Flight Planning: Time En Route, Fuel Burn & Reserves

Every cross-country flight rests on three numbers you work out before you ever start the engine: how long it will take, how much fuel it will burn, and how much fuel you must still have in the tanks when you land. Get these right and the flight is routine. Get them wrong and you are the next fuel-exhaustion statistic. This article walks through the arithmetic with real numbers, then covers the legal reserves and a practical pre-flight checklist.

Run the numbers yourself with the Flight Time Estimator and the Fuel Burn Estimator as you read along.

Time en route

The foundation of everything is a single relationship: time equals distance divided by ground speed. In aviation we measure distance in nautical miles (nm) and speed in knots (kt) — one knot is one nautical mile per hour — so the units cancel cleanly to give hours:

Decimal hours are awkward to read on a clock, so convert the fractional part to minutes by multiplying by 60. A flight that computes to 1.75 hours is 1 hour plus (0.75 × 60) = 45 minutes, or 1:45. Likewise 0.4 hours is 24 minutes. Always build your time estimate leg by leg, because ground speed changes with the wind on each leg.

Wind makes or breaks the plan

Here is the trap that catches new pilots: the speed in that equation is ground speed, not the airspeed the airplane flies through the air. Your true airspeed (TAS) might be a steady 120 kt, but the air itself is moving. The portion of the wind blowing straight down the route — the head- or tailwind component — adds to or subtracts from your TAS to give ground speed:

That same 240 nm leg takes 2:00 in still air, but 2:40 against a 30 kt headwind (240 ÷ 90) and only 1:36 with a 30 kt tailwind (240 ÷ 150). Because fuel is burned by the clock, the headwind does double damage: it both slows you down and makes you burn more fuel to cover the same ground. A crosswind that is not aligned with your course contributes only a fraction to the head/tail component — you resolve the wind into its along-track and across-track parts when you compute your heading and ground speed. The lesson: always plan each leg with the forecast winds aloft, never with airspeed alone.

Fuel burn

Once you know your time aloft, fuel follows directly from the airplane’s cruise burn rate in gallons per hour (gph):

Burn rate varies enormously with the airplane. These are typical cruise figures for common general-aviation types — always defer to your aircraft’s performance charts at your planned power and altitude:

Worked example: a Cessna 172 burning 8.5 gph on the still-air 2:00 leg needs 8.5 × 2.0 = 17 gallons just for the cruise portion. But that is not the whole trip. Add an allowance for engine start and taxi, and remember that the climb burns at a higher rate than cruise. Now re-run it into that 30 kt headwind: the leg stretches to 2:40 (2.67 hours), so cruise fuel rises to 8.5 × 2.67 ≈ 22.7 gallons — about 34% more fuel than the still-air case, just from the wind.

Reserves: the legal minimums

You may never plan to land with empty tanks. U.S. regulations set minimum fuel reserves that must remain after you reach your intended landing point. For Part 91 operations:

Read those carefully. Day VFR requires enough fuel to fly to your first intended point of landing and then continue for 30 minutes at normal cruise. At night the VFR reserve grows to 45 minutes. Under IFR (FAR 91.167) you must be able to reach the destination, then fly to the alternate airport when one is required, and then still have 45 minutes of cruise fuel left.

Endurance and the headwind trap

Endurance is how long the airplane can stay airborne on the fuel aboard:

Note usable fuel — every tank holds a small amount of unusable fuel that the engine cannot draw on, so plan from the usable figure in the POH, not the total tank capacity. Endurance is measured in hours; range (how far you can go) is endurance multiplied by ground speed, which is why a headwind shrinks your range even though the hours of fuel aboard are unchanged.

Now the classic trap: you plan a 4.0-hour flight in an airplane with 4.7 hours of endurance, leaving a comfortable-looking 0.7-hour (42-minute) cushion — just above the day-VFR minimum. Then you meet an unforecast 20 kt headwind. Your ground speed drops, the flight stretches to 4.6 hours, and your 42-minute reserve has quietly collapsed to roughly 6 minutes. This is exactly how well-meaning pilots run tanks dry. The defense is to plan generously, watch your actual fuel flow and ground speed in flight, and divert early if the numbers are not working out.

A practical pre-flight fuel checklist

• Measure the route in nautical miles, broken into legs.
• Get the winds aloft and compute ground speed for each leg from your TAS and the head/tailwind component.
• Compute time en route per leg (distance ÷ ground speed) and total it.
• Add taxi and climb fuel on top of the cruise burn, using POH figures.
• Multiply burn rate × time for trip fuel.
• Add the required reserve (30 min day VFR / 45 min night or IFR) — and a margin beyond it.
• Compare to usable fuel. If trip + reserve exceeds usable fuel, plan a fuel stop. Never “hope” the wind improves.
• Physically verify the fuel quantity before departure — dip or visually confirm the tanks; do not trust the gauges alone.

Tie it together with the Flight Time Estimator for your en-route times and the Fuel Burn Estimator for the gallons. Then cross-check how heat and altitude affect performance in Density Altitude Explained, and confirm the loaded airplane is within limits in Aircraft Weight & Balance.