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.