code: def alt40fms(msg):
      
      d = hex2bin(data(msg))
  
      if d[13] == '0':
          return None
  
      alt = bin2int(d[14:26]) * 16    # ft
      return alt
docs: Selected altitude, FMS
  
      Args:
          msg (String): 28 bytes hexadecimal message (BDS40) string
  
      Returns:
          int: altitude in feet