AIR TRACTOR AT-502A Instructions for use

AIR TRACTOR, INC.
Olney, Texas
AT-502A/502B/504
TABLE OF CONTENTS
MAINTENANCE SECTION
Page No.
ENGINE MAINTENANCE................................................................................................................. 01
Introduction .......................................................................................................................... 01
Maintenance......................................................................................................................... 01
Cleaning Your Engine Exterior............................................................................................. 01
Fuel ................................................................................................................................ 02
Figure 101 Fuel Nozzles...................................................................................................... 03
Oil ................................................................................................................................ 03
Figure 102 Location of Oil Drain Plugs................................................................................ 04
Air Filters.............................................................................................................................. 05
Miscellaneous ...................................................................................................................... 06
Compressor Washes............................................................................................................ 07
Figure 103 Gas Generator Case.......................................................................................... 07
Troubleshooting ................................................................................................................... 08
Starting Procedures ............................................................................................................. 08
Ground Run Procedures...................................................................................................... 09
Routine Maintenance Inspection........................................................................................ 010
INTRODUCTION................................................................................................................................. 1
FUEL REQUIREMENTS..................................................................................................................... 1
OIL REQUIREMENTS ........................................................................................................................ 1

AIR TRACTOR, INC.
Olney, Texas
AT502A/502B/504
TABLE OF CONTENTS (Continued)
MAINTENANCE SECTION
Page No.
TIRE INFLATION................................................................................................................................ 1
LIFTING AND JACKING..................................................................................................................... 1
MAIN WHEELS................................................................................................................................... 2
Main Wheel Alignment .......................................................................................................... 2
BRAKES.............................................................................................................................................. 3
Brake Master Cylinders.......................................................................................................... 3
Brake Bleeding....................................................................................................................... 4
Parking Brake......................................................................................................................... 4
TAIL WHEEL....................................................................................................................................... 4
Tail Wheel Fork...................................................................................................................... 5
Tail Wheel Fork Housing........................................................................................................ 5
Tail Wheel Lock Pin and Housing.......................................................................................... 6
MAIN AND TAIL GEAR ATTACH BOLTS .......................................................................................... 6
TAIL GEAR SPRING........................................................................................................................... 7
MAIN GEAR SPRING......................................................................................................................... 8
WINGS ............................................................................................................................................. 10
Wing Attachment to Fuselage.............................................................................................. 10
Wing Center Splice Connection........................................................................................... 11
Wing Walk............................................................................................................................ 11

AIR TRACTOR, INC.
Olney, Texas
AT-502A/502B/504
TABLE OF CONTENTS (Continued)
MAINTENANCE SECTION
Page No.
FUEL TANKS.................................................................................................................................... 12
Fuel Tank Senders............................................................................................................... 13
Fuel Tank Receiver.............................................................................................................. 13
FUEL SYSTEM DRAINS .................................................................................................................. 14
FUEL SYSTEM SCREENS AND FILTERS...................................................................................... 14
AILERONS........................................................................................................................................ 14
FLAPS ............................................................................................................................................. 15
AILERON AND FLAP RIGGING....................................................................................................... 16
AILERON AND FLAP BELLCRANKS AND SUPPORTS ................................................................. 16
FLAP ACTUATOR ............................................................................................................................ 17
Flap Actuator Troubleshooting............................................................................................. 18
Flap Actuator Removal Instructions..................................................................................... 19
Flap Actuator Installation Instructions.................................................................................. 19
Flap Motor Troubleshooting................................................................................................. 20
HORIZONTAL STABILIZERS........................................................................................................... 20
Stabilizer Struts.................................................................................................................... 21
Stabilizer Rigging................................................................................................................. 21
VERTICAL FIN.................................................................................................................................. 21
RUDDER........................................................................................................................................... 22
RUDDER CONTROLS...................................................................................................................... 22
ELEVATORS..................................................................................................................................... 23

