AIR TRACTOR AT-502A, Maintenance Section

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Maintenance Page 8 Air Tractor, Inc.
March 14, 2018 AT-502A/502B/504
The clamp bolts should be changed every 1,000 hours for row-crop work and every 500 hours if
operating from rough strips in rice operations. Torque the clamp bolts to 600 inch-pounds (torque wrench on
bolt head), and make sure there are sufficient shims between the clamp block and the fuselage frame to
prevent bending the clamp block as the bolts are torqued.
The tail gear spring will fatigue and break at some point in the life of the aircraft. The number of hours
before failure will vary considerably as some operators make many more landings per flight hour than others,
or carry heavier loads, or operate from very rough strips. Pilot technique is a factor also, since some pilots
are very rough on airplanes. For these reasons it is not possible to predict when a failure might occur.
Usually the tail spring will show signs of yielding just prior to failure and the tail of the aircraft will sit a
slight bit lower to the ground than previously. Sometime during the first 100 hours or so of operation a
measurement should be taken and recorded from the clamp block to the ground or hangar floor. The aircraft
should be fully fueled, and with a full hopper. At various intervals this measurement should be checked, and
after 1,000 hours check the measurement at least every 50 hours. Although service history for the Air Tractor
is relatively limited, it appears that the tail spring should be changed about every 1500 hours for worst
conditions of rice operations from rough strips, and every 2,000 hours for the best conditions of row-crop
work from smooth strips. Each operator should determine at what point in between these extremes his
operation fits and change the tail spring at the appropriate interval, unless the measuring process described
earlier indicates yielding. In that case the spring should be changed immediately. When ordering a new tail
gear spring, also order two ea. 40065-1 shims in case they are needed.
For everyone’s protection, take a cutting torch and destroy the old spring when it is replaced.
MAIN GEAR SPRING
The main gear spring is p/n 40086-1 and will fit on either side of the aircraft. It is machined of E-4340
aircraft quality steel, formed, heat-treated, shot-peened for fatigue resistance, and magnafluxed for cracks. It
is sandblasted and primed and finished with yellow polyurethane paint.
The inboard attachment of the spring to the fuselage frame is with a NAS156A88 bolt, MS20002C16
washer under the bolt head (be sure the chamfered side of the washer is next to the head), one or more
MS20002-16 washers under the nut, AN310-16 nut, and MS24665-360 cotter. The bolt is installed upside
down and torque should be 6,400 inch-pounds (torque wrench on bolt head). A simple adapter for your
torque wrench is a short piece of 1.0" hex stock to enter the bolt head, and a 1.0" socket for your torque
wrench.
The inboard bolt should be changed every 2,000 hours for row-crop work and every 1,000 hours for
rice operations. It is necessary to hoist the aircraft as described under “Lifting and Hoisting” when changing
the inboard bolt. Be sure to grease the bolt with general purpose grease before installing. Also, be sure the
bolt is marked “R” on the head to indicate the threads were rolled after heat-treat.
The gear spring is clamped to the fuselage frame with a p/n 40080-1 clamp block with one
NAS154A114 bolt and one NAS152DH43 bolt. MS20002C12 washers go under the aft bolt head and
MS20002C14 washers go under the forward bolt head (Make sure the chamfered side of the washer is under
the bolt head). Various numbers of 40055-1 and 40055-2 round shim washers (each a different thickness) go
between the clamp block and fuselage frame at the aft bolt location. The fwd bolt uses p/n 3088A 7/8 ID
shims. The shims are selected to allow the bolts to be fully torqued without bending the clamp block. The
40055-1 shim is .018 thick and the 40055-2 shims .030 thick. The 3088A shims are .010 thick. If too many
thick shims are used the gear connection will be too loose and will rattle during taxi or landing. If the gear leg
becomes loose during service even though the clamp block bolts are at full torque it is necessary to
exchange a thick shim for a thin one. Alternate shims for 40055-1/-2 are 3088A324 (3/4 ID).