Hobbs v Tacho

[Lagentium]

Good evening Ppruners,
Another bit of advice needed please. Our group agreements are all coming together nicely, but we seem to have hit a snag on whether we should bill the flying time on the Hobbs meter or Tacho, could someone please explain the pros and cons to me as I haven't got a clue!

Thanks in advance, Jim:ok:

[riverrock83]

Hobbs = time the aircraft is in a certain state, normally the time that the oil pressure is above a certain psi.
Tacho is a count of the number of revolutions of an engine which has been converted into a time based on the engine being at a specific (normally cruise?) RPM.

If you bill based on tacho, people are more likely to use a more gentle cruise speed and pilots are not going to hurry taxiing (as the rpm is lower and so you pay less). Of course - it will also depend on whether your bills include fuel (but I'm sure thats on a different thread).

In my share we bill on airborne time plus 5 min. This means there is no incentive to rush power checks, etc although it does mean that student pilots doing efficient (ie relatively slow and well leaned) cross country cruising probably slightly subsidise those doing aerobatics...
It also matches engine maintenance schedules as they are based on actual time rather than hobbs / tacho.
That and we trust each other to record the details accurately!

[BackPacker]

It doesn't really matter. Just add up all the costs you made in a certain period, except for the costs that can be allocated to a single flight (e.g. landing fees). Divide by the total tacho or hobbs time accumulated in that period. Then for each flight multiply that amount by the amount of tacho or hobbs that that flight generated.

You will find that the numbers will be very, very close together. The difference will mostly be because of rounding.

The only time where you will see a significance difference is where the members of your group have wildly different usage patterns. Like when one member does all-out aerobatics, and the other uses the aircraft as an aerial observation platform at minimum sustainable RPM.

[peterh337]

If you totally trust the group members, bill by airborne time.

[Maoraigh1]

Bill by tacho. As a group, it'll even out over time. Logging time in air is a nuisance - I'm sometimes busy with other things (like controlling a taildragger in a gusty wind) immediately before take-off and after touchdown.
Even if you trust each other at present, using tach means nobody starts to suspect anyone.