public string TakeOff(AerialVehicle a)
 {
     a.isFlying = true;
     a.FlyUp();
     aerialVehicles.Remove(a);
     MaxVehicles--;
     return($"{a} is taking off.");
 }
 public string TakeOff(AerialVehicle a)
 {
     a.StartEngine();
     a.IsFlying = true;
     a.FlyUp(50);
     Vehicles.Remove(a);
     return(a + " taking off!");
 }