I doubt "ACRO" stands for "airplane", since the only letters in common between the two are "A" and "R". The OP obviously knows that ACRO is the correct mode for an airplane, he's just asking what word it is based on. It seems like ACRO must be short for acrobatic, but why they whould choose that word for the airplane mode is beyond me. Since they seem to have a four letter limit for the mode names, I guess it looks/sounds better than shortening "plane" to "PLAN"!