Hobbs issue

H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

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rcpilot
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Hobbs issue

Post by rcpilot »

So I had my plane over to the avionics shop to have the bi-annual transponder check and to have a PTT button for the radio installed on the co-pilot side of the dash. My instructor and I had to fly the plane to another airport about 20 minutes away. My instructor flew back by himself. Before securing the plane, he checked the hobbs and noticed it had acquired about 9 hours since the trip to the shop. I verified this on the entry in my airframe log book it shows 533.9 hours. Now the hobbs shows 544.1 hours. Unless my instructor flew some kind of space/time distortion on his way back, that's odd. Anyone have any idea what may have caused this? We haven't had a chance to fly again to see if it's keeping accurate time(the anomaly was a one-time thing)or if it's running "fast".
ct4me
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Re: Hobbs issue

Post by ct4me »

Mechanical Hobbs, or Electronic?
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Re: Hobbs issue

Post by rcpilot »

ct4me wrote:Mechanical Hobbs, or Electronic?
Mechanical
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drseti
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Re: Hobbs issue

Post by drseti »

Actually, most Hobbs meters merely require that the oil pressure sensor switch be closed in order to operate. That normally occurs only when there is oil pressure, but switches can stick! I've had my Hobbs meter advance while the avionics shop was doing my static system, encoder, and altimeter checks -- and I know for sure the engine wasn't running.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV
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AvSport.org
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roger lee
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Re: Hobbs issue

Post by roger lee »

Many Hobbs now days require 15 psi oil pressure. Many US Rotax engines are in this category especially if they have digital instrumentation. This keeps people from just turning on the electrical system and accumulating time.
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rcpilot
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Re: Hobbs issue

Post by rcpilot »

drseti wrote:Actually, most Hobbs meters merely require that the oil pressure sensor switch be closed in order to operate. That normally occurs only when there is oil pressure, but switches can stick! I've had my Hobbs meter advance while the avionics shop was doing my static system, encoder, and altimeter checks -- and I know for sure the engine wasn't running.
That could explain it then because that's just what I had done. I guess I'll find out tomorrow when we fly.
MackAttack
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Re: Hobbs issue

Post by MackAttack »

The G3X displays both oil pressure time and engine time ... is that what causes the variation; the oil pressure is high enough to move that meter but the engine isn't running fast enough? the oil pressure time in my soon-to-be-new LSA is higher by 10-12 hours than the engine time ...
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Re: Hobbs issue

Post by drseti »

What you call oil pressure time is what we generally call Hobbs time. What you're calling engine time is usually referred to tach time, and is related to RPM. That is, one hour of flight at max RPM shows 1 hour tach time, while 1 hour flown at 50% RPM indicates 0.5 hours of tachieving time. It is important to note that Rotax maint manuals specify that all maintenance intervals a d TBOs are based upon Hobbs, not tach time. Continental and Lycoming maint intervals, OTOH, are generally based on tach time.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV
[email protected]
AvSport.org
facebook.com/SportFlying
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3Dreaming
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Re: Hobbs issue

Post by 3Dreaming »

I looked at the picture of your panel in the other thread, but I couldn't clearly see a hour meter. There are amost as many ways to wire an hour meter as there are different meters. They can simply be wired to run when the master is on, or by several different ways to switch them on. The most common is a oil pressure switch, but others include airspeed, chargind system, or squat switches.
The most likely cause of your hour meter being off is from the avionics shop doing something that had the hour meter powered up.
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Re: Hobbs issue

Post by rcpilot »

That could be since they were trying to figure out why the radio would cause the trim indicator lights to peg. Hopefully that the reason and it will behave now. The hobbs is all the way on the bottom right side of the panel.
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Re: Hobbs issue

Post by drseti »

It's not at all usual for radio frequency interference (RFI) to make something in an EFIS go wonky. If the problem goes away when you unkey the microphone, it's a non-issue. As Microsoft likes to say, "it's not a bug, it's a feature".
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV
[email protected]
AvSport.org
facebook.com/SportFlying
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Re: Hobbs issue

Post by 3Dreaming »

In your picture it must be covered by the Post-it note. You might try turning on the master switch and see if it runs while the switch is on. At least that would solve the mystery.
MackAttack
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Re: Hobbs issue

Post by MackAttack »

drseti wrote:What you call oil pressure time is what we generally call Hobbs time. What you're calling engine time is usually referred to tach time, and is related to RPM. That is, one hour of flight at max RPM shows 1 hour tach time, while 1 hour flown at 50% RPM indicates 0.5 hours of tachieving time. It is important to note that Rotax maint manuals specify that all maintenance intervals a d TBOs are based upon Hobbs, not tach time. Continental and Lycoming maint intervals, OTOH, are generally based on tach time.
Thanks for clearing that up. I knew it had to be one way or the other, being used to the other engines. Because Rotax engines are flown mostly at or close to max RPM (top of the green), then that explains why the two numbers are probably pretty close for most of these aircraft ...

Cheers!
roger lee
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Re: Hobbs issue

Post by roger lee »

Are you reading Hobbs time or Tach time. You're better off with Hobbs. Tach time can vary from 15%-18% depending on engine rpm and some older style tachs registered time with just the electrical on. Al Hobbs and Tachs should have an oil pressure switch installed. You usually can reset the proper time on either if needed.
Roger Lee
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rcpilot
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Re: Hobbs issue

Post by rcpilot »

roger lee wrote:Are you reading Hobbs time or Tach time. You're better off with Hobbs. Tach time can vary from 15%-18% depending on engine rpm and some older style tachs registered time with just the electrical on. Al Hobbs and Tachs should have an oil pressure switch installed. You usually can reset the proper time on either if needed.
My plane only has a hobbs. Next time my mechanic woeks on it I'm going to see if we can verify how it's actually connected. Flew yesterday and it seems to be working OK. I'll keep an eye on it.
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