Effects of aircraft tire pressures on flexible pavements

The ICAO ACN/PCN system for rating airport pavements has been in existence since 1981. The four tire pressure categories assigned to the PCN rating, which may have been representative of the aircraft existing at its inception, are no longer representative of the current fleet of large wide bodied aircraft operating with higher wheel loads and higher tire pressures. The four code letters Z, Y, X and W, correspond to the four tire pressure limits of 0.5 MPa, 1.0 MPa, 1.5 MPa and unlimited respectively Due to the fact that ICAO does not make formal recommendations regarding how an airport should assign these tire pressure limits, the Boeing Company decided to perform a series of tire pressure tests at the FAA William J. Hughes Technical Center, National Airfield Pavement Test Facility in New Jersey, USA, in an attempt to arrive at tire pressure limits more consistent with today's aircraft and based on actual flexible pavement behavior under high tire pressures. The tests consisted of three sections of varying asphalt surface thicknesses trafficked by a single wheel load device in order to contain pavement damage to tire pressure effects only. The test loading began at 18,144 kg (40,000 lb) and continued up to 24,947 kg (55,000 lb), with the tire pressure increasing 0.14 MPa (20 psi) every 500 passes. A pavement was declared failed when rutting had reached a medium severity level of 13-19mm (0.50-0.75 inches) or severely cracked such that the PCI level was 55 or below. The conclusion was reached that high tire pressures up to 1.65 MPa (240 psi) have no adverse effects on flexible pavements with asphalt and base material quality meeting the recommended minimum standards set forth by various agencies. Our recommendation for revised ICAO tire pressure categories, to be used in the PCN rating system, are also presented.