How does a Jet Engine achieve fuel saving at high Altitude ?

Engine fuel consumption decreases at hight altitude because of the higher propulsive efficiency of the engine , which brings it closer to its best spacific fuel consumption (SFC ) / Maximum Endurance .

Note : lower (SFC) means lower fuel burn .

Fuel Endurance : Is the maximum airborne time possible for a given quantity of fuel , which increases as well the Maximum attainable range .

Best fuel consumption ( SFC ) : FUEL BURN (ib) divided by ( Engine thrust (ib).

Maximum attainable range is defined as the greatest distance flown over the ground for a given quantity of fuel .

At higher true airspeed ( TAS ) for a constant indicated airspeed ( IAS ) which increase in maximum attainable range .this simply happens from the reduction in air density at high altitude , thus ground speed / ground distance are covered and range increased .

When are the best SFC and Maximum Endurance achieved ?

• At high altitude when the Minimum drag speed (VIMD) remains constant with altitude and because minimum aircraft drag requires minimum thrust ( thrust = Drag ) .
• As Mach number speed becomes limiting above FL 260-FL 280 and to maintain a constant MN the TAS AND Equivalant airspeed EAS decrease with altitude increase and the VIMD varies only with EAS .
• when the aircraft reach its best Endurance speed (VIMD) at higher altitude then the EAS reduced.