AIR TRACTOR AT-502A, Maintenance Section

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Air Tractor, Inc. Maintenance Page 05
AT-502A/502B/504 March 14, 2018
The oil cooler may be drained by removing both hoses and allowing the oil to run out. Removing the chip
detector (Figure 102 Item No. 2) will drain the gear reduction case, as will removal of the drain plug (Figure
102 Item No. 9) located in the lowest point of the accessory case. You cannot get it all out, but if you do that
much, you will be in good shape.
Checking
the oil should always be done within 10 minutes of shut down. Unlike the “OTHER” Pratts,
this one does not require filling before checking. The reason for checking the level right after shut down is
that it does not allow the oil time to escape to other areas of the engine. Sometimes bad seals allow the oil to
leak back into cavities inside the engine giving you a false indication on the dipstick. Once the engine starts
turning, the scavenge pump picks up the oil and returns it to the oil tank.
The oil filter, located on the right side of the air inlet case, is held in place by a cover with 4 nuts.
Removing the oil filter requires a special puller and should not be removed by any other means. Cleaning the
filter may be accomplished by gently agitating the element in clean solvent. After cleaning, allow the element
to stand in a clean environment until dry.
CAUTION: UNDER NO CIRCUMSTANCES MAY THE ELEMENT BE ULTRASONICALLY
CLEANED, PRESURE FLUSHED, OR DRIED WITH COMPRESSED AIR. SUCH
CLEANING AND DRYING WOULD CAUSE DAMAGE TO THE FILTER MEDIA.
Besides cleaning the filter, pull the chip detector on the bottom of the gear reduction case and keep it
cleaned off every 200 hours. The discovery of ANY magnetic particles should be investigated by qualified
personnel.
Air Filters
The PT6A engine requires a large amount of air at full power settings. To handle this load while
adequately cleaning the air, your Air Tractor is equipped with large, efficient air filters. The length of service
for these filters depends entirely on the operating conditions. When you replace these filters, use the original
equipment filters for replacement. Cheaper, aftermarket filters are not the same quality, so they neither
perform as well nor last as long. Remember, your engine requires a lot of air to perform properly. Check the
condition of the filters often to determine the replacement intervals for your flying conditions. Filters should
be inspected/cleaned/replaced in accordance with the Inspection Schedule is Section 3. Illumination of the
Air Filter CAUTION light indicates that normal air flow through the filter is restricted. Should the Air Filter
CAUTION light illuminate, land as soon as practical and service the air filter system including cleaning or
replacing the filter(s) as necessary. (See instruction below). Do not rely on this light to determine air
filter cleaning/replacement intervals.
For aircraft with the large, cylindrical commercial truck filters (Donaldson p/n P182043 (alternate is
P510337) for the AT-502B/504 and Donaldson p/n P181008 (alternate is P510335) for the AT-502A),
removal of these filters is accomplished by removing the airscoop assembly and the aft lower engine cowl.
There is a 3/8” bolt with a check nut on the aft center of each filter, and it is necessary to loosen this bolt
before the filters can be removed. The bolt is to press the filters against the forward wall of the filter box for a
tight seal. With the filters loose, the small support angles are removed and the filters will slide out the bottom
opening. These filters may be cleaned using the instructions in Air Tractor Service Letter 257. The filters are
installed in reverse order of removal.
For AT-502A/502B/504 aircraft (only 502A aircraft with the PT6A-140AG engine) with the Donaldson
p/n P613731 Ram Air filter, removal of the filter is accomplished by removing the lower cowl skin and the six
AN3 bolts that hold the filter access cover to the aft plenum bulkhead. Remove the filter by pulling it through
this access opening. These filters may also be cleaned using the instructions in Air Tractor Service Letter
257. The filters are installed in reverse order of removal.