OFP vehicle speed from velocity vectors

_plane = _this select 0
_x     = velocity _plane select 0
_y     = velocity _plane select 1
_z     = velocity _plane select 2
_xy    = sqrt (_x^2 + _y^2)
_speed = 3.6 * (if (_xy!=0) then {_xy/cos (atan (_z/_xy))} else {abs _z})
hint Format ["%1 %2", speed _plane, _speed]