Jet Endurance Calculator

How long can a jet stay in the air on a single fuel load? That number is its endurance, and it's one of the first things you check when sizing an aircraft or planning a mission. This calculator works it out with the Breguet endurance equation. Give it the engine's fuel consumption rate, the aircraft's aerodynamic efficiency, and the takeoff and landing weights, and you get the maximum flight time. It runs the other way too: if you need a 20-hour loiter, enter the rest and read off the L/D ratio you'd have to hit.

What Is Jet Endurance?

Endurance is the longest an aircraft can stay airborne before it runs out of usable fuel. Three things set that number for a jet: how efficiently the engine burns fuel, how efficiently the wing flies, and how much fuel it carries.

Jet engines make thrust by burning fuel. How much fuel they burn for the thrust they produce is the thrust specific fuel consumption (TSFC). A lower TSFC means the engine squeezes more flight time out of every kilogram of fuel. Modern high-bypass turbofans on airliners sit around 8-17 g/(kN·s); older turbojets run higher, closer to 25-30 g/(kN·s).

The wing's side of the story is the lift-to-drag ratio (L/D, or $C_L/C_D$ ), which measures how much lift it generates for the drag it creates. A higher L/D means you need less thrust to hold level flight, so endurance climbs. To wring out the most time aloft, a jet flies at the angle of attack that maximizes L/D, the loiter condition.

The weight ratio, initial weight over final weight, stands in for the fuel fraction. Carrying more fuel raises the ratio, but it also adds weight, which demands more thrust and burns fuel faster. That trade-off is why the equation leans on a natural logarithm: each extra bit of fuel buys you less time than the last.

How to Use This Calculator

Enter the engine's thrust specific fuel consumption (TSFC). Units can be kg/(kN·s), g/(kN·s), or lb/(lbf·h).
Add the lift-to-drag ratio. Commercial jets sit around 12-18; high-altitude surveillance drones reach 30 or more.
Give the initial weight, the gross takeoff weight with all fuel on board.
Give the final weight, what the aircraft weighs once the usable fuel is gone, counting structure, payload, and reserve fuel.
Read the endurance in your chosen time unit. To solve for a different variable, leave it blank and fill in the other four.

Understanding the Breguet Endurance Formula

The Breguet endurance equation ties flight time to a handful of engine and airframe numbers:

E = \frac{1}{c_t \cdot g} \cdot \frac{C_L}{C_D} \cdot \ln\left(\frac{W_i}{W_f}\right)

Where $c_t$ is the mass-specific TSFC, $g$ is gravitational acceleration ( $9.80665 \text{ m}/\text{ s}^2$ ), $C_L/C_D$ is the lift-to-drag ratio, and $W_i$ and $W_f$ are the initial and final weights.

Here's a real example. Take a surveillance drone with these specs:

TSFC: 14.2 g/(kN·s) = 0.0000142 kg/(N·s)
Maximum L/D: 20
Initial weight: 44,482 N (10,000 lbs)
Final weight: 26,689 N (6,000 lbs)

Step 1: Compute $\frac{1}{c_t \times g} = \frac{1}{0.0000142 \times 9.80665} \approx 7{,}181$ seconds per unit of L/D.

Step 2: Multiply by L/D: $7{,}181 \times 20 = 143{,}620$ .

Step 3: Compute the natural log of the weight ratio: $\ln(44{,}482 / 26{,}689) = \ln(1.667) \approx 0.511$ .

Step 4: Final result: $143{,}620 \times 0.511 \approx 73{,}390$ seconds, or roughly 20.4 hours of continuous flight.

That's more than 20 hours of uninterrupted surveillance. A couple of things fall straight out of the math: halve the TSFC and you double the endurance; push L/D from 20 to 30 and you add about 50%. The weight-ratio term is where diminishing returns live, since going from a ratio of 1.5 to 2.0 buys more flight time than going from 2.0 to 2.5.

Applications

Drone operators size loiter time for surveillance, communication relay, and maritime patrol missions.
In commercial aviation, it estimates how long an aircraft can hold before landing and helps weigh engine options early in design.
Military planners check combat air patrol duration for fighters and maritime patrol aircraft.
Design teams run trade studies to see how wing efficiency, engine choice, and fuel capacity move the endurance number.
Students use it to build intuition for how propulsion, aerodynamics, and weight pull against each other.

Tips for Maximum Endurance

Fly at the speed for maximum L/D, the loiter speed. It's usually slower than the cruise speed you'd pick for range.
Choose engines with the lowest TSFC for the mission. High-bypass turbofans are hard to beat for sustained loiter.
Trim structural weight where you can. A lighter airframe means a bigger fuel fraction for the same takeoff weight.
Endurance and range peak at different speeds: maximum endurance wants minimum thrust, maximum range wants minimum drag. Optimize for one and you give up the other.

Frequently Asked Questions

What is the difference between endurance and range?

Endurance is about time in the air; range is about distance covered. You get maximum endurance by flying slowly, at the speed where thrust is lowest, and maximum range by flying faster, where drag is lowest. For any given aircraft those are two different airspeeds.

Does this equation account for climb and descent?

Not on its own. The Breguet equation assumes steady, level flight at constant altitude, so it leaves out climb and descent. For a full mission profile, work out climb, cruise, loiter, and descent separately and add them up.

What TSFC values should I expect?

Modern high-bypass turbofans land around 8-17 g/(kN·s), or 0.3-0.6 lb/(lbf·h). Military low-bypass turbojets run higher, 20-30 g/(kN·s) or 0.7-1.2 lb/(lbf·h). Turboprops are lower still, but they need a different equation.

Can I use this calculator for propeller aircraft?

It won't work for props. Propeller aircraft use a different form of the Breguet equation, one built around power-specific fuel consumption and propeller efficiency instead of thrust-specific fuel consumption.

Why does adding more fuel have diminishing returns?

Extra fuel adds weight, which means more thrust to stay level, which burns fuel faster. The natural log in the formula captures that feedback loop, so every additional unit of fuel adds a little less flight time than the one before it.