Hobbs vs Tacho: what actually counts when splitting costs

Every shared-aircraft treasurer eventually hits this question: do we bill by Hobbs or Tacho? The two meters sit side by side on most panels and count time differently. The difference shows up on every invoice. It's worth being explicit about which one you use — and why.

What each meter measures

Hobbs runs whenever the engine is running (and typically while oil pressure is above a threshold, or while the master is on — it depends on the airframe and installation). In practice, Hobbs is wall-clock engine-running time.

Tacho runs at a rate proportional to engine RPM, calibrated so that one "Tacho hour" roughly equals one hour at cruise RPM (commonly around 2,300 RPM on a four-cylinder Lycoming). Taxi and low-power ground operations accumulate Tacho time slowly; cruise accumulates it 1:1.

Same block of engine time, two different numbers. A flight with long taxi and a short cruise leg might read 1.2 Hobbs and 0.9 Tacho.

Which one reflects engine wear?

Tacho is closer. Engine wear is driven mostly by RPM-hours at power, not by minutes of ground idle. The whole point of tachometer-based time is to measure the work done by the engine — which is what correlates with time to overhaul.

If your aircraft's overhaul reserve is being funded per flight hour, Tacho is the more defensible unit. You're charging in the same currency you're saving toward.

Which one is "fairer"?

It depends on who you're being fair to.

• Tacho favours the pilot who does a lot of training, pattern work, or short hops. Ground and taxi time don't count as heavily.
• Hobbs favours the club that wants a simple, one-to-one accounting of "aircraft in service" time, including time spent getting to and from the runway.

Many long-running partnerships settle on a hybrid: bill Tacho for hourly rates, but use Hobbs to trigger maintenance intervals. Pilot-Next supports this — each aircraft can have both methods configured, and you pick which one drives invoicing.

What most clubs actually do

From 15 years of running partnership software and talking to clubs:

• Flight schools and partnerships with heavy taxi / pattern operations tend toward Tacho.
• Cross-country-heavy ownership groups tend toward Hobbs — the difference is small, and Hobbs is easier to read and verify.
• Clubs that fund overhaul reserve by flight hour tend toward Tacho.

None of these are wrong. The important thing is that the group is explicit about it, consistent about it, and that every invoice reflects the same meter that was recorded.

In Pilot-Next

• Each aircraft has a defaultBillingMethod — HOBBS or TACHO.
• Each pilot can have a preferredBillingMethod, in case your club offers both options.
• At settlement, the app reads the start and end of whichever meter applies, multiplies by the rate, and produces one invoice line.

No drama. Just consistent numbers feeding the ledger.

The real lesson

Don't agonize over the right meter. Pick one, write it into your club bylaws or partnership agreement, and apply it consistently. The difference between Hobbs and Tacho across a year of flying is usually 5–10%. The difference between well-run books and messy books is much more expensive than the choice of meter.