AIR TRACTOR, INC.
Olney, Texas
AT-502A/502B/504
TABLE OF CONTENTS (Continued)
MAINTENANCE SECTION
Page No.
ELEVATOR CONTROL .................................................................................................................... 24
COCKPIT CONTROLS..................................................................................................................... 25
ELEVATOR TRIM TABS................................................................................................................... 26
TRIM TAB CONTROLS .................................................................................................................... 26
TRIM LEVER..................................................................................................................................... 27
FUSELAGE REMOVABLE SKINS.................................................................................................... 27
FUSELAGE FIXED SKINS................................................................................................................ 27
FUSELAGE COCKPIT SKINS.......................................................................................................... 28
FUSELAGE FRAME ......................................................................................................................... 28
WINDSHIELD.................................................................................................................................... 28
CANOPY DOORS............................................................................................................................. 28
SEAT ............................................................................................................................................. 28
ENGINE DRIVEN AIR CONDITIONER ............................................................................................ 29
Cautionary Information......................................................................................................... 30
R134a Information................................................................................................................ 30
Application Specifications .................................................................................................... 32
Diagnosis Confirmation of Compressor Failure................................................................... 32
Compressor Repair.............................................................................................................. 36
Compressor Replacement ................................................................................................... 40

AIR TRACTOR, INC.
Olney, Texas
AT-502A/502B/504
TABLE OF CONTENTS (Continued)
MAINTENANCE SECTION
Page No.
Service Procedures.............................................................................................................. 41
Common Causes of Compressor Failure ............................................................................ 46
HOPPER........................................................................................................................................... 47
HOPPER GATE BOX........................................................................................................................ 47
HOPPER LID .................................................................................................................................... 48
GATE BOX CONTROLS................................................................................................................... 48
SPRAY LEVER CONTROLS............................................................................................................ 48
SPRAY PUMP................................................................................................................................... 48
SPRAY PLUMBING .......................................................................................................................... 49
BOTTOM LOAD PLUMBING............................................................................................................ 49
SPRAY NOZZLES ............................................................................................................................ 49
STRIPPING AND RE-PAINTING PARTS......................................................................................... 50
Stripping and Repainting Aluminum Parts........................................................................... 50
Priming Aluminum Parts with Chromated Alkyd Primer....................................................... 51
Stripping & Repainting Steel Parts....................................................................................... 51
Materials Used for Stripping, Painting, & Preservation........................................................ 52
TORQUE VALUES FOR SHOP USE ............................................................................................... 53
ELECTRICAL SYSTEM.................................................................................................................... 54
General ................................................................................................................................ 54
Starter/Generator................................................................................................................. 54

AIR TRACTOR, INC.
Olney, Texas
AT-502A/502B/504
TABLE OF CONTENTS (Continued)
MAINTENANCE SECTION
Page No.
Voltage Regulator ................................................................................................................ 54
Charging System Troubleshooting Guide............................................................................ 55
Troubleshooting Procedures................................................................................................ 56
Lighting System Troubleshooting......................................................................................... 58
Checking the Batteries......................................................................................................... 59
Oil Cooler Blower................................................................................................................. 60
Engine Instruments.............................................................................................................. 60
Engine Overspeed Solenoid ................................................................................................ 60
Voltmeter.............................................................................................................................. 60
Boost Pump.......................................................................................................................... 60
Fuel Gauging........................................................................................................................ 60
Wing Flaps........................................................................................................................... 61
Stall Warning Horn............................................................................................................... 61
Windshield Washer.............................................................................................................. 61
Windshield Wiper................................................................................................................. 61
Cockpit Lighting.................................................................................................................... 61
Flap Light ............................................................................................................................. 61
Position & Strobe Lighting.................................................................................................... 62
Taxi Lights............................................................................................................................ 62
Night Working Lights............................................................................................................ 62

AIR TRACTOR, INC.
Olney, Texas
AT-502A/502B/504
TABLE OF CONTENTS (Continued)
MAINTENANCE SECTION
Page No.
AVIONICS......................................................................................................................................... 63
LIFE LIMITED ITEMS ....................................................................................................................... 63
PROPELLER MAINTENANCE ......................................................................................................... 64
TIE-DOWN INSTRUCTIONS............................................................................................................ 65
TOWING INSTRUCTIONS AND LIMITATIONS............................................................................... 66
DRAINING THE STATIC SYSTEM................................................................................................... 67
LOW FUEL WARNING SYSTEM ..................................................................................................... 67
Test Function ....................................................................................................................... 67
Corrective Action.................................................................................................................. 67
AMSAFE AIRBAG SYSTEM............................................................................................................. 67
LEVELING......................................................................................................................................... 68
WEIGHT AND BALANCE ................................................................................................................. 68
ITT INSTRUMENT CALIBRATION CHECK ..................................................................................... 68

