The following is an explanation of .air file record 1515.
1515=Wing Planform - MAC/CoG ect,record
1515h=Sets FS variables for CoG gauge, etc.||FS2002: Sweep, Root Chord, Taper||This data (except MAC length) is overruled by parameters in [airplane_geometry] section of Aircraft.cfg||FS9: used only by Cessna 182 and 208, Extra 300, Mooney Bravo, Learjet, DH Dash-8, Concorde||CFS3: not used.
1=00,double,MAC Length (ft)
1h=Not overruled by anything if greater than 0.000000. FS uses this figure as the MAC length when calculating the position of the CG Ball in the Fuel and Payload Planner regardless of how unreal the figure might be (FS7 Concorde 90.39)||If the real MAC length is known it should be used here||FS9-FSX MD83 beta 13.17 would require a wing_root_chord length of 19.325086 ft.
2=08,double,MAC Leading Edge position (ft)
2h=Mean Aerodynamic Chord Leading Edge longitudinal distance from FS_Reference_Position||overruled by wing_pos_apex_lon in [airplane_geometry] section of Aircraft.cfg.
3h=should be the equivalent in radians of, but overruled by, wing_sweep (deg) in [airplane_geometry] section of Aircraft.cfg||FS will accept negative wing_sweep for wing leading edges that sweep forward instead of back.
4=18,double,wing_root_chord length (ft)
4h=Measured at aircraft's longitudinal centreline, not at its usual position at the junction of wing and fuselage||should be the same as, but overruled by, wing_root_chord in [airplane_geometry] section of Aircraft.cfg||FS will accept a wing_root_chord length that is shorter than the Standard Mean Chord (SMC) length but if the wing_root_chord in the Aircraft.cfg is longer than twice the SMC length, FS overrules it and uses the SMC length as the wing_root_chord length||FS9-FSX MD83 beta 23.1 is too long as MD83 beta maximum wing_root_chord length is 22.430427 ft, according to 1 above and 5 below should be about 19.33.
5h=wing_tip_chord length/wing_root_chord length||wing_tip_chord length=(SMC length*2)-wing_root_chord length||appears to have no effect||FS9-FSX MD83 beta 0.16 would require a wing_root_chord length of 19.336570 ft.