Definition
Direct operating cost answers a narrow question precisely: what did this hour of flying cost, over and above what the aircraft would have cost sitting still? The components are the ones that scale with hours flown:
- **Fuel**, at actual burn rather than book figures, since a training profile with repeated circuits burns differently from a cross-country. - **Oil**, including consumption between changes. - **Engine reserve** — the overhaul cost divided by the TBO interval, accrued per hour. - **Propeller reserve**, on the same basis. - **Airframe and scheduled maintenance reserve**, covering the 100-hour and annual inspections and the components with hourly or cyclic lives. - **Unscheduled maintenance**, which is the component operators most often omit and the one that most reliably ruins a cost model.
What DOC deliberately excludes is everything fixed: hangarage, insurance, the annual's labour where it is calendar-driven rather than hours-driven, financing, and the overheads of running a school. Those belong in the fixed side of the model.
The reserve components are what make DOC an accrual rather than a cash figure. An engine overhaul is a single large payment years apart, but the cost is incurred every hour the engine turns. Booking it per hour is what stops a school reading a low-maintenance year as profit and then meeting the overhaul with nothing set aside.
Why It Matters for Flight Schools
DOC is the number that should sit underneath a wet rate, and it is the number most often estimated once and then left to go stale. Fuel price moves, overhaul quotes move, and TBO assumptions change with how the engine is actually operated — a training fleet doing repeated circuits accumulates cycles and heat cycles quite differently from a private owner flying cross-countries.
It is also the figure that makes fleet decisions legible. Two aircraft with similar rental rates can have materially different DOC, and the one with the lower DOC is the one that should absorb the marginal hour. Without a per-tail DOC, dispatch decisions default to whatever is convenient rather than whatever is cheapest to fly.
How Aviatize Handles This
Aviatize holds maintenance events and forecasts against each individual airframe rather than the fleet as a whole, so the hourly reserve components of DOC can be built from what a specific tail number has actually cost and is actually due. Because utilisation and billing sit alongside that record, a school can compare cost per hour against revenue per hour per aircraft rather than across an averaged fleet.
Frequently Asked Questions
- What is included in direct operating cost?
- Only the costs that scale with hours flown: fuel at actual burn, oil and its consumption, and the per-hour accruals for engine overhaul, propeller overhaul, scheduled airframe maintenance including the 100-hour and annual, and unscheduled maintenance. Hangarage, insurance, financing and school overheads are fixed costs and sit outside DOC.
- Why are engine reserves counted as an hourly cost?
- Because the cost is incurred by the hour even though it is paid in a lump. An overhaul divided by the time between overhauls gives the amount each hour of operation consumes. Accruing it per hour is what prevents a school reading a maintenance-light year as profit and then reaching the overhaul with nothing set aside for it.
- What is the difference between direct operating cost and cost per flight hour?
- Direct operating cost counts only what an hour of flying adds. Cost per flight hour is the fuller figure — it takes the fixed costs the aircraft incurs whether or not it flies, such as hangarage, insurance and financing, and spreads them across the hours actually flown, then adds DOC on top. DOC is a property of the aircraft; cost per flight hour depends on how much you fly it.