Air Tractor, Inc. Maintenance Page 01
AT-502A/502B/504 March 14, 2018
ENGINE MAINTENANCE
Introduction
This section of the Maintenance/Inspection manual written by Van B. Dardis, Lubbock, Texas.
The Pratt & Whitney PT6A series of engines have been proven to be one of the most reliable
powerplants available today. The engine is a lightweight, reverse flow, free turbine engine utilizing two
independent turbine sections. One section drives the compressor in the gas generator section and the
second drives the propeller shaft through a gear reduction box.
Air enters the engine through an annular plenum chamber, passing through a three stage axial and one
stage centrifugal compressor. In the combustion chamber, fuel mixed with air is ignited. The expanding
gases are directed through the two turbines and out the exhaust ports on opposing sides of the engine.
While an unfamiliar and complex looking piece of equipment, the engine is easy to understand and,
with a little time, will begin to talk to you like all the others before.
Maintenance
Maintenance on your Pratt & Whitney PT6A engine will be one of a preventative nature. Time and
experience have proven that an engine properly maintained will give excellent service with a minimum of
down time and expense. Establish a routine maintenance schedule and adhere to it as closely as possible,
taking into consideration the problems of busy schedules in mid-season.
Equally important to proper maintenance is the keeping of records. Maintenance records on a turbine
engine are extremely important and every effort should be made to see that all the work performed is
properly recorded. A record of the number of starts and flights MUST be kept for figuring cycle counts on the
rotor components in your engine at each annual. The cycles are figured using the number of starts and
flights, with a formula found in the applicable engine Service Bulletin. When these parts reach their life limit in
either hours consumed or cycles, they must be removed and discarded. Few of you will ever see a part
reach that limit but the records must be kept for your logbooks to be in proper order.
While on the subject of record keeping, it is a good idea to keep a small pad in the cockpit to record
engine parameters on a regular basis. For example, on the return from your first load each day, at the same
altitude and with the same torque and propeller RPM setting, record the following: hourmeter time, OAT, ITT,
Ng, oil pressure, and fuel pressure. When trouble appears, this information will save lots of time
troubleshooting and will also give you a good picture of the trends in your engine. Remember, no two
engines will run the same. The change in your engine parameters over a period of time is what really
matters. Keep it written down because your mind will deceive you over the span of a season and it is the
best way for maintenance personnel to make an accurate diagnosis when problems arise.
Cleaning Your Engine Exterior
The cleaning of your engine may be accomplished only when the engine is cold. NEVER WASH A HOT
ENGINE. You may wash the engine exterior with water or with a petroleum base solvent. While cleaning
your engine, it is a good time to inspect all the engine externals; tubing, wiring, control linkages, hose
assemblies, etc. You should look for evidence of wear, chafing, cracks, corrosion, fuel and oil leaks, and the
security of hoses, brackets, clamps and connections. Also inspect your air inlet system for signs of leaks that
would allow unfiltered air into the engine.

