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only two multiple-gear aircraft in the LEDFAA 1.2 library; the C-5 and the B-747. Each is treated
differently. Responses for the C-5 are computed as if the two six-wheel groups on each side are one gear,
and response is computed using 12 wheels with the two six-wheel groups in tandem. Responses for the B-
747 are computed using only one four-wheel group (wing and body gears have the same geometry and
assumed wheel loads).
2. Compute maximum vertical strain at the top of the subgrade.
3. Assume that the maximum response computed in step 2 acts across the width of each tire footprint applied
to the pavement by the aircraft. For flexible pavements the tire footprint is an equivalent “elliptical cone of
influence” projected down from an assumed ellipse at the surface onto the top of the subgrade. The number
of equivalent tires at the top of the subgrade decreases when there is overlap at the top of the subgrade (see
figures 4 and 5). The area of the ellipse at the surface is the wheel load divided by the tire pressure, and the
ellipses have aspect ratios of 1.6 in both cases.
4. Compute the transverse component of pass-to-coverage ratio for each ten-inch strip across the pavement
using the footprint width determined from step 3 for all wheel tracks in the gear configuration (that is,
wheels in tandem are counted as one footprint moving along the pavement).
5. Compute the longitudinal component of pass-to-coverage ratio for the gear used to compute response. For
flexible pavements this is found from the projected cones of influence at the top of the subgrade and the
longitudinal spacing of the wheels on the gear, see figure 6.
6. For each aircraft, compute the number of coverages applied to each ten-inch strip over the design life.
7. For each aircraft, compute the number of coverages to failure from the failure model for the computed
maximum vertical strain.
8. For each aircraft, compute the CDF for each strip by dividing the applied coverages by the number of
coverages to failure.
9. For each strip, add the CDFs for all the aircraft in the mix.
10. The CDF used for design is the maximum across all strips.
In LEDFAA 1.3, the same procedures are used for all aircraft as in version 1.2 except for the multiple-gear
commercial aircraft, B-747 and A380. The multiple-gear aircraft are given new designations so that the new
procedures can be selected correctly during program execution. The designations are:
1. WFBF: Wing = dual-tandem, Body = dual-tandem.
2. WFBN: Wing = dual-tandem, Body = triple-dual-tandem.
Designations for two-gear aircraft are “F” for dual-tandem (see reference 10) and “N” for triple-dualtandem.
Other multiple-gear aircraft configurations which might be added in the future will require additional
designations and alterations to the source code. The designation for four-wheel strain response and 16-wheel passto-
coverage computation for B-747 aircraft is J. This designation still exists in the program and the 1.2 procedures
for B-747 CDF computations can still be done if the data is formatted correctly.
For aircraft with WFBF and WFBN designations, response is computed with all of the wheels in the main
gear acting together. Evaluation points used to compute vertical strain are entered into the aircraft library data
structure in a defined order.
1. The first eight points are the standard locations for the wing gear type.
2. The second eight points are the standard locations for the body gear type. The standard locations for tripledual-
tandem gears have only six points. The number was increased to eight for the WFWN designation so
that the dual-tandem and triple-dual-tandem data structures are compatible. (Aircraft with two triple-dualtandem
gears (designation N) still have six evaluation point locations as standard.)
3. The remaining points comprise a rectangular grid enclosing the plan view area of the wheels on one side of
the aircraft and extending to the centerline of the aircraft. The current grid is six points wide and five points
long.
The evaluation point locations for the B-747 and the A380 aircraft are shown in figures 7 and 8.
6
After computing response at all of the evaluation points, the maximum response is found among the wing
gear and leftmost three rows of evaluation points. This response is assumed to apply to each wheel in the wing gear.
The maximum response is found among the body gear and rightmost three rows of evaluation points and assumed to
apply to the body gear. CDF is then computed as in version 1.2 for two separate aircraft: one having only the wing
gears and the second having only the body gears.
RIGID DESIGN PROCEDURE CHANGES
Rigid pavement thickness design in LEDFAA 1.3 is unchanged from version 1.2 except that the B-747 is now
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