The International Cessna 170 Association

So... I did it. I left my master switch on for two days and ran down my battery. That ended up not being a big deal. I connected a charger and not much later it started up like a champ.

My dilemma is, apparently my Hobbs meter runs when the master switch is turned on and I gained 48 hours of Hobbs time. How do I (or do I need to) document this?

I’m new to airplane ownership. Does Hobbs time even matter or is everything airframe and engine based on Tach Time?

Thanks!
Doug

Airframe and engine time recorded in the logbooks are actual times calculated from tach time. If the tach is original to the plane from the beginning, tach and actual time will be the same. But if the tach was replaced at some point with a zero time tach (like happened with my plane), then time must be added to the tach time to get the true total time. So, for example, my tach was replaced with a zero time recording tach at 212 hrs, so I must add 212 hrs to tach time to get actual total time. My logbook entries show tach time, then total time, which is tach time + 212. Hobbs time is irrelevant. A lot of planes (like mine) do not even have a hobbs meter. I hope this is not clear as mud.

This is one of the things I just love about the piano switches.

As long as they all line up, the Master is off.

Sorry to hear about yours being left on Doug

I have learned too

Walk into hanger - turn on Master Switch - look at panel - some instruments light up. Don't know why.

Walk around to front of Wing - Push up on STALL switch - like the sound of Stall switch while it is on the ground tied DOWN!

Back to cockpit - TURN OFF the Master switch.

What ever I am doing - Will check for M.S. OFF at least four(4) Times.

Pays to be Methodical.

I think every body has done that [forgot to switch off the master switch ] ,me included . It’s when you are tired ,distracted or you are thinking about that beer in the fridge at home is when most mistakes are made . What I have done is my rotating beacon is always switched on and is only switched off when power is cut from the ship by the master switch. And to think I was the only human on the planet that makes mistakes.

We have all done that. I leave my original beacon on as a warning. It is so LOUD you can hear it with the hangar doors closed while I sit on the couch in WaBuchanan’s hangar and drink his beer.

Back in the day, one of our favorite things to do at 170 fly-in's after a long day at the airport and on our way to a motel in the airport van, was to call out, "hey did you turn your master off "........ya got a lot of those blank eyed looks

I did a really dirty deed one time as a practical joke that nearly backfired... When flying for the state of Tx we had a trip to LBB that required a flight of 4 King Air 200 aircraft. Our “chief” pilot (we had a lot of chiefs and few indians) was a guy named Bob and he was the first to arrive so he took the crew car and left for the hotel leaving the rest of us to stand around waiting for a crew-bus to get the rest of us to the hotel.

I walked up to the cute little counter-girl at the FBO and asked her if the first King Air to arrive left any contact info... and she replied “Yes, he gave me his cell phone number!” (of course I knew he’d make certain the cute FBO girl would have his cell number).

So I told her that there was “some flashing lights inside the cockpit of that first King Air... perhaps she might wish to call him and let him know” .... (The King Airs had “master caution” flashers in the cockpit if the master were “on” and a discrepancy existed, such as no generator-charge, no oil pressure, etc, etc.,...or if a Master Switch were left in the “on” position and the airplane abandoned.)

I knew that this would cause the selfish “chief” pilot to have to drive back to the airport in the crew car with which he’d selfishly absconded/commandeered ... while the rest of us pilots/co-pilots were enjoying a beverage in the hotel cocktail lounge.

It was a completely made-up scenario ...he’d not left the master “on”... I just wanted to cause him the grief/inconvenience of having to return to the airport as punishment for being so un-thoughtful of the rest of us who arrived only minutes behind him.... and give the rest of us some needed-laughter at his expense.

Back-Fire! I did not know that the Director of our agency (the state “aircraft-pool”) was actually in LBB ahead of our flights and that the “chief” pilot Bob would be hob-knobbing (sucking-up as usual) with him... so, Guess Who drove back to the airport with Bob to discover it was only a practical joke perpetrated by .... ME!

Ahhh....the best laid plans o’ mice and men....

OK Doug... you’ve now learned the secret of forgetful pilots... we leave our strobes/anti-collision lights ON and never turn them off except via that aircraft Master-Switch. (When you get to the FBO entrance-door... the correct answer to the linecrewman’s question about that flashing light is: “Oh, Yes.... I was just checking to make certain it still flashes...I was just about to return to the airplane to turn it off. Thanks!”

As for the comment that the engine times (or airframe times) are “always” based upon the tach-time (and not the hobbs/hour meter).... that may or may not be true. Aircraft which have both types of gauges typically have logbook entries entered in either OR BOTH... in which case you should look at your records to see what your mx guys have been doing. Hopefully there is a correlation which can be made which can be used to notate the now-disunified readings and your logs can be so noted.
Another thing you might do is to make up a small sticker/lable which you can place on the hobbs/hour mtr which says “subtract 48 hrs” or something similar.
Yet another thing is to DISCONNECT that hobbs/hour mtr for the next 48 hours of operational time of the aircraft to bring the two back into agreement.
Lastly, you might wish to install another automatic-switch which must be activated in order for the hobbs/hour mtr to begin timing. This can be an oil-pressure switch (very common) placed at the engine oil pressure gauge fitting. When engine oil pressure exists...the hobbs/hour mtr receives electrical power and begins to “run” and when the engine oil press falls at shut-dn the meter stops. Another method is to install a “wind-driven” switch... a small vane-type switch that only runs when relative wind from flight closes the switch that supplies elect-pwr to the hobbs/hour mter. (Less common on light-aircraft but available in the market place.)
I suggest the oil pressure type which can be bought from usual aircraft suppliers as well as the National Aircraft Parts Agency (acronym’d NAPA... which also sells auto parts.). I actually have such a switch removed from my Baron (previous owner had some crazy plan of having a hobbs for each engine which I removed.). You can have it for the asking...send me your shipping address via PM or email.