Maintenance Page 02 Air Tractor, Inc.
March 14, 2018 AT-502A/502B/504
After cleaning your engine, blow it off with compressed air and run the engine until it is dry. Keeping the
outside of your engine clean will help in spotting problems, make controls easier to operate, and limit the
wear on the beta block and beta ring. Remember also to keep your oil cooler cleaned on a regular basis to
maintain the efficiency of the cooler for operation in hot weather.
Fuel
The Pratt & Whitney PT6A engine will run on Jet A, kerosene, diesel, or Avgas. Avgas may be used
only in the case of an emergency and for a duration of no longer than 150 hours between overhauls. Diesel,
though approved by Pratt & Whitney, is not recommended. Diesel does not burn as clean as Jet A or
kerosene, causing problems with fuel nozzles, combustion liners, and hot section parts.
Air Tractor recommends the use of Jet A fuel that has been treated with a Fuel System Icing Inhibitor
(FSII) such as Prist. Some Jet A comes from the supplier with FSII in it and some does not, so check with
your supplier to be sure. It is also recommended that fuel be treated with a biocide (such as BioBar) to
prevent the growth of bacteria and fungus in your fuel storage tank(s).
On your ground equipment, install a fuel filter/water separator that will shut off the fuel flow when water
is present. Fuel MUST be kept clean, water free, and free of bacterial and fungal growth. Care should be
taken when fueling the aircraft in dusty conditions to keep the dirt from entering the fuel tanks.
Should something get into the system, your aircraft is equipped with a fuel filter located on the firewall
between the batteries. This fuel filter may be either a Fram FS 1133-PLM or Air Tractor p/n 53114-1 fuel
filter. Both fuel filters use an Air Tractor p/n 52351-1 fuel filter element. See Service Letters #229 and
#229A for more details on fuel filter inspection and maintenance.
The engine also has a disposable fuel filter on the high pressure pump located at the 3 o’clock position
on the accessory case. This element is an AN6235-3A. This filter should be changed anytime the firewall
filter is found contaminated or every 300 hours. If you find ANYTHING in either filter, you should investigate
until the problem has been found and corrected.
Drain the fuel header tank sump DAILY to remove any moisture in the fuel system from condensation
or contamination. Draining the sump daily also will prevent the drain valve from freezing up from contact with
fertilizer and chemicals.
Your Air Tractor is equipped with an electric fuel boost pump and an engine driven low pressure fuel
pump. The electric boost pump, installed as a back-up for the engine driven low pressure pump and for filling
the lines prior to starting, can provide fuel to the high pressure pump, which must have a minimum of 5 PSI
at all times.
Your plane has a negative pressure switch and warn light with the switch on the outlet side of the fuel
filter. If this warning light should come on, turn on the electric boost pump and have the problem corrected
promptly. (A check of your fuel filters is in order here, you may have one starting to clog up due to fuel
contamination.)
Fuel nozzles are the most important routine maintenance item on your engine. No one single item has
as much direct effect on the condition of the inside of your engine. Dirty fuel nozzles can ruin combustion
liners, guide vanes, turbine blades, and the gas generator case. Fuel nozzles are not cleaned WHEN they
are dirty, but to keep them from BECOMING DIRTY. Your fuel nozzles may be cleaned by your maintenance
personnel or they may be sent to a number of locations for cleaning and returned to you for installation. Air
Tractor recommends nozzle cleaning at 300 hour intervals as long as you are using Jet A fuel, and you are
getting good reports from the personnel cleaning the nozzles you remove.

Air Tractor, Inc. Maintenance Page 03
AT-502A/502B/504 March 14, 2018
The earlier model of PT6A engines came with 7 primary and 7 secondary nozzles. The engine starts on
the primary nozzles and at about 35% Ng the secondary nozzles come in. The engineers at Pratt & Whitney
have discovered that the engine will start cooler by reducing the number of secondary nozzles, hence
Service Bulletin 1372. All the new engines from Pratt & Whitney come in the 10 primary / 4 secondary
configuration. Most of the older engines have been converted, but if you have one that has not, converting it
over will cool down your starting ITT temperatures considerably.
Before you remove the fuel nozzles, write down the location of the primary and secondary nozzles. The
primary nozzle may be identified by a weld spot on the flat area next to one of the bolts. See figure 101. DO
NOT MARK ON THE GAS GENERATOR CASE WITH A PENCIL.
To remove the fuel nozzles, remove the safety wire from the retaining bolts and remove the bolts with a
twelve point 1/4 inch socket. After removing the bolts and transfer tube retainer, the transfer tube will slide
one direction or the other into the nozzle body. After sliding the transfer tubes clear, the fuel nozzle may be
removed from the engine. The fuel supply lines will have to be disconnected from the lower nozzle.
When installing the fuel nozzles, use anti-seize or moly-lube on the bolts and torque them to 36 in. lbs.
and safety the bolts. Extreme caution should be used in handling the clean fuel nozzles. DO NOT TOUCH
THE NOZZLE TIP WITH YOUR FINGERS.
Be careful when working around the T5 wires on the right side of the engine. These wires and their
attaching points are fragile and should not be bumped or moved.
IMPORTANT: The quality of fuel and the condition of the fuel and fuel nozzles will have a direct relation
to the condition and the longevity of your engine.
FIGURE 101
Oil
Oil used in the Pratt & Whitney PT6A engine is a synthetic base oil designed for turbine and jet
engines. Unlike petroleum base lubricants, turbine oil cannot be mixed, one brand with another. If you have
to change brands, you should drain the oil tank, oil cooler, gear reduction case, and accessory case. See
figure102 for the drain locations.
The oil tank drain (Figure 102 Item No. 7) is located on the bottom of the air inlet case and may be
removed by removing the clevis pin (Figure 102 Item No. 5), inserting a 1/2-20 NF bolt into the threaded plug
and pulling. The drain plug just has an O-ring (Figure 102 Item No. 6) on it and will come out with a little pull.

