AIR TRACTOR AT-502A, Maintenance Section

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Air Tractor, Inc. Maintenance Page 9
AT-502A/502B/504 March 14, 2018
The method employed at the factory to determine the amount of shims required is to install the clamp
block and fully torque the bolts before installing the large inboard bolt. The block is checked for straightness
with a small straight edge (a six-inch steel scale is OK) to be sure the block is not bent by not having
sufficient shims. Then the gear leg is grasped at the lower end, given an outboard tug by hand. A moderate
tug should move the inboard end of the leg down from the attach bushing in the fuselage frame a distance of
1/8" to 1/4". If it is a greater distance than 1/4" the connection is too loose, and a thinner shim is tried and the
process repeated. Once the proper shims are determined, the bolts are loosened and the large inboard bolt
is installed. Then the clamp block bolts are tightened to full torque (3,000 inch-pounds on aft bolt, 3,500 inch-
pounds on fwd bolt).
An adapter for your torque wrench may be made from a short piece of 5/8" hex stock to enter the aft
bolt head, and a 5/8" socket for the torque wrench. Be sure to grease the bolts before installing. The clamp
block bolts should als
o have the letter “R” stamped on the head to indicate the threads were rolled after heat-
treat.
For row-crop operations the forward clamp bolt should be changed every 2,000 hours and the aft
clamp bolt every 2,000 hours. For rice operations from rough strips the forward bolt should be changed every
1,000 hours and the aft bolt every 1,000 hours.
If an obstruction is hit with the main gear change the forward clamp bolt immediately and check the
clamp block for straightness. The forward clamp bolt can be changed without hoisting the aircraft. The
parking brake should be set, and use care so as not to disturb the shims that are in place.
Since each main gear leg is slightly different in thickness, and since each clamp block is machined
within a specific tolerance, it is necessary to make a shim selection as described earlier if either the gear leg
or the clamp block is changed.
As in the case of the tail gear spring, the main gear spring will fatigue and break at some point in the
life of the aircraft. Again, the number of hours before failure will vary considerably as some operators make
more landings per hour than others, or carry heavier loads, or operate from very rough strips. Pilot technique
is again a factor, as some pilots are very rough on airplanes. For these reasons it is impossible to predict
when a failure might occur.
Service history is still relatively limited but in the interest of safety and the high financial consequences
of a broken main gear leg, it would be advisable to take a highly conservative approach and change the main
gear springs well before the probable fatigue life. The main gear springs should be changed in accordance
with Service Letter #104.
When the main gear springs are changed, use new bolts and nuts at all locations. The axle attach bolts
are NAS148-66. Install the springs to the fuselage in accordance with the instructions in this section, and
install the axles to the spring using the procedure to check camber and toe-
in outlined in the section “Main
Wheel Alignment”. Be sure to make log book entries when bolts or gear springs are changed.
When ordering new main gear springs or clamp blocks also order 8 ea. 3088A325 shims and 8 ea.
3088A324 shims in case they are needed. In addition, the large aluminum tapered shims at the axle
attachment might need an addition or change so it would be a good idea to order 2 ea. 40077-1 shims and 2
ea. 40077-2 shims in case they are needed.
Whenever the airplane is ground-looped or involved in an incident that puts a permanent bend in one
gear spring, the other spring must be changed as it may have been overstressed.