Specific Range and Endurance

Defining Range and Endurance

• Range is the maximum distance an aircraft can fly on a given fuel load.
• Endurance is the maximum time an aircraft can remain airborne with its available fuel.

Specific Range Explained

• Specific Range (SR) is the distance (typically in nautical miles) an aircraft can travel per unit mass of fuel (e.g., NM/kg). It is calculated as true airspeed (TAS) divided by fuel flow.
• Specific Range over the Ground (SRG) uses groundspeed instead of TAS, accounting for wind effects, and is expressed as nautical ground miles per unit of fuel.

Specific Endurance

• Specific Endurance is the time an aircraft can remain airborne per unit of fuel. It is calculated as 1/fuel flow (hours per kg or similar units).

Speed for Maximum Range and Endurance

• For jets, maximum endurance is achieved at minimum drag speed (VMD), while maximum range is at approximately 1.32 times VMD.
• For propeller-driven aircraft, maximum endurance is at minimum power speed (VMP), and maximum range at minimum drag speed (VMD).
• The speed for maximum endurance is always lower than that for maximum specific range.

Effect of Weight and Altitude

• Heavier aircraft require lower optimum altitudes for maximum range due to increased thrust or power required.
• For jets, optimum altitude for maximum range is typically near or just above the tropopause, balancing engine efficiency and TAS.
• For piston aircraft, range is theoretically independent of altitude if optimum SFC is maintained, but practical limitations apply.

Operational Use

• Pilots may choose maximum endurance for holding patterns or loitering, and maximum range for ferry flights or fuel-critical situations.
• Flying at maximum endurance speeds can have drawbacks, such as high angle of attack and reduced speed stability, especially in propeller aircraft.

Wind and Endurance

• Wind affects range but not endurance. Endurance depends only on fuel flow, not groundspeed, so wind has no direct impact.