Maintenance Page 04 Air Tractor, Inc.
March 14, 2018 AT-502A/502B/504
LOCATION OF OIL DRAIN PLUGS
2. CHIP DETECTOR 7.OIL TANK DRAIN 10. ACCESSORY CASE DRAIN
Figure 102

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.

Maintenance Page 06 Air Tractor, Inc.
March 14, 2018 AT-502A/502B/504
For AT-502A (all engines except the PT6A-140AG) aircraft with the p/n 52427-17 Ram Air filter,
removal of the filter is accomplished by removing the lower and side cowl skins from the aircraft. Then
completely remove the lower plenum cover to gain access to the filter. From the righthand side of the
airplane, remove the filter retaining band on the aft plenum bulkhead by removing the eight bolts that hold it
in place. The heads of these bolts are on the aft side of the aft plenum bulkhead. Lastly remove the filter by
loosening the four worm drive clamps that hold the filter in place. This filter may be cleaned using the
instructions in Air Tractor Service Letter 257. The filters are installed in reverse order of removal.
For aircraft with the Brackett BA-409E (AT-502B/504 and AT-502A with PT6A-140AG engine) or
BA-413E (AT-502A with PT6A-45R/60AG/65AG engines) foam Ram Air filter, removal of the filter element is
accomplished by removing the lower and side engine cowls and the lower engine plenum cover. Remove
the center filter support rods by removing the screw on the forward end and sliding them out of their retaining
brackets at the aft end. Remove the old filter element and replace with a new one, ensuring that the filter
element completely fills the stainless steel frame, especially in the corners. Replace the filter support rods,
plenum cover, and cowl skins in reverse order of removal. This filter should not be cleaned, as this may
remove the wettant and decrease the filtering ability of the foam. Any excess dirt or debris that has
accumulated on the forward face of the filter may be brushed off with a bristle brush, like a paintbrush, and
vacuumed out of the plenum. The Brackett air filter has a maximum recommended service life of 12
calendar months, regardless of the actual flight time. The reason is to ensure that acceptable levels of
wettant remain in the filter to adequately capture dust and debris.
Miscellaneous
The starter-generator spline requires greasing regularly. The quick disconnect band may be removed
by loosening the nut on the retaining bolt. After removing the band, slide the starter out exposing the spline.
Get a good hold on the starter. THE STARTER IS VERY HEAVY. Grease the spline with a MIL-M-7866 type
of grease and reinstall the starter. DO NOT JAB THE STARTER INTO THE HOLE! ! ! The gear in the engine
has a carbon seal inside that will be damaged if the gear should receive a hard blow. The process will be a
little awkward the first time or two but you will get the hang of it with practice. It is a lot easier and cheaper to
grease the spline than to replace the gear in the engine.
The controls and control cables should be kept clean and dry. Do not put grease on the controls as it
attracts dirt causing excessive wear and binding.
The batteries on your turbine will require REGULAR servicing. Your batteries will last longer if you
maintain the proper water level and the proper voltage on the voltage regulator. Too high a voltage on the
regulator will boil the batteries. Most operators use a digital voltmeter to set the regulator to a measured
voltage of 27.5 in the summer or 28.5 in the winter.
One of the hardest things on your batteries is neglect in the off season. Air Tractors with Crophawk flow
meters or anything else wired direct to the battery should be disconnected during the off season so as not to
drain the battery. Periodic checks should be made during the off season to insure that the batteries stay
charged. You may turn on the master switch and read battery voltage on the voltmeter. Batteries below 22
volts should be put on the charger and charged back to full strength. If you will keep them serviced during the
season and give them a little maintenance in the off season, their longevity will increase. Don’t forget to drain
the battery acid sump at regular intervals.
When leaving your airplane outside, be sure and cover the exhaust ports and keep the propeller
tethered. The propeller needs to be tethered to prevent the windmilling of the power section. The reason for
this is that the oil pump is in the rear of the engine and will not turn with the prop and therefore, will not
provide oil to the front of the engine. Don’t be too embarrassed the first time you start the engine with the
propeller tethered, I can assure you that you are not the first.

