Hobbs hours vs plog hours?

[LH2]

I'm adding up my hours from the last few days, and there is a difference of 25 minutes over 19 hours (11 flights). The strange thing is that the Hobbs reading is less than the hours as counted from the plog times (brakes-off, brakes-on).

I did not misread the Hobbs, as the difference between first and last reading equals the partials. The brakes-off/brakes-on hours are contrasted against the ADF timer, so while they might be off by a minute or so, the total flight time is likely to be correct (short if anything as I'm not counting the time required by post start-up checks).

Is it possible for the Hobbs to be slow?

Any other ideas appreciated.

[NuName]

You dont say what type it is but if it is retractable the the hobbs may well be inhibited on the ground by the squat switch. This ensures flight time only is recorded.

[gfunc]

Hi LH2,

In the 172 I float around in the Hobbs time is always less than the tach time, I've been told that the Hobbs is calibrated in terms of RPM, in the case of the aircraft I fly, 2400RPM (cruise setting). In other words if the engine is running at 2400RPM for an hour the Hobbs will read 1 hour, whereas if running at 1200RPM for an hour the Hobbs will read 1/2 an hour. I guess your discrepency is because of the time on the ground, descent etc.

I think is so the operator can gauge fuel costs and avoids me having to pay extra for other people who try and save cash by cruising around at full throttle! Sort of gives you an incentive to try thermalling in a 172 (it works!).

Cheers,

Gareth.

[dublinpilot]

gfunc,

I believe what you are describing is the Tacho, not the Hobbs. The tacho counts engine revolutions.

dp

[LH2]

I've been told that the Hobbs is calibrated in terms of RPM

Bingo! I guess that's what it is. If anything, it beats the explanation advanced by my partner (no, I didn't run out of fuel )

Sort of gives you an incentive to try thermalling in a 172 (it works!)

I know

Thanks for the replies.