How we calculate

Every figure on HobbsHour comes from a published source with a date, and every formula is on this page. If you can't trace a number back to a source, that's a bug we want to hear about.

The two kinds of cost

Flying costs rise with every hour on the Hobbs meter:

• Fuel: burn in gallons per hour times the price per gallon of 100LL.
• Engine reserve: the overhaul cost divided by the engine's TBO (time between overhauls) in hours.
• Propeller reserve: a per-hour amount set aside for propeller overhaul.

Fixed costs arrive every year whether you fly or not: insurance, tie-down or hangar rent, the annual inspection including typical findings, oil changes and small parts, and fees and subscriptions.

The formulas

Flying cost per hour = fuel burn × fuel price + overhaul cost ÷ TBO + propeller reserve

Fixed cost per year = insurance + storage + annual inspection + small items + fees

Cost per Hobbs hour = flying cost per hour + fixed cost per year ÷ hours flown per year

Cost per year = fixed cost per year + flying cost per hour × hours flown per year

Loan payment per month = P × r ÷ (1 − (1 + r)⁻ⁿ), where P is the amount financed, r is the yearly rate ÷ 12, and n is the number of monthly payments.

Headline figures leave the loan out, because part of every payment builds equity in the airplane. The calculators show the cost with the loan as a separate line.

Low, typical and high cases

Real costs vary with the airplane, the airport, the pilot's experience and how hard the engine is run, so each line has three values. The low case uses the cheaper end of each published range, the high case the expensive end, and the typical case a middle value. The assumptions behind each case are written out on the model page. Model pages use 100 hours a year unless stated.

What is not included

• Depreciation or appreciation of the airplane, and the return the cash could earn elsewhere.
• Avionics upgrades, paint and interior work.
• Large unplanned repairs, such as a cylinder replacement or corrosion work found at an annual. The annual inspection line covers typical findings only.
• Sales tax and registration costs at purchase.

Sources and how often they are checked

• 100LL prices: monthly national averages reported from iFlightPlanner data. Checked monthly.
• Engine TBO: manufacturer service instructions, such as Lycoming SI 1009. Checked when a new revision appears.
• Fuel burn: manufacturer figures and published fact sheets. Checked once per model.
• Overhaul, inspection and insurance costs: published prices and ranges from shops, brokers and aviation publications. Checked twice a year.
• Storage: published rate schedules from public airports. Checked yearly.
• Purchase prices: published guides, labeled as rough estimates. Aircraft values are the weakest input on the site because no free official data exists.
• Loan rates: published rates from aviation lenders. Checked monthly.

Each page shows the date its data was last checked. Changes to figures are listed in the data changelog.

Corrections

If you have a current quote, rate schedule or invoice that shows a figure is out of date, send it in. Corrections are checked against a source before they change a page.