Peculiarities of PCN for the A321
The Airbus 321 series creates an obscure and very technical problem in terms of the calculation of PCN. It is not relevant to pilots (except for arcane technical interest), but it is to airport authorities and airlines.
It became known in 2008 that there is an anomaly in the calculation of ACN/PCN. The procedure for computing PCN on rigid pavements given in the FAA AC 150/5335-5A sometimes gives unreasonable PCN values when the aircraft mix contains:
• a large number of narrow body aircraft (A321, etc.).
• a relatively small number of large wide bodies such as the Airbus A330 or Boeing 747 or 777.
The procedure tends to produce a PCN for one of the narrow bodies as the critical aircraft, indicating that the wide body operations should be restricted. This is not a true representation of the situation. The anomaly exists at several airports.
As an example, for a 400mm thick PCC pavement, the ACN of the A321-100 is 54 at maximum weight of 83.4 tonnes and the A330-300 ACN is 62 at maximum weight of 230.9 tonnes. The design for A330 at 239.5 tonnes which is an ACN of 65 is for a PCC slab thickness of 399mm (about equal to the 400mm PCC slab). Therefore a PCN of 65 is acceptable, even though the strict application of the method would find the A321-100 as the critical aircraft and thus a PCN of 54 should be used. The published PCN should be 65, not 54.
This problem seems to have reared its head with the A321 particularly because its undercarriage is so heavily loaded and it is such a popular aircraft.