The range on the airplane icon is in yards. Altitude is measure in feet. Three feet to a yard. So an airplane directly above you with an icon showing 1k or 1000 range would be 9000-10,000 feet higher than your altitude.
Also, the range shown in the airplane icon is approximate. If I remember correctly, a range icon of 400 means the airplane is between 450 and 350 yards away, or something like that.