What, you may ask, is a middle altitude? A wiseacre might say that it is the airspace between low and high altitudes and he would be right.
The highest level for the middle might be Flight Level 250. The aircraft certification standards change above that altitude. Or it might be Flight Level 230 where air traffic control airspace sectors generally change from low to high. Or it might be Flight Level 180 where the airspace rules change and Victor airways become jet routes.
