just went to the flight club and brought up the issue to them. Talked with their operations manager, and one of their CFI said 2.9 hr hobbs seems normal and that FlightRadar is not always accurate. I also looked at the paper time log for my entry and noticed my tach is 2.04 hr. THe CFI said if you lean and use less-than-full power setting, the tach will be lower than hobbs. As a new pilot, I'm not sure if my 2.04 tach is a big difference or not from 2.9 hobbs. But I do remember leaning to rich of peak at 5,500 FT MSL. He also said time running on the ground is usually 20 minutes (0.3 in hobbs format). He may or not be right, altough I personally felt it wasn't that long. The class C and class D airports that I flew in and out of were not busy. Because I didn't use a timer, I can't say for sure as a new pilot, my sense of time may not be as refined. Lesson learned...

I could've not mistaken my time records because I verified the start tach and the hobbs on the paper (time log) with BOTH the start tac/hobb time on the computer AND the tac/hobb time in aircraft before my preflight.

I did not take pictures of the start tac/hobbs in the aircraft, but like I previously said, I did verify them during the preflight. However, I did take pictures of the end tac/hobbs time. The time values could not have been misread (they aren't XXXX.9 or XXXX.0). The previously pilot who flew before me did not log such values neither.
 
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Can you speak with other members in the flight club about this issue? Perhaps you’re not the first who has had this problem. If you are the first, then others will know to keep an eye on it themselves when they fly that airplane. If several people report a similar problem then the operations manager can’t ignore it anymore.
 
LastXdeth said:
As a new pilot, I'm not sure if my 2.04 tach is a big difference or not from 2.9 hobbs.
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Very little tach time, if any, accumulates during taxi.

If you fly that plane again, you could write down the times of engine start and engine stop from your watch or smart phone. That would tell you if the Hobbs meter is running too fast.

Did the club personnel tell you whether the Hobbs is connected to an oil-pressure switch or directly to the master?
 
Not saying this is necessarily your issue, but I've found that a lot of pilots (myself included) completely underestimate the time it takes from engine start to takeoff, and then from landing to engine shutdown. Especially as a new pilot, Hobbs time from engine start to takeoff can easily be 0.3. And another tenth or two after landing. And then repeat again for the flight back. If the Hobbs is tied to the master (I owned an airplane like this once), it could be even longer.

Then there is the implicit rounding of the Hobbs, where if it ticked over to the next digit 10 seconds after startup and 10 seconds before shutdown, there's an effective automatic extra 0.2. Of course, for the previous renter and then next renter, they get the benefit of that, and so will you at times. Sometimes you win, sometimes you lose, it all averages out.

LastXdeth said:
THe CFI said if you lean and use less-than-full power setting, the tach will be lower than hobbs.
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If the CFI actually said that leaning will lower the tach reading, that is completely incorrect. Now, operating less than full power, yes, that will lower the tach time. But in most cases, the tach time will almost always be lower the Hobbs time. You'd have to have a really short taxi/runup and a long flight at high power to see otherwise on most rental airplanes. So if your tach time was 2.09, then your Hobbs time was almost certainly higher than that. A rule of thumb is about 20%, so that would make a Hobbs estimate of 2.5. And in the possibility of the rounding error mentioned above, and that's 2.7. A little longer on the ground than you expect, and you're at 2.9.

But of course the best solution is twofold:
1) Find out if the Hobbs is on an oil pressure switch, the master switch, or some other switch.
2) Time your next flight with a clock, starting and stopping at the same time as the Hobbs starts and stops.

I do fly a lot of airplanes without a Hobbs, so I am in the habit of using @EricBe 's MyFlightBook logbook app to semi-automatically log time. I press "engine start" and "engine stop" on the app and it does the math. Granted, I don't rent and this would not be a replacement for the rental records, but it certainly would identify a problem in the Hobbs itself.
 
OIl Switch, (one or both engines)
Master Switch, (stupid on a rental)
Squat Switch,
Gear Switch,

Airspeed is a new one, but certainly possible. It doesn't really matter as long as everyone understands and it is priced accordingly.
 
LastXdeth said:
my tach is 2.04 hr
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LastXdeth said:
2.9 hobbs
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Thanks, honestly this ties out.. people don't realize how much time the airplane is "on" vs "flying" .. if you get to the airport at 2 you're rarely in the air before 2:30

As someone else said the tach basically doesn't move when you're idling, and if you pull the rpm back (has nothing to do with lean, but rather, engine revolutions) you can run the tach slower than real time.

So there's 0.9 to account for.. assuming you did a full stop:

0.1 to preflight
0.3 to taxi for departure from airport A
0.25 to taxi back for departure from airport B
0.25 to taxi back to parking at airport A

and voila, you have your 0.9

..also, if that's what the hobb says log it. You're cheating yourself a lot of time by only logging tach or what flightaware says. You can log anytime the plane is moving under it's own power so long as it's moving with the intention of flight, and ends up flying
 
2.04 tach vs. 2.9 hobbs is a significant differential (especially for a cross-country flight) which could only be explained by a really long taxi at both ends or excessive ground idling. The best thing to do is make sure other pilots are recording tach and hobbs times on each flight and check to see if there is a pattern.
 
dmspilot said:
excessive ground idling
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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
 
Tantalum said:
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...
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I have a client (not on this board that I know of, so not you Mike...) who I have seen take 30 minutes between engine start and takeoff, at an uncontrolled field. He is very cautious and meticulous, perhaps to an excess, but it's his airplane and I'm not about to rush him through his checklists. Doesn't hurt that he's paying me by the hour, of course. I do purposefully block extra time when I'm flying with him.
 
dmspilot said:
2.04 tach vs. 2.9 hobbs is a significant differential (especially for a cross-country flight) which could only be explained by a really long taxi at both ends or excessive ground idling. The best thing to do is make sure other pilots are recording tach and hobbs times on each flight and check to see if there is a pattern.
Click to expand...
I generally get tach time at about 75% of flight time. So for flight time of 2.9, I would expect tach time of ~2.18. So 2.04 doesn't seem that far off, especially if a lot of time was spent idling or at less than full power.
 
Tantalum said:
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
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When I said excessive, I didn't mean an extra 15 or 20 minutes. I meant more like an hour, or hour and a half. The tach is still running, just slower.
 
The 172 I used to rent the hobbs worked on the master. Typically the hobbs is approximately 1.2 X the Tach but that is not always the case it depends on what you are doing but that is good number to get you relatively close.
 
Palmpilot said:
Of the many rental planes I have flown, only two that I know of ran whenever the master switch was on, as opposed to an oil-pressure switch.
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Same here, actually, but the people in control of this decision have the most to gain by choosing the aggressive configuration, so I always assume master = meter running.
 
For whatever its worth, my home FBO has a policy that if the Hobbs fails for some reason, rental time is tach * 1.3. So following that formula for the sake of argument, 2.04 tach imputes 2.65 Hobbs or so - not too far off from 2.9. Assuming a rental rate of $175/hour multiplied by the delta between 2.65 and 2.9 (~$44) - the problem may or may not be worth the consternation.
 
The engine doesn't need to be running to close the oil pressure switch. Just cranking the engine will do it and it will continue to be closed for a bit after the engine stops turning (whether it was doing it under its own power or by the starter). I chewed up a good bit of Hobbs time trying to start a balky rental 152 once.
 
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