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What happens to jet endurance with altitude?
  • A
    Endurance is maximum at low levels with MCT set.
  • B
    Endurance is maximum at a high level, low OAT, and design RPM.
  • C

    Endurance can be maximized by flying with minimal fuel load at the highest possible altitude.

  • D
    Endurance increases above the optimum altitude due to reduced density.

The endurance of an aeroplane is the time it can remain airborne on a given quantity of fuel or, put another way, endurance can be expressed as fuel used over a given airborne time.

For a jet aeroplane, Fuel Flow = Fuel Flow per unit Thrust (SFC) x Total Thrust

The aim is to reduce the amount of fuel used to produce each thrust unit (SFC needs to be small). For a jet engine this occurs when ambient temperature is very low and engine rpm is very high. This can only occur at high altitude. So, flying at high altitude, minimizes the fuel used per unit of thrust.

In level flight the forward acting force of thrust is controlled and balanced by the rearward acting force of drag. If drag is small, the aeroplane need fly with only a small amount of thrust. Therefore, on the formula above, “Total Thrust” can be replaced by “Total Drag”. To minimize drag, the jet aeroplane simply flies at the velocity for minimum drag.

  • In summary then, for a jet aeroplane to maximize its endurance by minimizing its fuel flow, the pilot would fly the aeroplane at VMD and fly as high as possible.

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