Hobbs Time!!
Thread Starter
Join Date: Nov 2016
Location: Droitwich
Posts: 12
Likes: 0
Received 0 Likes
on
0 Posts
Hobbs Time!!
Hi all. Maybe someone on here can give an definitive answer to the rumour that I've heard going around. And that is:
Does flying at a higher RPM cause the Hobbs meter to actually go faster, therefore giving the impression more hours have been flown?
Does flying at a higher RPM cause the Hobbs meter to actually go faster, therefore giving the impression more hours have been flown?
Join Date: Sep 2004
Location: Down at the sharp pointy end, where all the weather is made.
Age: 74
Posts: 1,684
Likes: 0
Received 3 Likes
on
3 Posts
Hobbs meters are basically fairly accurate clocks. They can be started or stopped in a variety of ways but are all independent of engine RPM. The term 'Hobbs' has become ubiquitous, like 'Hoover' has for any vacuum cleaner. Datcon timers are pretty popular, too. They are most commonly activated by the aircraft electrics and so record time from master 'on' to master 'off' Somes are activated by engine oil pressure whilst a very few are turned on and off by airspeed (typically 40 knots, so they effectively measure airborne time).
Don't confuse a timer like the Hobbs or Datcon with the tachometer, which measures engine RPM and is usually associated with the RPM gauge. Typically, a sensibly operated engine will register around .75 or .80 per operating hour.
TOO.
Don't confuse a timer like the Hobbs or Datcon with the tachometer, which measures engine RPM and is usually associated with the RPM gauge. Typically, a sensibly operated engine will register around .75 or .80 per operating hour.
TOO.
Join Date: Jan 2008
Location: Canada
Posts: 631
Likes: 0
Received 0 Likes
on
0 Posts
To expand on TOO's post, the hour meter in a completely mechanical tachmeter expresses hours, though is really counting revs. Most of those tachometers have an RPM printed on the side at which the hour reading will be precise. I recall 2350 RPM as being fairly common. So yes, if you're running the engine at a faster RPM than the value printed on the mechanical tach, it will accumulate "hours" very slightly faster than real time. On the other hand, you won't be able to sustain level flight at a power significantly less than the "value", so it's not really possible to get free flying that way.
The "Hobbs" is entirely electric, and will either be wired to the aircraft master switch, or to an oil pressure switch from the engine. They will always be right on hour for hour.
The "Hobbs" is entirely electric, and will either be wired to the aircraft master switch, or to an oil pressure switch from the engine. They will always be right on hour for hour.
Following on from Step Turn, a lot of Cessna's use 2300 RPM (120, 140, 170, 180, 182, 188, 336, 337) and others 2566 RPM (150, 152, 172, 177, 206, 207,210, 310, 320, also a number of Bonanza models), 3000 RPM (Cessna 175), 1800 RPM (P & W Stearman). Check your own aircraft though.
Join Date: Jul 2012
Location: Barbados
Posts: 411
Likes: 0
Received 0 Likes
on
0 Posts
The Hobbs records time - the time can be from when the master is switched on or the engine started (or some other detectable event).
Personally I have a pressure switch on my engine so when I get oil pressure the Hobbs kicks in - I suppose that if the engine quits I will "miss" the 0.1 or so as I glide to where I land.
The one which varies with engine speed is the tach - so, with a non-CP, if one is pootling around at 1700 rpm one will get tach hours less than Hobbs - similarly running full throttle and one gets tach higher than Hobbs.
Personally overall I find my Hobbs to be higher than tach - but when renting wet (billed on Hobbs) to those who are doing a long cross country the figures come back very close as they run nuts out and they get no benefit from less fuel burned.
Personally I have a pressure switch on my engine so when I get oil pressure the Hobbs kicks in - I suppose that if the engine quits I will "miss" the 0.1 or so as I glide to where I land.
The one which varies with engine speed is the tach - so, with a non-CP, if one is pootling around at 1700 rpm one will get tach hours less than Hobbs - similarly running full throttle and one gets tach higher than Hobbs.
Personally overall I find my Hobbs to be higher than tach - but when renting wet (billed on Hobbs) to those who are doing a long cross country the figures come back very close as they run nuts out and they get no benefit from less fuel burned.