Air Tractor, Inc. Maintenance Page 07
AT-502A/502B/504 March 14, 2018
Compressor Washes
Compressor washes are performed to remove dirt and deposits which hamper the efficiency of the
compressor. The quality of the Air Tractor induction system is so good that most engines require a
compressor wash only once a year. An inspection of your engine inlet is a good indicator of when to perform
a compressor wash. If the cone on the inside of your inlet is dirty, then the compressor is too. Anything going
into the compressor has to go by here. If the cone is clean, you can be sure the inside of your engine is in
pretty good shape. Don’t do a compressor wash just to be doing one.
Regardless of how many you do, there is only one way to accomplish the task with maximum results.
Remove the engine cowling and the air inlet cover over the engine inlet. Connect and start the battery
charger. Remove the P3 filter and leave the filter bowl off. If your engine does not have a P3 filter, remove
the compressor discharge line from the right hand side of the FCU. Remove the two gas generator case fuel
drain valves to allow the dirty water to escape rapidly. See Figure 103.
Figure 103
If you do not remove the valves, what happens inside your engine strongly resembles the bathtub after
the kids finish their bath. The rings left in the engine will do more harm than the washing does good.
After you get the engine ready, take a CLEAN open top 5 gallon bucket and mix 1 qt. of an approved
cleaning agent, (see Note *), 1 gallon of jet fuel, and 3 gallons of water. While motoring the engine, pour 1/2
of the contents into the engine inlet. Wait 15 minutes and pour the remaining mixture into the engine. Then at
20 minute intervals, pour in 2 to 3 gallons of clean water at a time until the water running out of the engine is
clean.
After the water stops dripping out of the engine, replace the fuel drain valves and safety them. Replace
the air inlet cover, P3 filter and engine cowling. When everything has been replaced, start the engine and run
for at least 15 minutes to dry the engine out. Never put the airplane up without running the engine until it is
dry.
Note *: B&B 3100 Cleaner: B&B Chemicals Co., P.O. Box 796, Miami, Florida 33166
Turco 4217 Cleaner: Turco Purex Industries Corp., 24700 South Main Street, Carson, California
90749

Maintenance Page 08 Air Tractor, Inc.
March 14, 2018 AT-502A/502B/504
Troubleshooting
Troubleshooting on the PT6A engine is not that difficult, once you get acquainted with the engine. The
most important thing you can do as an operator is keep an accurate record of the engine parameters. All
troubleshooting starts with the information you will have on that little tablet. Maintenance personnel will need
to know in what way the engine parameters have changed. As you become more experienced with the PT6A
engine, all these parameters will come to have more meaning. Until all these things begin to come together,
consult qualified personnel.
Starting Procedures
Starting the PT6A turbine engine is not complicated, but it is the best opportunity you will have to
damage your engine if the proper procedures are not followed. A battery cart will be a good investment. You
can take two truck batteries wired in series for 24 volts, put them in a cart for ease of movement, and
connect them to cables with a plug compatible to your Air Tractor. It is also convenient to wire a trickle
charger to a female socket like your airplane has and you can keep the batteries plugged in when not in use
so they will always be ready when needed.
Before you begin the starting procedure, make sure all the levers and switches are in their proper
positions. The throttle against the idle stop, the start lever in the cut off position, the propeller in feather, and
the ignition switch in the middle position. Hot batteries or the battery cart will spin your engine between 18%
and 20% Ng. NEVER TRY TO START AN ENGINE TURNING LESS THAN 15%. Once you have obtained
an Ng speed above 15%, turn on your ignitors, wait for the Ng speed to peak, then bring the start lever to the
RUN position. Start counting 1001, 1002, 1003, etc, your engine should ignite before you reach 1005. If it
does not, let off the start switch, pull the start lever back to cut off, and investigate. In any case, allow the fuel
in your engine time to drain. You do not want to start an engine with a lot of excess fuel inside. Most engines
will “light” around 1003. There will be a pause in most engines around 35% Ng, then a continuation on to
*50% to *52% Ng. The ITT will usually peak around 45% and be coming down as the engine reaches its idle
speed. Engines with the 10-4 fuel nozzle configuration will start well under the red line, (usually more than
100oC) while the 7-7 configuration will start somewhat hotter. In no case should an engine normally exceed
the red line on a start, although the manual allows temperatures above the red line for 2 seconds.
Once the engine is above 50% Ng, release the starter switch. You may leave the ignitor switch in the
up position since the ignitors are OFF as soon as the start switch is released. Turn on the generator. Right
after a start, especially without a battery cart, your generator will be carrying a heavy load while recharging
the batteries, and can drag an engine down. If your Ng speed begins to fall below *50% when you turn on the
generator, you should advance the power lever as required to maintain a Ng speed above *50%.
Starting an engine that is turning slow with weak batteries is asking for a disaster. During the start
process, if anything seems questionable, pull the start lever back to the cut-off position with the starter still
engaged and allow the starter to run till the ITT has fallen to safe limits. As I stated at the beginning of this
section, starting the engine is not hard, but you have to do it right or you will find yourself in a trap.
* These are values for the PT6A-15AG/-34AG. For the PT6A-45R/-60AG/-65AG/-140AG expect values of
56-58%.

