[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
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.