The main thing is to keep track of both the airplane datum, and the static_cg_height, and that GSX works in METERS and its X and Y values are exchanged (GSX wants lateral first, then longitudinal, then height)
Nose gear is simple, because it's just a longitudinal distance so, in case of the default A321 is:
longitudinal position of the front gear contact point: 43.00 ft
longitudinal datum - 4.5 ft
( 43.00 - 4.5 ) = 38.5 ft = 11.73 meters. We set the pushback at 12.25 meters because it doesn't go exactly on the contact point, also because several airplanes, including defaults, have them set not very precisely, so you'll need to adjust it a bit anyway.
For a door, it's a bit more complex, because:
- you need to take into account the datum in X, Y and Z
- the height from ground is also depending on the static_cg_height
- the aircraft.cfg indicates a position in the middle of the door (to attach the jetway), but GSX needs the doorstep so, you usually have to subtract about 1.20 meter from that.
Taking the main passenger exit, for example, the A321 aircraft.cfg set it at:
45.50 ft (longitudinal), -6.0 ft (lateral), 7.0 (height)
The airplane datum is -4.5, 0, 0 and the static_cg_height is 9.2 ft
GSX wants lateral, longitudinal and height, and in meters so, we'll fist eliminate the datum, which is -4.5, 0, 0 so, only the longitudinal distance will be affected by the datum, which will result in:
( 45.50 - 4.5 ) = 41 ft
( -6 - 0.0 ) = -6.0 ft
( 7.0 + 9.2 ) = 16.2 ft (note the static_cg is being added to the door's height)
Converted in meters, it results in the door set at:
12.49 m, -1.82 m, 4.93 m
After we subtract half of the door's height ( 1.2 mt ) from 4.93 to get a reasonable point for the doorstep, and invert the X and Y, because GSX wants lateral first, then longitudinal, then height, we arrive at the following values for the GSX.CFG file
-1.82 12.49 3.73
Which is fairly close to the values set in GSX:
-1.95 12.4 3.7
The 13 cm difference in the lateral distance it's because the jetway attach point in the aircraft.cfg is usually a bit "inside" the door, while GSX needs the exact point on the fuselage surface where the stair will attach to. The 9 cm difference in longitudinal is to better fit the stair shape to the door when it's opened, and the 3 cm difference in height is basically not noticeable...
