Setting Hobbs after it went down

B

Bill Jeffreys

Filing Flight Plan
Joined
Jan 16, 2022
Messages
17
Display Name
Display name:
breezyD77
What happens when a hobbs meter goes down and thus stops recording. When you fix it, how do you know what to set it to? Would you do the same for calculating PIC time accumulated while hobbs was down?
 
As far as I know, the Hobbs is only useful for measuring elapsed time from engine start to engine stop for a particular flight. Subtract "Hobbs out" from "Hobbs in" and the difference is what you pay for, at most rental places anyway, and what you can write in your logbook. Otherwise, unlike the recording tachometer, there's no significance to the numbers displayed. Fix the Hobbs (if you want to) and start it at zero, or anything else. I have one in my airplane, but I doubt if I've even looked at it to see if it works. I use tach time for my logbook - yeah, a little less than what a Hobbs would show, but close enough, and I don't have to squint to read it.
 
Bill Jeffreys said:
how do you know what to set it to?
Click to expand...
Some new hobbs meters can be preset to a specific time, however, most new hobbs are installed with "0" and the log entry will explain the change in time indicated. On a side note, there could be other causes of a hobbs not working such as a bad oil press switch (if installed), broken wire, blown fuse, etc.
Bill Jeffreys said:
Would you do the same for calculating PIC time accumulated while hobbs was down?
Click to expand...
FYI: outside of pilot time recording, if the hobbs was used for maintenance tracking or aircraft Time-in-Service recording you'll need to ensure the continuity of those times as there are regulatory requirements for such tracking.
 
Ok, I guess it's a non-issue then. I will just keep continue the Hobbs from the point where it stopped and use the tach time for my Pilot logbook. thanks all.
 
Mine stopped working. I had it removed. I own the plane. All MX revolves around calendar time or tach time.

Why do you want to keep it?
 
Bill Jeffreys said:
Ok, I guess it's a non-issue then. I will just keep continue the Hobbs from the point where it stopped and use the tach time for my Pilot logbook. thanks all.
Click to expand...
You ALWAYS want to use Hobbs for flight time and the Tach for maintenance. The Tach runs about 80 percent slower than the Tach. So if you have required maintenance every 100 hours, wouldn't you want to use the tach just because it will save your money?
 
Greg Bockelman said:
The tach runs about 20% slower than the Hobbs.
Click to expand...
That's not a universal constant. Most mechanical recording tachs read revolutions divided by some nominal RPM.

Flight time (pilot time) or time in service (maintenance time) really want clock time, but the FAA is pretty lenient with how you determine that as long as it's close and consistent. My plane has had at least four different tach/hour meters over the 72 years. Each time we just note in the log book how to correct the value the current one is giving to the total time.
 
Bill Jeffreys said:
Ok, I guess it's a non-issue then. I will just keep continue the Hobbs from the point where it stopped and use the tach time for my Pilot logbook. thanks all.
Click to expand...
I'm not sure what you are saying here.

Tach numbers measure engine time based on tach readings for aircraft recurring maintenance and ADs.

Hobbs is a meter (invented by John Weston Hobbs) to measure use independent of ups and downs of tach readings, it's primary purpose is pretty much rental. Some start electrically with the master, some with an oil pressure switch, some with....

Both have been used (tach typically with a fudge factor) as an acceptable estimate of "flight time, " which, if you want to be exact, is writing the time down "when an aircraft moves under its own power for the purpose of flight" and "when the aircraft comes to rest after landing."
 
If the Hobbs is your only timekeeper (some airplanes do not have a recording tachometer) then you would have to manually keep track of the hours flown until the Hobbs is fixed. At that point you would either have to maintain that correction factor to calculate total time or you could simply hook 12 volts up to the meter and let it run for the number of hours needed to catch up.
 
midlifeflyer said:
Tach numbers measure engine time based on tach readings for aircraft recurring maintenance and ADs.
Click to expand...
Not universally. Modern Bonanzas didn't come with a recording tach. My Navion hasn't had one in over ten years. The time in service is computed using an hour meter (Hobbs).

Mechanical recording tachs don't measure time at all. All they do is divide the revolutions by some nominal value. Some of the modern electronic replacements actually do measure time above a certain thershold.

The FAA doesn't really care as long as you are consistent. The actual definition of TIME IN SERVICE which is the thing used for various maintenance functions is the time the aircraft is off the ground. A Hobbs meter connected to the squat switch on the gear is a great way of doing that, but much of the fleet doesn't have that, so they will accept the tach as an approximation.
 
kshaw said:
You ALWAYS want to use Hobbs for flight time and the Tach for maintenance. The Tach runs about 80 percent slower than the Tach. So if you have required maintenance every 100 hours, wouldn't you want to use the tach just because it will save your money?
Click to expand...
How does the tach run slower than the tach?
 
Silvaire said:
or you could simply hook 12 volts up to the meter and let it run for the number of hours needed to catch up.
Click to expand...
Or have the hour meter vendor preset the time to your specific time.
 
flyingron said:
Not universally. Modern Bonanzas didn't come with a recording tach. My Navion hasn't had one in over ten years. The time in service is computed using an hour meter (Hobbs).

Mechanical recording tachs don't measure time at all. All they do is divide the revolutions by some nominal value. Some of the modern electronic replacements actually do measure time above a certain thershold.

The FAA doesn't really care as long as you are consistent. The actual definition of TIME IN SERVICE which is the thing used for various maintenance functions is the time the aircraft is off the ground. A Hobbs meter connected to the squat switch on the gear is a great way of doing that, but much of the fleet doesn't have that, so they will accept the tach as an approximation.
Click to expand...
Interesting. Thanks.
 
Bill Jeffreys said:
Ok, I guess it's a non-issue then. I will just keep continue the Hobbs from the point where it stopped and use the tach time for my Pilot logbook. thanks all.
Click to expand...
Pretty much, a non issue. Like what Pilwat said, it just a number. It doesn’t represent the age of anything.

My hobbs had the fuse removed and was for years so it was no where near tach time when I bought it.

Long story short, I got a new hobbs when I got a new panel. So now I know how many hours I have flown my new panel at glance! Lol
I use my hobbs for my logbook. I compare my hobbs and the timer in my 650 gps before I shut down the avionics switch.
 
I have the EI MVP50 for maintenance time now. It was set to the hours that were on the Hobbs that it replaced. I should set it to the actual total airframe time. One of these days I got to talk to EI Dave about doing that.
 
FWIW: it one wishes the most accurate method to record true maintenance time in service, connect a hobbs or similar meter to a landing gear squat switch or to an airspeed switch. This will ensure the only time recorded is when the aircraft is airborne as per the TIS definition.
 
Just write down the time from when you start your taxi to when you park on the ramp again. Easy enough to do.
 
You must log in or register to reply here.
Back
Top