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:
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
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: