Block Time vs Hobbs: Why Your Billing Basis Shouldn't Be Dictated by Your Software

Every flight a school bills has to be measured in some unit, and that unit — the billing basis — quietly shapes what students pay, how instructors and aircraft owners are compensated, and whether your invoices match your agreements. It is a genuine business decision. Yet at a great many schools it was never actually decided; it was inherited from whatever the scheduling software supported out of the box, which in much of the market means Hobbs time and nothing else.

That is backwards. The basis you bill in should follow from how your operation works — where you fly, how your aircraft are supplied, what your owner or lease agreements specify, and how you want to be fair to students — and the software should then apply whatever basis you chose. When the tool only knows one basis, the causation flips: the software dictates the business, and the school ends up working around its own billing convention because the system cannot express the right one.

This matters more than it sounds, because the differences between bases are real money and real fairness. This post lays out the four ways flights get measured for billing, why the US market defaulted to Hobbs and software followed, the concrete situations where Hobbs is the wrong choice, and what it looks like when the basis is something you configure rather than something you are stuck with.

There are a few distinct units in play, and they are not interchangeable — the same flight produces different numbers depending on which you use.

Hobbs time is the elapsed time recorded by a Hobbs meter from engine start to engine shutdown. It is the standard billing unit at most US flight schools: simple, visible on a meter in the panel, and intuitive to students. It counts every minute the engine runs, including taxi and any time spent holding.

Tach time is engine time recorded by the tachometer, which counts hours based on engine RPM relative to a calibrated cruise setting rather than real elapsed time. Because it is tied to RPM, tach time typically runs lower than Hobbs — a flight with a lot of low-RPM taxi and idle records less tach time than Hobbs. It is also the basis most tied to actual engine wear, which is why maintenance intervals are often tracked on it.

Block time is measured block-to-block — from the moment the aircraft first moves under its own power to when it comes to rest at the end — which is how civilian flight time is logged internationally. It is the norm in much of the world outside the US and maps naturally onto how many operators and aircraft owners think about and contract for usage.

Finally, there is the prepaid block-hour account — often just called a block account — which is less a measurement than a billing structure: a student or renter pre-funds a balance, frequently at a discount, and flights draw it down as they are flown on whichever underlying basis the school uses. It changes the cash-flow and commitment dynamics rather than the per-flight measurement itself.

The dominance of Hobbs in US flight training is a historical convention that hardened into an assumption. The Hobbs meter is ubiquitous in the American training fleet, it is easy for a student to read straight off the panel, and rental has been billed that way for generations. There is nothing wrong with it — for a US school renting its own fleet to students, Hobbs is simple and perfectly defensible.

The problem is what happened next. Flight-school software built primarily for the US market encoded Hobbs as the billing basis, sometimes the only one, because it matched the dominant customer. That was a reasonable product decision for a US-only tool, but it baked a regional convention into the software's DNA. As those tools spread to operators who do not fit the US-rental-fleet mold — schools outside the US, clubs, operators flying aircraft they do not own — the hard-coded Hobbs assumption came along for the ride, and it stopped fitting.

The result is a market where a lot of software treats "how you measure a billable flight" as a settled question with one answer, when for a growing share of operators it is not settled at all. A European ATO, a club matching an owner's lease terms, or a school billing a fleet it doesn't own can all have perfectly good reasons to bill on something other than Hobbs — and can find their software simply won't let them.

There are several concrete, common situations where a basis other than Hobbs is the correct choice, not a preference.

You operate where block time is the convention. Outside the US, block time is how flight time is logged and how operators think about usage. A school in that environment billing on Hobbs is swimming against the local convention its students, instructors, and regulators already use — block time is simply the native unit.

Your aircraft agreements are written in a specific basis. When you fly aircraft you don't own — under a lease or from multiple owners — the owner's agreement dictates how usage is counted and compensated. If an owner's lease is written in block time or tach, and your software only bills Hobbs, your student invoices and your owner reconciliations are measuring different things, and someone has to reconcile the gap by hand every month. The basis has to match the agreement.

You want to bill on actual engine wear. Some operators prefer tach-based billing because it tracks engine RPM and therefore real wear more closely than wall-clock engine time, which some argue is fairer to a student who spent twenty minutes stuck at low RPM behind traffic.

You want the commitment and cash flow of prepaid blocks. Selling training as prepaid block-hour packages improves cash flow and student commitment, but it interacts with revenue timing: prepaid funds only become earned revenue as instruction and flight hours are actually delivered, a point we cover in our note on how flight-training revenue is recognised. And the basis interacts with the wet-versus-dry question of whether fuel is bundled into the rate. None of these are exotic — they are ordinary operating choices that a Hobbs-only tool cannot express.

Step back and the issue is not really Hobbs versus block versus tach. Any of them can be the right answer. The issue is who gets to decide. When software supports only one basis, the school no longer chooses its billing convention on business grounds — it accepts whatever the tool imposes, and then contorts its operation, its student communication, and its owner agreements to fit.

That contortion has real costs. It shows up as manual reconciliation between what the software billed and what an owner's lease actually says. It shows up as student confusion when the invoice unit doesn't match how the flight was logged. It shows up as a school that cannot take on aircraft from an owner whose agreement is written in the 'wrong' basis, because its system can't handle it — a self-inflicted limit on the business. And it shows up as the quiet erosion of trust that comes when the numbers never quite line up and everyone assumes the software is 'just like that.'

The deeper principle is one we return to often: a flight school's software should adapt to the school's operation, not the other way around. Billing basis is one of the clearest tests of that principle, because it is so fundamental and so often hard-coded. If changing how you measure a billable flight means fighting your software or abandoning the idea, the tool is making a business decision that belongs to you.

The alternative is a system where the billing basis is a setting you control, not a constant you inherit. In practice that means a few things working together.

You set the basis to match your operation — and, critically, you can set it per aircraft where you need to, because a school flying aircraft from several owners may genuinely need block time for one owner's aircraft and Hobbs for another's, each matching that owner's agreement. On top of the basis sits itemised pricing, so the aircraft charge, the instruction charge, fuel or surcharges, and fees are separate line items rather than a single blended number — which is what lets the student see a clear invoice and lets you reconcile cleanly with owners and your accounting. Prepaid block accounts draw down against whatever basis you've set, with the balance visible in real time and revenue recognised as flights are actually flown.

The test of a good billing system is that a school never has to say 'we'd like to bill that way, but the software won't let us.' The basis, the itemisation, the prepaid handling, and the owner reconciliation should all bend to how the school actually runs. This is the same configurability argument behind our post on pricing that fits how a school runs — billing basis is just the most foundational example of it.

Aviatize was built on the premise that these decisions belong to the school, which is why its billing is configured rather than fixed. The versatile rate engine lets a school define how flights are priced and on what basis, itemise aircraft and instruction and fees as separate line items, run prepaid block accounts with real-time balances, and reconcile usage back to aircraft owners — so the billing expresses the operation instead of constraining it. That is the practical meaning of software that adapts to your SOPs: we invoice the flight the way you need it done, in the unit your operation and your agreements actually use.

So if you are evaluating tools, or wondering why your current system makes billing feel like a fight, ask the foundational question directly: can it bill on the basis I actually operate on — block time, Hobbs, or tach — and match what my owner agreements specify? If the honest answer is 'it only does Hobbs,' the software has been quietly making one of your most fundamental business decisions for you. Take it back. The billing basis is yours to set, and the right system is the one that simply asks you which one you want.