30 June 2001
The procedure for computing the pass-to-strain conversion percentage has been computerized, and the
factors can easily be computed for single, twin, single-tandem, twin-twin, twin-tandem, or other gears.
(c) The distribution of the pass-to-strain conversion percentages as a function of point
location and effective thicknesses is given in Appendix E. These pass-to-strain conversion percentages
can be used to convert, for any point location, the number of aircraft operations to effective strain
(2) Aircraft loading. The aircraft loading and gear characteristics are used in the response
model for computing the magnitude of strain. The information needed includes the number of tires, tire
spacing, load per tire, and contact pressure. The radius of the loaded area is computed based on the
assumption of a uniformly loaded circular area, i.e.,
r = radius of loaded area, millimeters (inches)
L = load per tire, Newtons (pounds)
p = contact pressure, MPa (psi)
Note: units should be consistent with units of the section parameters.
In principle, all main tires should be used in computing the strain, but usually only the tires on one
landing gear need to be used. The distance between gears for common aircraft is sufficiently great to
prevent interaction between gears. Within a main gear, some searching for the maximum strain may be
needed. For most cases the maximum strain will occur under one of the tires, but for closely spaced
tires or strains at a great depth, the maximum may move toward the center of the tire group.
(3) Traffic grouping. The traffic is grouped so that within each group each individual pass of an
aircraft will cause damage similar a pass of any other aircraft in the group. That is, the pattern of strain
of every pass of the group would be almost the same; then the value of the allowable number of passes
(N) would be the same. For this to be true, the loading characteristics for aircraft within a group must be
similar, and the single set of material properties must be applicable for all passes within the group.
Grouping reduces considerably the design effort, and it is advantageous to reduce traffic to as few
groups as possible. Grouping of the aircraft by similar pass-to-strain conversion percents has already
been accomplished in Appendix E. Additional subgrouping would be necessary to account for other
differences, such as load magnitude and tire pressure. Also, other groupings may be necessary to
account for changes in material properties such as changes in subgrade modulus caused by thaw and
changes in asphalt modulus caused by temperature. For pavements that are relatively unaffected by
changes in temperature and are designed based on a single critical aircraft, it may be possible to reduce
the aircraft operations to a single group. In this case, the design procedure simplifies to determining
allowable strains for the design aircraft and to adjusting the pavement thicknesses to obtain the