Wow... It is good to hear that I'm not alone.

It was one of those deals where I was just taxiing my plane around the airport and was out of the ordinary. I should know better than to shortcut anything. I can say I've learned my lesson now.

Past planes that I have flown all had the oil pressure switch to drive the Hobbs. I made an assumption this one did as well. gahorn, thanks for the offer, I'll take you up on that and will send you my address in a PM.

When I bought this plane a few months ago I went through every logbook entry and tried to make sense of tach time vs airframe time vs. engine time vs. SMOH time. It seemed like each mechanic had their own preferred way of documenting it. Luckily I was able to make sense of it. I think will be adding an entry to my log book stating that the master switch was left on and that 48 hours went by with no change in tach time.

Thanks again, everyone!
Doug

Or you can just simply remove the Hobbs Meter, unless you are using it for training, and be done with one more item that most of our vintage planes don't have or need.

If I may piggy-back on George's comments, there are two types of oil pressure switches. Old autos with "idiot lights," used a switch that connected its terminal to the housing (ground) when the oil pressure fell too low, thus turning on a warning light in view of the driver. This would NOT be the type of oil pressure switch to use for a Hobbs application. The other type of oil pressure switch is one that OPENS when the oil pressure is low, but CLOSES when pressure is present. This was less common, but should still be available. If someone out there has a part number, perhaps they can chime in. The easiest way to connect such a switch into the Hobbs circuit is to connect the Hobbs ground wire to the pressure switch terminal, and it's power wire to a circuit that is powered when the master is on. Thus it will have a +12 VDC any time the master is on, but will be grounded (running) only when the engine is generating oil pressure. This same concept works equally well if you wish to track running time for tractors, stationary generators, and the like.

Much to my own embarrassment over my disorganized junk-parts boxes... I have not yet found that oil pressure switch for Doug...

But just to help anyone else looking for this item it can be found at Aircrft Spruce for about $35.

https://www.aircraftspruce.com/catalog/ ... ckkey=3855

One more comment on this subject ... another option is to remove your hobbs and buy a new (zero-time) one ... and hook it up to a battery for sufficient time to bring the new one into compliance with the desired reading. (My grandkids left the key on in their go-cart and this was the solution I chose...bought a cheap $5 one off the internet and hooked it up to a battery charger for a almost a week so I could keep appraised of the oil-change-schedule.)

Don't know if this issue has been remedied yet, but the really obvious solution is to simply disconnect the Hobbs meter, noting how much extra time it is showing. Then, fly the airplane, with a large note posted by the tachometer listing the tach time when the Hobbs must be RECONNECTED so it will be back to its correct reading. Of course some will point out that the tach is only accurate at its calibrated RPM, but this should get it close enough for most of us mortals.

Karl Towle wrote:Don't know if this issue has been remedied yet, but the really obvious solution is to simply disconnect the Hobbs meter, noting how much extra time it is showing. Then, fly the airplane, with a large note posted by the tachometer listing the tach time when the Hobbs must be RECONNECTED so it will be back to its correct reading. Of course some will point out that the tach is only accurate at its calibrated RPM, but this should get it close enough for most of us mortals.

My A&P and I also discussed this solution... Would have been acceptable, but I like to use the Hobbs meter to log time (I take a picture of it after each flight so I can do my logbook entry when I get home). I ended up just putting a short entry in my aircraft log with the before and after times stating no flight during that time. I also added an oil pressure switch at the next annual. It is good to go now.

dstates wrote:

Karl Towle wrote:Don't know if this issue has been remedied yet, but the really obvious solution is to simply disconnect the Hobbs meter, noting how much extra time it is showing. Then, fly the airplane, with a large note posted by the tachometer listing the tach time when the Hobbs must be RECONNECTED so it will be back to its correct reading. Of course some will point out that the tach is only accurate at its calibrated RPM, but this should get it close enough for most of us mortals.

My A&P and I also discussed this solution... Would have been acceptable, but I like to use the Hobbs meter to log time (I take a picture of it after each flight so I can do my logbook entry when I get home). I ended up just putting a short entry in my aircraft log with the before and after times stating no flight during that time. I also added an oil pressure switch at the next annual. It is good to go now.

Use a “jumper” to activate that oil pressure switch while it sits in the hangar and that meter will eventually “catch up”. (But don’t do what I did to the grandkids go-cart…. forget it’s out there building time….for when I was merely trying to get the new meter to read approx 25 hrs (the estimated time since the go-cart was new)…I forgot about it for almost 800 hours! (I had taken a perfectly good meter and made it almost worthless for gauging when to change the oil. This is sorta like the definition of “Inflation”….. when the gov’t taking a valuable commodity….like paper….. And makes it worthless …by applying ink.