The difference in one paragraph
A Hobbs meter counts elapsed clock time whenever oil pressure is present. Thirty minutes holding for an IFR clearance is 0.5 hours, whether you are at idle or at full power. A tachometer counts engine revolutions, calibrated so that it reads real time only at a specific cruise RPM — usually somewhere around 2,300 to 2,500. At idle it turns at roughly a third of that rate, so the same thirty-minute hold records about 0.15 to 0.2 hours.
Every partnership has to pick one as the billing meter. It sounds like a bookkeeping detail. It is actually a decision about who pays for whose flying.
Why the choice moves real money
The two meters diverge in proportion to how much time an aircraft spends below cruise power. On a two-hour cross-country at 75% power, Hobbs and tach run almost one to one — the difference might be 0.1 hours. On an hour of pattern work at a busy towered field, tach can run 20 to 25% below Hobbs.
Now put two partners in the same aeroplane. One flies $100 hamburgers and family trips: long legs, high power, minimal ground time. The other is building hours towards a commercial certificate: circuits, holds, taxi delays, practice approaches at reduced power.
Bill on tach and the circuit pilot's hour costs materially less than the cross-country pilot's hour, even though the circuit pilot cycled the engine harder, made twenty more landings on the tyres and brakes, and put the airframe through far more stress. Bill on Hobbs and the cross-country pilot subsidises nobody, but the circuit pilot pays for taxi time in which the engine is barely working.
Neither is objectively fair. Both are defensible. What is not defensible is choosing without discussing it, because the partner who flies the other pattern will eventually notice.
The case for tach
- It tracks engine wear. Overhaul timing, cylinder life and oil consumption follow revolutions, not clock time. If your hourly rate exists mostly to fund the overhaul, tach is the honest denominator.
- It matches the maintenance log. Most piston engine time in logbooks is tach time. Billing on the same basis means your reserve fund and your engine's remaining life move together.
- It cannot be gamed by ground delays. Nobody gets an expensive bill because ground control put them number six for departure.
The case for Hobbs
- It is what everyone already understands. Rental aircraft bill Hobbs. New partners arrive expecting it.
- It captures airframe and cycle costs. Brakes, tyres, nosewheel shimmy dampers, starter and alternator wear all correlate with time and cycles, not RPM.
- It is harder to dispute. A Hobbs reading is a number on a clock. Tach readings on older aircraft can be genuinely ambiguous, and some tachs are simply inaccurate.
- Recovery is easier. Because Hobbs hours are higher, the same rate collects more money — which matters if your group habitually under-reserves.
What most well-run partnerships actually do
The common resolution is to bill on tach and set the rate accordingly. If you would have charged $90 an hour on Hobbs, and your tach typically runs 12% low, then $102 on tach collects the same money over a year. Partners flying patterns pay less per clock hour, which is arguably right because they are wearing the engine less, and the fund still fills up on schedule.
The failure mode to avoid: switching from Hobbs to tach without adjusting the rate. That is an instant 10 to 20% cut in revenue into the reserve, and nobody notices until the overhaul quote arrives.
Getting the conversion right
Do not guess your ratio. Measure it. For the next ten flights, record both readings. Divide total tach by total Hobbs across all ten. That is your group's real factor, and it will be specific to your aeroplane, your airport and your mission mix — a partnership based at an uncontrolled field in Kansas will see a very different number from one at a Class B satellite.
Typical observed ranges:
- Long cross-country flying, uncontrolled field: tach ≈ 0.93 to 0.97 × Hobbs
- Mixed personal flying: tach ≈ 0.88 to 0.93 × Hobbs
- Training and pattern work, towered field: tach ≈ 0.75 to 0.85 × Hobbs
Write it down
Whichever you choose, put three things in your operating agreement: which meter is the billing meter, what the rate is, and the procedure when the meter fails. Meter failures happen, and the time to agree how to handle them is not the evening someone lands with a dead tach and no idea what to write in the book.
A workable failure rule: if the billing meter is inoperative, bill on the other meter using the group's measured conversion factor, and note it on the flight record so the correction is visible later.
One thing worth automating
Whichever meter you pick, the arithmetic that follows is the same every flight: meter delta, times rate, minus any fuel the pilot bought, with the reserve portion split off into the engine fund. Doing that by hand once a month for three or four partners is exactly the job that quietly becomes one person's unpaid second job. Set the meter type once and let the software do the rest.