Air Tractor, Inc. Maintenance Page 09
AT-502A/502B/504 March 14, 2018
Ground Run Procedures
The following are Air Tractor’s recommendations on performing ground engine runs. These
recommendations apply to all Air Tractor aircraft:
1. It is recommended that the hopper be filled with water prior to performing ground runs.
2. The airplane should be positioned on the tie-down such that FOD/prop wash damage to other
aircraft, structures, personnel, or equipment will not occur, nor be directed at or across an active
runway. Also pay close attention to the ground surface below the propeller arc. Sand, gravel, dirt,
tools, debris, etc. can be picked up by the prop and can cause damage to the propeller blades.
3. The ideal tie-down anchor is a steel loop or eye set in concrete. A 24” diameter x 24” tall column of
concrete buried 36” in the ground is usually adequate. Soft or sandy soil may require additional
concrete.
4. The airplane should be tied down at the tail using a polyester or nylon tow strap or lift strap looped
around the tailwheel housing attach block and routed through the tailwheel fork. See Figure 1 below
for a suggested strap routing. Exercise extreme caution that the tie-down strap does not contact the
steering pin mechanism at any time during the ground run. Tie-down strap should be minimum 5000
lbs. breaking strength. The airplane should be positioned as close as possible to the tie-down
anchor to provide sufficient vertical tension in the tie-down strap.
5. Chocks should be removed to prevent the aircraft from rotating on the main wheels and nosing into
the ground in the event of tie-down and/or brake failure. Personnel conducting the ground run
should anticipate forward movement of the aircraft in this event.
6. Only qualified personnel should conduct the ground runs.
7. The airplane brakes should be set or held during the ground run.
8. The control stick should be held full aft (nose up) during the ground run.
Figure 1 - Suggested Tail Tie-Down Method

Maintenance Page 010 Air Tractor, Inc.
March 14, 2018 AT-502A/502B/504
Routine Maintenance Inspection
The following is a guide to your routine maintenance inspections. It is not an absolute, but intended to be a
guide to help you in establishing a schedule that will maintain your engine in the proper manner. For
complete maintenance inspection schedule see the appropriate Pratt & Whitney Maintenance Manual.
100 Hour Intervals
__________ GREASE PROPELLER
200 Hour Intervals
__________ REMOVE AND CLEAN OIL SCREEN
__________ REMOVE AND CLEAN FUEL NOZZLES
300 hour intervals are OK if you are using Jet A and you are getting good reports on the
nozzles you remove.
__________ REMOVE AND CLEAN CHIP DETECTOR
Annual Inspection
The annual inspection should include the 100 hr. and 200 hr inspection items listed and the following:
__________ CHANGE OIL IN ACCORDANCE WITH APPLICABLE PRATT AND WHITNEY SERVICE
BULLITIN.
__________ WASH ENGINE
__________ PERFORM POWER RECOVERY WASH IF NEEDED
__________ REMOVE AND REPLACE INDUCTION AIR FILTER (BRACKETT FOAM AIR FILTER
ONLY)

