Why is the Airbus A320 permitted to carry 180 passengers, instead of the maximum 179 allowed?
See Page 9 of TCDS A.064 ANNEX - Airbus A318, A319, A320, A321 - Special Conditions


  TCDS A.064 ANNEX - Airbus A318, A319, A320, A321 - Special Conditions  

EQUIVALENT SAFETY FINDING         E-2107: Passenger Extension to 180
APPLICABILITY:                    A320
REQUIREMENTS:                     JAR 25.807
ADVISORY MATERIAL:                N/A

  
  Statement of Issue:  
  
  An application was made on September 30, 1992 to JAA for certification of maximum number of passengers of 180.
  
  Paragraph 25.807(c)(1) of JAR specifies for passenger seating configuration, the number and
  types of Emergency Exits for each side of the fuselage.
  The maximum seating capacity allowable under JAR 25.807(c)(1) for A320 exit configuration is
  179 (2 Type I and 2 Type III).
  
  However, the exits size for front and rear doors on the A320 is such (32x73 inches) that they
  are oversized Type I and therefore can be considered as a non standard exit arrangement.
  
  The same exit size is being used on a derivative of the A320, for which Latin Square Tests
  have been conducted in order to establish an appropriate rating for that kind of exits. These
  tests have shown that, when associated with the performance of the slide installed at these
  exits, a rating of 55 passengers is appropriate.
  
  It should also be noted that the full scale emergency evacuation demonstration of the A320
  with 179 passengers has demonstrated compliance with JAR 25.803(c) with sufficient margin
  to justify 180 passengers.
  
  Under those conditions, a maximum capacity of 180 passengers is requested for the A320.
  
  JAA Conclusion:
  Based upon the demonstrated higher rating for the floor level exits and the adequate margins
  for 179 passengers shown in the 90 seconds demonstration, and used as a basis to show
  compliance to JAR 25.803(c), the JAA accept 180 passengers as the maximum capacity for the
  A320 on the basis of an Equivalent Safety Finding.