# Maximum (Best) Range Speed v.s Maximum (Best) Endurance Speed

[Ref. The Pilot’s Manual PM2 Ground School Page 62-63, 71 ; Air Pilot’s Manual 1 – Flying Training Page 94-95 ; Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 11-9 – 11-11 ; PPL Instruction Kit Vol1 (306) – 9th Edition, V9.2 P.13-14]

Maximum (Best) Endurance Speed
= Minimum Power Airspeed

• minimum fuel consumption
• Maximum Time for the aircraft at airborne

Maximum (Best) Range Speed
= L/Dmax Speed = Minimum Drag Speed

• fuel/distance is least – minimum fuel burn to cover a given distance
• Maximum Still air Distance  (Still = calm = nil-wind)

WHAT IF we put the Altitude into consideration?

To achieve the Best Range Speed  – related to the distance – TAS :

• Both the engine and propeller are most efficient at low altitude where the air is more dense.
• However, for the same IAS, TAS increases with altitude (IAS = ½ ρ TAS²)
• therefore, at a constant IAS, a greater distance can be covered at high altitude.

To achieve the Best Endurance Speed :

• Both the engine and propeller are most efficient at low altitude where the air is more dense AND
• fuel flow will be least at low altitude
• therefore, at a constant IAS, a greater endurance can achieve at low altitude.

[Extension: The Airbus safety magazine (January 2016) – #21 Safety First P.9]

Green Dot (GD) = L/Dmax Speed (for those fly-by-wire aircraft – i.e Airbus products)

• show on PFD speed scale
• is based on the aircraft weight – calculated by the Auto Flight System (AFS) and needing ZFW information which is inserted in FMS during flight preparation

(Capture from The Airbus safety magazine (January 2016) – #21 Safety First P.9)

Point 2 (in the first regimeis) is a stable equilibrium: in cruise, no matter speed above or below point 2, the aircraft’s speed will naturally back to Point2 ;

Point 1 (in the second regimeis) is a unstable equilibrium: in cruise, no matter speed above or below point 2, the aircraft’s speed will will continue to accelerate or decelerate ;

Once the speed below Point 3, no thrust margin available to accelerate while maintaining a stabilized level flight. Then the only way to stop the deceleration is to lose altitude in order to accelerate beyond point 3.

However, Point 3 is not shown on PFD and so from operation point of view, in cruise, GD IN A NUTSHELL
Do not fly below.