Air Tractor, Inc. Maintenance Page 011
AT-502A/502B/504 March 14, 2018
Miscellaneous
__________ VERIFY ACCURACY OF ITT GAUGE EVERY TWO YEARS
__________ OVERHAUL STARTER-GENERATOR AT 1000 HOURS
__________ DYNAMIC BALANCE PROPELLER AT 1000 HOURS
__________ REPLACE AN47-22A EYEBOLTS ON HOROZONTAL STABLIZER (REFERENCE SL 129)
TORQUE TO 600 IN-LBS AT 1350 HOURS.
__________ REPLACE OR OVERHAUL COMMERCIAL AIRCTRAFT PRODUCTS P/N D-145-00-35
ACTUATOR AT 1500 HOURS.
__________ REPLACE THE RE4M6 BEARING ON THE AFT END OF THE FLAP PUSH-RODS AT 2000
HOURS.
__________ OVERHAUL PROPELLER PER HARTZELL REQUIREMENTS (See Hartzell Service Letter
HC-SL-61-61Y, Rev 1 or later).
__________ OVERHAUL THE FUEL CONTROL UNIT (FCU) IN ACCORDANCE WITH THE PRATT
AND WHITNEY CANADA MANUAL.
__________ CHANGE MAIN GEAR BOLTS IN ACCORDANCE WITH SCHEDULE IN “MAIN AND TAIL
GEAR ATTACH BOLTS” OF THIS SECTION
__________ CHANGE TAIL GEAR BOLTS IN ACCORDANCE WITH SCHEDULE IN “MAIN AND TAIL
GEAR ATTACH BOLTS” OF THIS SECTION

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Air Tractor, Inc. Maintenance Page 1
AT-502A/502B/504 March 14, 2018
INTRODUCTION
This section is presented to describe maintenance functions that are applicable to the AT-502/504 airplane.
Airframe maintenance items that are not specifically covered in this section are addressed in FAA Advisory
Circular (AC) 43.13. Additional maintenance functions for the engine, propeller, and other accessories can
be found in the specific manufacturer’s maintenance documentation.
FUEL REQUIREMENTS
See Approved Airplane Flight Manual.
OIL REQUIREMENTS
Oil used at the factory is denoted inside the oil door on the top engine cowling. If a different brand is
used, the engine and lines should be drained and a flushing procedure with the new oil should be carried out
in accordance with the Pratt & Whitney PT6A Maintenance Manual. Also see Flight Manual.
TIRE INFLATION
The main tires are inflated to a pressure of 50 psi (unloaded) or 52 psi (loaded). The tail wheel tire is
inflated to a pressure of 50 psi (loaded or unloaded).
LIFTING AND JACKING
A jack pad is provided with each aircraft for the main gear. This jack pad is to be used for changing or
removing a wheel. To install the jack pad, lay a soft rag on the inside so that it will not chip the paint from the
main gear leg and slide the jack pad up the lower end of the main gear leg until it is snug. It will be necessary
to remove and replace the lower plastic band that holds the brake line in place. The band is a Tyton T150M-0
and should be stocked as a standard spares item. After the jack pad is in place, install the bolt and nut
through the pad flanges with the bolt on the top side of the gear leg. This is to keep the pad from spreading
apart and slipping. Use a small hydraulic jack to lift the wheel off the ground. Be sure the tail wheel is locked,
and chocks are in place on the other main wheel. The hopper should be empty before jacking.
Alternately, to service the main wheel, tires or brakes, the pane may be jacked utilizing a typical
tripod general aviation wing jack of at least 3000 lb capacity and the jack point included in the delivery kit. To
accomplish this, remove the main landing gear cuff and expose the main gear spring clamp. Insert the jack
point into the head of the socket head bolt attaching the clamp block. You may need to place a dab of
grease or putty in the socket head of the bolt to temporarily hold the jack point in place. Position the jack
under the jack point and lift the wheel off the ground. Before lifting the plane be sure the other main gear is
chocked and that the tail wheel is locked. Use caution making sure the jack is stable understanding that the
main gear spring deflects latterly as the plane is lifted and lowered.
CAUTION: Care should always be taken when the aircraft is lifted to
prevent damage to the aircraft or harm to people near it.
To jack the tail wheel, place a small board (a short 2x4) under the tail spring about mid-way between
the forward spring attach bolt and the main clamp block that attaches the spring to the fuselage. Place the
jack on the board and lift the wheel. Be sure the parking brake is set before jacking.