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treated as a multiple-gear aircraft for response computation. This is for compatibility with the A380, which has
different numbers of wheels in the wing and body locations and forces a change in the program implementation.
However, the landing gear trucks can be treated independently for stress computation without significant error in the
computed values (see reference 11), unlike with flexible pavement strain computations (as discussed above).
Changes to the program were also required for compatibility with the new aircraft designations. CDF computations
are exactly the same as for flexible pavements except for the following.
1. The pavement response predictor of failure is maximum horizontal stress (maximum principal stress) at the
bottom of the concrete slab for new pavements. For concrete overlays on rigid pavements, maximum
horizontal stress is computed at the bottom of the overlay slab and at the bottom of the existing slab.
2. The transverse component of pass-to-coverage is computed in the same way as for flexible pavements
except that an ellipse at the surface of the pavement is assumed.
3. The longitudinal component of pass-to-coverage is unity for tandem wheel spacings less than or equal to 72
inches (1.83 m) and 0.5 for tandem wheel spacings greater than 72 inches (1.83 m). (For wheel spacings
less than 72 inches rigid pavement pass-to-coverages are the same as in AC 150/5320-6D except for small
differences due to the assumed width of the contact patch).
4. Multiple-gear aircraft are treated completely as two separate aircraft, for both response computation and
CDF computation. The same data structure as for flexible pavements is used to define the evaluation points
but only the first eight points are used to compute the wing gear response and only the second eight points
are used to compute the body gear response. In contrast to the belly gear aircraft implementation, the
multiple gear aircraft implementation for rigid pavements does not explicitly appear as two aircraft in the
design aircraft list. The separation is completely transparent to the user unless the output text file is opened
and inspected.
Items 1 through 3 above apply to both versions of LEDFAA. Item 4 applies to version 1.3 only.
SUMMARY AND FUTURE DEVELOPMENTS
The FAA’s computer program for airport pavement thickness design, LEDFAA, has been upgraded from version 1.2
to 1.3 to retain compatibility with new operating systems for personal computers and to accommodate the latest
commercial jet aircraft, in particular the Airbus A380-800. The main changes in the update have been to reprogram
in Visual Basic 6.0, slightly improve the user interface, change the layered elastic computational program, expand
the aircraft library, change the failure criteria for subgrade failure in flexible pavement design, and change the way
in which vertical strain in the subgrade of flexible pavements is computed for multiple-gear aircraft.
The use of a layered elastic model to represent a rigid pavement does not allow design for a jointed
pavement and LEDFAA converts interior stresses to equivalent edge stresses by an approximate correlation
technique. A new program, called FEDFAA, has been developed for rigid pavement thickness design in which a 3D
finite element model is used for jointed rigid pavement structures. Flexible pavement design in FEDFAA is exactly
the same as in LEDFAA 1.3, and any changes which may be made in LEDFAA before FEDFAA is ready as a
replacement will be incorporated in the final version.
A further change which will be made to LEDFAA for flexible pavement design is to treat belly-gear
aircraft such as the MD-11 and the A340 in the same manner as the large multiple-gear aircraft. That is, all wheels in
the main gear will be assumed to contribute to the computation of vertical strain. This will not significantly change
the thickness designs produced by the program, but will be implemented so that the multiple-gear aircraft are all
treated the same. The fact that the belly-gear wheels have different tire pressures than the wing-gear wheels
complicates the implementation and is the reason for the delay. Minor changes may also be made to the flexible
subgrade failure criteria as a result of an ongoing correlation study between the different design procedures. Changes
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in thickness designs will not be significant. (A description of the correlation study is contained in a companion
paper.)
ACKNOWLEDGEMENTS
The work described in this paper was supported by the FAA Airport Technology Research and Development
Branch, AAR-410, Dr. Satish K. Agrawal, Manager. Thanks are also due to Rodney Joel of the FAA Office of
Airport Safety and Standards, AAS-100, for information on Change 3 to AC 150/5320-6D, and for other technical
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