# What is an airliner's "green dot speed", and how does it vary with weight, and why?

A related question told us, quoting from an Airbus performance manual,

Green dot speed (minimum gradient) is a function of weight.

Comments under the question stated that the "green dot speed" increased as weight increased.

So-- what is "green dot" speed? Does it reference some fixed, optimal angle-of-attack? Perhaps the one that gives the best L/D ratio (glide ratio)? If so, it would obviously increase with weight, generally being proportional to the square root of the wing loading. If this interpretation of how green dot speed varies with weight is not the correct one, then what is?

• "If this is the case, what is the meaning of the phrase "the rate and gradient of descent magnitudes are reduced at higher weights"?"-- this part of the question is now answered here aviation.stackexchange.com/questions/75181/… . Perhaps doesn't address the main thrust of the question comprehensively enough to justify posting this comment as an actual answer. Mar 10 '20 at 17:24
• In fact I think I'll just delete that part of the question as this would not affect the validity of the existing answer anyway. Mar 10 '20 at 17:43
• "(from green dot to VMO), the rate and gradient of descent magnitudes are reduced at higher weight" Well of course: when heavier the descent rate is reduced. And since you are flying at fixed speed, a lower rate means reduced gradient. This is only valid above g/d and since they assume you never fly above Vmo they range this :from green dot to Vmo :-) Mar 11 '20 at 9:47

Green Dot (GD) speed is a term used by Airbus. It is named after the symbol on the speed tape. It refers to the speed that results in the best climb gradient in case of an engine failure, but it is close the speed resulting in best L/D ratio with all engines operating.

Airbus defines it as follows:

Definition

GD speed is the engine-out operating speed in clean configuration. In other words, it corresponds to the speed that allows the highest climb gradient with one engine inoperative in clean configuration. In all cases (all engines operative), the GD speed gives an estimate of the speed for best lift-to-drag ratio. It is also the final take-off speed and it represents the operational speed of the clean configuration and the recommended speed in holding in clean configuration. It is represented by a green dot on the PFD speed scale and displayed only when the slats / flaps control lever is in the ‘0’ (CLEAN) position and landing gears are not compressed

How is GD determined?

GD speed is computed by the Autoflight systems and is based on the aircraft weight. The GD formula has been set up so that the resulting airspeed provides the best lift-to-drag ratio for a given altitude, air temperature and aircraft weight, in clean configuration with one engine out. In some phases of flight, GD is computed to minimize drag and thus, the fuel consumption (for example during the HOLD phase).

(Airbus Safety First Magazine)

The GD speed is increasing linearly with aircraft weight and slightly increases with altitude above FL200:

(Airbus A320 QRH - 4.01 In Flight Performance - Speeds)

Here is a plot of the GD speed as a function of weight for different altitudes: