AIR TRACTOR AT-402A User manual

AIR TRACTOR, INC.
Olney, Texas
AT-402A/402B
TABLE OF CONTENTS
DESCRIPTION SECTION
Page No.
GENERAL .................................................................................................................1
CERTIFICATION BASIS...............................................................................................1
INSTRUCTIONS FOR CONTINUED AIRWORTHINESS (ICA’s)..................................1
FUSELAGE .................................................................................................................2
WINGS .................................................................................................................2
AILERONS .................................................................................................................3
Bendable Aileron Trim Tabs.................................................................................3
FLAPS .................................................................................................................3
EMPENNAGE ..............................................................................................................4
LANDING GEAR AND BRAKES...................................................................................4
Landing Gear.......................................................................................................4
Brake System ......................................................................................................4
BRAKE SYSTEM SCHEMATIC....................................................................................5
ENGINE .................................................................................................................6
INDUCTION SYSTEM..................................................................................................6
EXHAUST SYSTEM.....................................................................................................6
FUEL SYSTEM.............................................................................................................6
FUEL SYSTEM SCHEMATIC.......................................................................................8
TO INSTALL FERRY FUEL SYSTEM ..........................................................................9
TO REMOVE FERRY FUEL SYSTEM .........................................................................9

AIR TRACTOR, INC.
Olney, Texas
AT-402A/402B
TABLE OF CONTENTS
DESCRIPTION SECTION (Continued)
Page No.
ENGINE CONTROLS...................................................................................................9
PROPELLER AND GOVERNOR................................................................................10
ELECTRONIC STARTING AND CHARGING SYSTEM..............................................10
Description.........................................................................................................10
Theory of Operation...........................................................................................11
ELECTRICAL SYSTEM..............................................................................................12
COCKPIT ...............................................................................................................12
CONTROL SYSTEM ..................................................................................................13
ENGINE DRIVEN AIR CONDITIONER.......................................................................13
DISPERSAL SYSTEM................................................................................................14
DIGITAL PAYLOAD HOPPER QUANTITY INDICATION SYSTEM............................15
HOPPER RINSE SYSTEM.........................................................................................16
COCKPIT HEATER....................................................................................................16

Air Tractor, Inc. Description Page 1
AT-402A/402B March 1, 2018
GENERAL
The Air Tractor AT-402B is an all-metal Cantilever low-wing monoplane designed especially
for agricultural operations. It is powered by a Pratt & Whitney PT6A turboprop engine which is
highly suited to ag flying. The standard engine for the AT-402A/402B is the PT6A-15AG, which is
rated at 680 SHP at sea level and 71°F. Alternate engine is the PT6A-34AG, which is flat rated at
680 SHP. Installation losses are very low and the full horsepower can usually be obtained at
temperatures up to 71° for the -15AG and up to 100° for the -34AG engines. The engine for the
AT-402A is the PT6A-11AG Build Spec (BS) 943. It is rated at 550 SHP up to 90°F.
The propeller is a Hartzell three-blade 106-inch diameter constant speed prop with reversing
capabilities. The hopper is made of fiberglass and has a 400-gallon (U.S.) capacity. The horizontal
stabilizer is all metal and strut-braced for added rigidity. The vertical fin is cantilever. The elevators
and rudder are of all-metal construction and sealed to prevent chemical entry. The fuselage
features removable skin panels for ease of maintenance and cleaning. High-lift flaps are
incorporated and are interconnected with the ailerons to provide short take-off and landing
distances.
CERTIFICATION BASIS
The AT-402A/402B is structurally certified to meet the requirements of FAR 23. The engine
installation along with the systems and equipment meet FAR 23 requirements except in cases
where inappropriate for agricultural operations. In these cases placards are installed on the
instrument panel, which prohibit flying under those special conditions. The AT-402A/402B is
certificated by the manufacturer for a gross weight of 7,000 pounds starting with S/N 402A-1021.
However, under the provisions of Part 8 the operator may select his own maximum operating
weight, and field experience shows that higher operating weights can be carried safely with
adequate margins of performance and structural strength. See the aircraft Type Certificate Data
Sheet (TCDS) and Service Letter #304 for more information.
Certification basis is FAR 21.25 (a)(1) including certain paragraphs of FAR 23 applicable to
turbine engine installations.
INSTRUCTIONS FOR CONTINUED AIRWORTHINESS (ICA’s)
Maintain the airplane in accordance with the following manuals and documents:
LIST OF MANUALS AND DOCUMENTS
“Owner’s Manual- Air Tractor Model AT-402 Agricultural Airplane”
Document #03-0105
Air Tractor Inc.
Municipal Airport
P.O. Box 485
Olney, Texas 76374 U.S.A.

Description Page 2 Air Tractor, Inc.
March 1, 2018 AT-402A/402B
“Propeller Owner’s Manual & Log Book”
Manual 139
Hartzell Propeller Inc.
One Propeller Place
Piqua, Ohio 45356-2634 U.S.A
“Maintenance Manual- Turboprop Gas Turbine Engine- Model PT6A-15AG/-11AG”
Manual Part No. 3030442, Vols. 1 & 2
“Maintenance Manual-Turboprop Gas Turbine Engine-Models PT6A-34AG”
Manual Part No. 3021242, Vols. 1 & 2
Pratt & Whitney Canada
1000 Marie-Victorin
Longeuil, Quebec
Canada J4G 1A1
FUSELAGE
The fuselage structure is of heli-arc welded 4130N steel tubing. All fittings and bushings are
also of 4130N steel. Fuselage repairs may be made in accordance with appropriate FAA repair
procedures and gas welding is permissible. There are no heat-treated members in the fuselage.
The front section of the fuselage from the rear spar forward and including the engine mount has
been oven stress-relieved in order to prevent stress concentrations from the welding operation. For
corrosion control oil passage holes are drilled at the intersections of all tubes and clusters. Hot
linseed oil is then pumped into the fuselage frame and drained. This oil adheres to the inside walls
of the tubing. The exterior of the fuselage frame is then sandblasted and painted with AG Air
Yellow Epoxy, which is resistant to nearly all chemical action.
The fuselage lower and upper skins are attached to the fuselage frame with stainless screws
and nuts. The fuselage side panels are of heavy gauge 2024T3 alclad and are attached to
stainless camloc receptacle. This allows the side panels to be removed in minutes, leaving the
fuselage frame open for thorough cleaning and inspection. All skins are mounted with clearance
between the skin and the fuselage tubing so that chemicals will not collect and cause corrosion.
WINGS
The wing structure is full cantilever, with the main spar carrying all of the bending loads.
Construction is of 2024T3 alclad for skins and spar webs. The upper spar cap is also 2024T3
material, but the lower spar cap is made of alloy steel. The wing has a constant chord of 72 inches
and a span of 51.0 ft. The wing utilizes a NACA 4415 wing section and has 2 degrees of washout twist.
The leading edge is extra heavy with a leading edge doubler bonded internally to minimize denting.
Each wing panel has the leading edge divided into five different pieces to allow easy replacement.
Universal head rivets are used to facilitate the removal and replacement of leading edge sections.

Air Tractor, Inc. Description Page 3
AT-402A/402B March 1, 2018
The fuel tanks are located in the inboard sections of the wing and are an integral part of the
structure. The closely riveted seams and heavy skins make them burst resistant in the event of a
crash. Sealing is accomplished by an application of Product Research PR-1422A2 sealing
compound*. The inboard end of the wing is sealed against chemical entry and the aileron pushrod
has a flexible boot attached to prevent entry of chemical at the pushrod location. A complete wire
bundle for night working lights is installed inside the wing during assembly so that lights may be
added at a later date.
The upper spar cap is painted with yellow polyurethane paint and the lower spar is cadmium
plated and primed with Chromanoxide primer for corrosion protection. All other parts inside the
wing are treated with alodine and primed with zinc chromate primer prior to assembly.
*Alternate sealer is TBM PKMMC236A 1/2.
AILERONS
The ailerons are of all-metal construction and hinged on ball bearings. The bellcranks and
pushrod ends operate on ball bearings, producing smooth operation and low system friction. The
stick forces are light and the ailerons are very responsive. The light stick forces are a result of the
installation of aerodynamic servo tabs installed on the outboard end of each aileron.
The ailerons are interconnected with the wing flaps so that as the flaps move down through
20°, the ailerons droop 8°. The interconnection is non-linear so that most of the aileron droop
occurs with the first half of the flap travel. This provides optimum conditions for short take-off
performance.
Each aileron has a large balance weight on the outboard end of the aileron just forward of the
leading edge. These weights must never be removed as they are necessary for flutter prevention.
Bendable Aileron Trim Tabs
Small bendable aileron trim tabs are installed on the trailing edge of each aileron. These tabs
allow the pilot to make minor adjustments to the airplane’s roll trim by adding a small amount of up
or down force to the trailing edge of the aileron in flight.
To make an aileron trim adjustment, fly the airplane at normal cruise power and normal cruise
speed in smooth air. While flying the airplane in coordinated flight (ball centered), evaluate the
airplane’s natural rolling tendency with the controls free, and evaluate the amount of stick force
required to fly the airplane wings level. In the case of “left wing heavy”, the pilot should bend the
left bendable tab trailing edge UP, and the right tab trailing edge DOWN. Use a hand seamer or
other similar tool to make a clean, even bend. Avoid excessive bend angle, because the bendable
tabs will stall and become ineffective at bend angles greater than 20 degrees. Make tab
adjustments in small increments, and re-fly the airplane to re-evaluate roll trim.
FLAPS
Extra large Fowler-type flaps are incorporated on the Air Tractor. The flaps are electrically
operated and may be stopped at any position from 0° to the maximum of 20° travel. The flaps are
controlled by a switch conveniently mounted just below the throttle quadrant. The flaps have
external markings which may be viewed from the cockpit with each of the first two marks being 10°
of travel.
The flaps are very effective for both take-off and landing. For a short take-off roll, 10° of flap is
normally used. The flaps are also useful during turns, although generally less than 10° is used.

Description Page 4 Air Tractor, Inc.
March 1, 2018 AT-402A/402B
EMPENNAGE
The horizontal stabilizers are all-metal and strut-supported for rigidity. The struts are
constructed of streamlined 4130N tubing. The struts have a stainless clevis where the attachment
to the stabilizer is made and the clevis is adjustable in order to rig the stabilizers properly with the
wing and in a straight line from side to side.
The vertical fin is cantilever and is of all-metal construction. It has a relatively short span in
order to minimize the hazards of working under telephone lines or other obstructions. A heavy-duty
wire deflector cable is attached to the top of the vertical fin and extends to the top of the canopy
where it is anchored.
The control surface hinges have a single stainless steel ball with a Teflon lining. Long bearing
life can be expected and it is not necessary to lubricate the hinge.
The elevator trim tabs act as Servo tabs in addition to providing longitudinal trim for the Air
Tractor. Because of the large tab size, the trim tabs provide a considerable amount of trim
authority. Removable bronze bushings are provided at each joint. As the bushings wear, they
should be replaced so that trim tab free play may be kept to a minimum. The all-metal rudder has a
servo tab that also serves as an adjustable trim tab.
LANDING GEAR AND BRAKES
Landing Gear
A spring type main and tail landing gear is used on the Air Tractor. This type of landing gear
provides a minimum of maintenance, low drag and considerable energy absorption for hard
landings. In addition, a very smooth ride is provided for operations from rough strips.
The main gear axles are made of 4130N steel, and cadmium plated for corrosion protection.
Tapered aluminum shims are used between the axle and the main gear leg to provide the proper
camber and toe-in. The main gear wheels are Cleveland p/n 40-133 with a 29x11.00 10-ply rating
tire installed. Main wheel tire inflation pressure is 50 psi (unloaded) or 52 psi (loaded).
The tail wheel is a Cleveland 40-168. A grease fitting is provided in the side of the wheel in
order to grease the bearings.
The tail wheel tire is 5.00x5. Be sure it is 6-ply rating. Tire pressure is 50 psi loaded or
unloaded.
The tail wheel fork is of welded 4130N tubing construction. Bronze bushings and a brass wear
plate are incorporated where the fork enters the tail wheel housing. A tail wheel locking mechanism
is provided and the tailwheel may be unlocked by pushing the control stick forward. This lifts the
stainless locking pin out of the tail wheel fork and allows the tail wheel assembly to swivel 360°. If
during taxi the aircraft tends to steer to the left or to the right with the controls neutral, it is possible
to readjust the lock pin position by loosening the 4 screws that attach the lock pin housing to the
fork housing plate. The lock pin can then be moved in the desired direction and the screws
tightened.
Brake System
Brakes are the same, left or right, and are Cleveland p/n 30-98C. The brake cylinders are
Cleveland p/n 10-23A. Brake fluid is the conventional red petroleum-base Mil-H-5606A fluid. The
brake fluid reservoir is a Volkswagen p/n 113611301 L and is mounted on top of the lower
instrument panel where the fluid level is always visible. The parking brake valve is a Scott p/n
4500A-2. Stratoflex hoses are used from the pressure side of the master cylinder to the parking
brake valve and stainless steel lines are routed from the valve to the bulkhead fitting next to the
main landing gear. A high-pressure Stratoflex hose connects the bulkhead fitting to the wheel
cylinders.

Air Tractor, Inc. Description Page 5
AT-402A/402B March 1, 2018

Description Page 6 Air Tractor, Inc.
March 1, 2018 AT-402A/402B
ENGINE
The standard engine for the AT-402B is Pratt & Whitney PT6A-15AG turboprop engine. This
engine is rated at 680 HP for take-off at sea level and 71°F. Maximum torque at take-off is 1628 ft-
lbs. Maximum ITT for take-off is 725°C. Maximum Ng is 101.5% for take-off. Maximum Np for all
operations is 2200 RPM. See the flight manual for other engine limits and the special conditions.
The standard engine for the AT-402A is the PT6A-11AG, BS 943. This engine has a gas
generator similar to the -15 AG with a propeller gearbox rated at 550 SHP. Maximum torque for
take-off is 1315 ft-lbs. ITT maximum is 700° C.
For long engine life it is recommended that the ITT is kept well below the maximum limits. Use
only enough power on take-off to operate safely from the strip being used.
Read the engine maintenance manual for instructions as to care and servicing of your engine.
INDUCTION SYSTEM
Engine air is drawn in through the inlet scoop, through a barrier type filter and into a sealed
plenum which is located between the forward and aft engine firewalls. There are three different
filter installations that Air Tractor has used on AT-402A/402B aircraft. Each of these filter
installations is over 99% efficient and cause only a negligible pressure drop due to the ram air
pressure developed by the air scoop.
The earliest air filter system used two large, cylindrical commercial truck filters. These filters
are Donaldson p/n P182043 (alternate is P510337). Aircraft equipped with the Air Tractor Ram Air
system have the airscoop moved from the lower side of the engine cowling to the nose of the
aircraft, just below the propeller. This Ram Air configuration may have one of two different filter
systems. The first system uses a Donaldson P613731 pleated filter element. This filter looks like a
paper element filter, but is actually synthetic. The second type of Ram Air filter is a Brackett p/n
BA-409E air filter. This filter is a disposable foam element filter that contains a sticky residue
“wettant” that traps and holds dirt and debris. This is a lightweight, flexible filter that is installed in a
stainless steel frame inside the engine plenum.
A pressure differential switch is installed on the top R/H side of the aft firewall with one probe
in the engine area and the other probe in the plenum. At such time that the pressure difference
between the air in the engine area and the air in the plenum is 13 inches of water or more, the air
filter light on the instrument panel will come on. This light has a “push to test” feature and should
be checked before the engine is started.
EXHAUST SYSTEM
The exhaust pipes are Air Tractor p/n 50597-3 (L/H) and p/n 50670-1 (R/H). These pipes are
attached with stainless MS20034-1 bolts, MS20500-428 nuts, and AN960C416 washers. There are
exhaust covers provided with each aircraft and these covers should be installed when the engine
has cooled.
FUEL SYSTEM
Standard aircraft have two fuel tanks with a capacity of 85 U.S. gallons each. Both tanks
gravity feed into a small header tank which is located behind the chemical hopper and below the
rear spar attach tube. Optional fuel capacity is either 216 or 234 gallons.

Air Tractor, Inc. Description Page 7
AT-402A/402B March 1, 2018
The fuel valve handle is located within easy reach of the pilot and is marked “Main” and “Off”.
There can be no tank selection since both tanks are interconnected. The fuel valve is placarded in
“Gallons Usable”. The fuel gauge receiver, which is located on the instrument panel, is marked in
fractions of usable fuel.
Finger strainers of coarse wire mesh are located in each fuel tank. Fuel flows from the wing
tanks into the header tank, through the fuel valve and to the airplane firewall. From the firewall, a
fuel line is connected to a large fuel filter. This fuel filter may be either a Fram FS1133-PLM or Air
Tractor p/n 53114-1 fuel filter. Both fuel filters use an Air Tractor p/n 52351-1 fuel filter element.
The p/n 52351-1 fuel filter element should be cleaned at 100-hour intervals or as required,
depending on how clean the fuel is that is being used. A vacuum pressure switch is installed on the
outlet side of the filter. If the filter becomes clogged, the switch will close and the “FUEL FILTER”
warning light will illuminate in the cockpit.
The fuel filter screen is removed by removing the center bolt on top of the firewall-mounted
filter unit. The steel filter bowl and screen can then be dropped off. The fuel in the bowl should be
examined for sediment or water and the filter screen cleaned or replaced if necessary. (See S/L
229 and S/L 229A)
When the fuel filter bowl has been removed for screen inspection it must be remembered that
the fuel lines from the engine fuel control to the header tank have been drained, and these lines
will be air-locked if a start is attempted without first priming the lines with fuel. Use the electric
boost pump for this.
The fuel lines continue from the firewall mounted fuel filter to the airframe pump, which is
mounted on the engine vacuum drive pad. This pump is P/N 51076-1, set for CCW rotation. The
elbow on the discharge side of the airframe pump is modified to accept installation of a 15 psi
pressure switch (p/n M4003-15).
The airframe pump has a pressure adjustment that has been set at the factory for
approximately 18 psi with the engine operating at 80% power. If a new airframe pump is installed,
and if a negative pressure is read on start-up, it is a sign that the airframe pump is not set up for
proper rotation, and the body of the pump can be removed and rotated 180° to obtain a positive
reading. The engine manufacturer requires that a minimum pressure of 5 psi is being delivered to
the fuel control at all times, with a maximum pressure of 20 psi. The fuel pressure switch and warn
light has been installed so that if the fuel pressure warn light comes on, or if fuel pressure drops
below 15 psi, the electric fuel boost pump should be turned on. The pilot should land as soon as
possible and investigate. Possible cause of a drop in fuel pressure would be a dirty fuel filter, or
loose fuel line.
The fuel lines continue from the airframe pump to the fuel heater body. Lines from that point
on are furnished with the engine.
There are four quick drains in the fuel system: one in each wing tank, one in the header tank
and one in the fuel valve. These drains should be activated daily. It is good practice to pass the
fuel from these drains through a clean white rag held in the hand so that a check can be made on
the spot as to the cleanliness of the fuel and evidence of water.
Each fuel tank has an overboard vent located near the wing tip. With both tanks completely
full, it is possible during flight to vent a small amount of fuel overboard by skidding the aircraft or
rolling sharply.
Fuel type is called out in the Flight Manual. Fuel System Icing Inhibitor (FSII) such as Prist
should be added to the fuel to provide .06% - .15% of FSII by volume when operating below 40°F.
An optional ferry fuel system (hopper fuel) may be installed by following the procedures and
using the parts described on drawing 50280-8.

Description Page 8 Air Tractor, Inc.
March 1, 2018 AT-402A/402B

Air Tractor, Inc. Description Page 9
AT-402A/402B March 1, 2018
To Install Ferry Fuel System
1. Remove spray pump by disconnecting coupler at gatebox, disconnect brake cable at
pump, disconnect side discharge tube at spray valve and tube support on side of
gatebox, and uncouple pump at disconnect on pump strut.
2. Remove the bottom load tube between the gatebox and spray valve. Plug opening in
Gatebox with furnished plug and tighten clamp.
3. Install screen per Note 5 Drawing 50280 in gatebox opening that supplied pump. Install
coupler, fittings and filter per Drawing 50280. Assemble all fittings with PR-1422-A2. If
filter was used previously, disassemble and inspect for contamination. Safety coupler
arms with tyrap per drawing.
4. Remove inspection cover from lower belly skin and route hose fuel filter through hole to
plugged side of fuel valve. Hose must clear all flight controls.
5. Turn fuel valve pointer to “OFF” position. Remove plug from fuel valve, install fitting and
attach hose.
6. To use hopper fuel, rotate fuel selector handle CCW to second detent past “ON” position.
To Remove Ferry Fuel System
1. Turn fuel valve pointer to “OFF” position.
2. Remove hose and fitting at fuel valve and install plug.
3. Replace inspection cover in belly skin. Remove coupler, fittings, screen and filter from
gatebox.
4. Remove plug from bottom load tube opening in gatebox and install tube.
5. Install spray pump, plumbing and brake cable in reverse sequence to paragraph 1 under
To Install Ferry Fuel System above.
ENGINE CONTROLS
A quadrant on the left-hand side of the cockpit incorporates the Power lever, the Propeller
control lever, and the Start control lever. The Power lever has an idle stop and cannot be moved
into the reverse range without moving the trigger at the top of the lever forward. Do not allow the
power lever to be moved into the reverse range unless the engine is running or else the control
linkage could be damaged.
The Start lever has a latch that prevents inadvertent movement below the “Run” position that
would shut off the fuel in flight. The Start lever should always be in the “Flight Idle” position during
flight to allow the engine to remain spooled up enough during approach to allow quick acceleration
in case a go-around is required. Refer to the Start Procedures in the Flight Manual for proper
positioning of the Start lever when starting. It is important to remember that only with the Start lever
full back at the most aft stop position is the fuel cut off.

Description Page 10 Air Tractor, Inc.
March 1, 2018 AT-402A/402B
The Propeller lever may be placed in the full forward position during the start sequence for
more convenient movement for the Start lever providing the temperatures are warm. For cold
weather starts the Prop lever should be aft, as called out in the starting instructions, as initial oil
pressure will go to engine bearings instead of the propeller dome.
All engine controls utilize push-pull cable assemblies. These cable assemblies are 66-3635-
2154.
These cables have very little friction and require no lubrication.
PROPELLER AND GOVERNOR
The propeller used is a Hartzell HC-B3TN-3D/T10282+4 or T10282NS+4 three-blade 106”
diameter constant speed with reversing capabilities. The primary prop governor is furnished with
the engine.
The overspeed governor is a Woodward p/n A210507. The overspeed governor takes over
automatically in case of failure of the primary governor and reduces RPM from 30 to 60 RPM below
the maximum of 2200 RPM.
Overspeed Governor –CHECK (first flight of the day and after maintenance)
The procedure for checking the overspeed governor is as follows:
POWER LEVER...............................................................................................IDLE
PROP LEVER (P) ..........................................................Full forward for 2200 RPM
PROP OVERSPEED TEST SWITCH.................................................................ON
POWER LEVER......................................................................................ADVANCE
PROP RPM........................................... ENSURE Np stabilizes at 2025 ± 50 RPM
POWER LEVER..................................................REDUCE to less than 2000 RPM
PROP OVERSPEED TEST SWITCH.....................................OFF and GUARDED
POWER LEVER...........................ADVANCE (Ensure that 2200 RPM is available)
POWER LEVER...............................................................................................IDLE
Propeller pitch settings are -7.8° (reverse), 18° (low pitch) at the 30” station.
ELECTRONIC STARTING AND CHARGING SYSTEM
Description - Refer to your parts manual and to schematics found in this manual, the system
consists of:
(1) The Starter/Generator (S/G) - the armature is continuously engaged in the engine’s
assessory gear box. The S/G functions as an electric motor for starting the engine and
then as an engine driven DC Generator.
(2) The Generator Control Unit (GCU) - The GCU is located under the cockpit floor.
(3) The Line Contactor Relay (LCR) - located on the right side of the firewall.
(4) The Start Relay - located on the right side of the firewall.
(5) The Start Switch (ST SW) - located on the Lower Instrument Panel.
(6) The Generator Switch (GEN SW) - located on the Lower Instrument Panel.
(7) The Pilots Panel Voltmeter (VM) - located on the Lower Instrument Panel.
(8) The Low Voltage Warning Light (LV LITE) - located on the Upper Instrument Panel (when
equipped)
(9) 15 Amp GCU Circuit Breaker (15A C/B) - located on the right lower engine mount tube
just ahead of the firewall.

Air Tractor, Inc. Description Page 11
AT-402A/402B March 1, 2018
Theory of Operation
Starting - Actuating the Start Switch energizes the Start Relay which applies battery
voltage to term C of the S/G which operates as a series wound DC motor. Note that at the same
time battery voltage is applied to pin N of the GCU which signals it to remove field excitation from
the S/G. The purpose of this feature is to prevent the poor start performance that would otherwise
result if the Generator Switch were left in the ON position.
Note: It is recommended that the Generator Switch be in the OFF position during START because
of the high inrush current to the batteries immediately upon Start termination will result in higher
than normal gas temperature (ITT). It is better to allow the engine to stabilize at Ground Idle before
turning the Generator Switch ON.
Generator Mode - Following engine starting and prior to turning the Generator Switch ON
(The engine at ground idle) the voltage output of the S/G (Terminal B) should be .5 to 1.5 volts. This
is a residual voltage that results from residual magnetism in the S/G even though the field excitation
from the GCU is Zero.
Turning the Generator Switch ON causes this residual voltage to be applied to pin J which the
GCU in turn outputs on pin M. As a result the S/C output will build up rapidly and stabilize at the
27.5 V voltage setting of the GCU (externally adjusted from 26 to 30 volts). Sensing for Voltage
Regulation is between pins B and G.
Line Contact Relay (LCR) Control - The GCU monitors the S/G output and the Bus
Voltage on pins B and A respectively. As the S/G voltage builds up to within approximately .2 volts
of the Bus Voltage the GCU will output system voltage on pin H resulting in activation of the LCR.
Power for this circuit is derived from pin J so turning the Generator Switch OFF disables the LCR
as well as the S/G.
Reverse Current Protection - If the S/G voltage drops below the Bus voltage (battery) due
to normal conditions such as engine shutdown or abnormal conditions such as loss of field
excitation the S/G will draw reverse current (discharge the battery). The GCU senses this condition
via pin D which is connected to terminal D of the S/G. Terminal D output is a positive voltage
proportional to reverse current.
Overvoltage (OV) Protection - In the event that system faults occur, either internal or
external to the GCU, that results in overexcitation of the S/G the OV circuit will prevent the Bus
voltage from going above approximately 32 volts. This protection is accomplished by a “Crowbar”
circuit in the GCU which internally shorts pin J to D. This results in immediate removal of voltage
from the S/G field and the LCR coil. At the same time high current flow through the 15A C/B
causing it to trip. The OV Circuit has a time delay to prevent nuisance trips due to system
transients.

Description Page 12 Air Tractor, Inc.
March 1, 2018 AT-402A/402B
ELECTRICAL SYSTEM
All turboprop Air Tractors have a 250 amp 28 volt starter-generator which is a Lear-Siegler p/n
23048-021 and uses a p/n 23046-520 QAD mounting kit. The system is derated to a 150 amp
system by use of a 150 amp PDLM-150 circuit breaker. The voltage regulator is a Precision
Airmotive p/n 42-1001, and the starter and master relays are Cutler-Hammer p/n 604H202A.
Alternate relay is the Kissling p/n 29-314-12-903. The batteries are Teledyne Gill Electric p/n G-
246AT and are wired in parallel for extra cranking power. Amp-hour rating of the two batteries
combined is 42 amp-hours. The batteries are each 24 volt lead-acid batteries. A ground start
receptacle is installed on the left-hand side of the lower cowling since an external power supply
should be used for starting at any time the batteries have a charge of less than 24 volts. Fully
charged batteries should allow the gas generator to peak to about 18% Ng before the Start control
lever is advanced to the “Run” position. At this high an Ng speed, a “cool” start with a ITT peak of
around 610 to 620°C is possible which is desirable.
A wiring diagram of both the starting system and the instrument wiring is furnished in the
Maintenance section of this manual.
COCKPIT
The AT-402A/-402B has exceptional visibility on the ground as well as in the air. The optional
windshield is three piece, the two corner pieces being Plexiglas while the flat center piece is
automotive safety plate. Cockpit entry is made through either of the two large canopy doors. A wire
deflector is incorporated into the windshield support structure.
The rudder pedals may be adjusted on the ground by removing the “T” pin and sliding the
rudder pedals in the aluminum channel to the desired position and reinserting the “T” pin.
The seat is fixed in one position at the factory. However, the operator may lower the seat if
desired by removing the two pins and drilling new holes through the seat rail. The seat structure is
of welded 4130N tubing, oiled internally, and sand-blasted, primed, and painted. The covers are
removable and may be changed as required. The covers are of a coarse mesh material for comfort
during hot weather. A special installation tool is required for installing new covers and may be
borrowed from the local Air Tractor dealer. At the bottom extreme of the seat a tube structure
supports a thick Ensonite pad to protect the pilot should the covering material fail in a severe crash.
A three inch wide safety belt is attached to the structure. The shoulder harness is attached to
the primary fuselage structure behind the cockpit. The shoulder harness is a necessary part of the
restraint system and must always be worn. If the aircraft is equipped with AmSafe Airbags, these
airbags are built into the shoulder harnesses. The airbag system is equipped with its own internal
battery, so it is always active. There is no pilot action necessary to arm or operate the system.
The airbag system requires the use of the installed crotch strap for proper pilot protection.
A control lock is located under the panel and hinges up to secure the control stick.
The instrument panels are in two sections, with the upper panel containing the instruments
most often used during flight. A large hopper window between the panels allows the pilot to
determine the amount of dry fertilizer or seed remaining in the hopper. Instruments provided with
the basic airplane include a sensitive altimeter, airspeed indicator, ITT gauge, oil temp gauge, oil
pressure gauge, torque-meter, propeller tachometer, gas generator tachometer, compass, boom
pressure gauge, fuel gauge, and voltmeter.
A stall warning horn is provided and the lift detector on the wing is set to activate the horn at
approximately 5 miles per hour above stall. A hopper quantity gauge is installed on the right-hand
side of the upper instrument panel. This gauge is marked to read in U.S. gallons with the aircraft in
the ground attitude. In addition the hopper is marked in 25 gallon increments where visible to
ground crew.

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AT-402A/402B March 1, 2018
The hopper window in the rear wall of the hopper is supported with an aluminum frame and is
sealed with silicone sealer which is impervious to most chemicals and is available at most industrial
stores.
The cockpit has aluminum enclosure skins and control push-rod boots which isolate the
cockpit from the rest of the aircraft and prevent the entry of most chemicals. If the aircraft is to be
used for dusting however, the operator should install boots about the rudder cables where they
pass through the cockpit wall and re-seal small cracks and seams with silicone sealer. In addition,
all areas along the aircraft bottom should be sealed.
CONTROL SYSTEM
Push-pull tubes are used in both the aileron and elevator system. Rod-end bearings are
installed in each push-rod to minimize control system friction. The torque tube which is mounted on
the cockpit floor is supported at each end with bronze bushings. These bushings have a grease
fitting installed so that grease may be applied as necessary. As these bronze bushings wear, fore
and aft slack may develop which can be removed by the addition of shims made of .010 thick steel.
These shims are added to the forward end of the torque tube. The stops for the elevator control
system are located on each side of the elevator horn. The aileron control stops are welded to the
floor structure at the forward end of the torque tube. Both the aileron and the elevator down stop
employ neoprene washers which act as shock absorbers for the system.
The rudder stops are located at the rudder horn. The rudder controls consist of large stainless
cables. There is a spring interconnected between the rudder and the aileron system which allows
banking the aircraft with rudder application alone.
ENGINE-DRIVEN AIR CONDITIONER (DWNG. 60906)
The AT-402A/402B uses a gas-cycle air conditioning system for climate control. The
functional layout of the air-conditioning system is shown in Drawing 60749.
This versatile air conditioning system allows the pilot to select ram air from outside, or to
recirculate cockpit air, or a combination of the two. The ram-air control handle is located on the aft-
cockpit wall, to the right-hand side of the pilot.
All control switches for the air conditioning system are located on the pilot’s panel. The pilot
can use the main switch to turn the air conditioner ON or OFF or to enable only the evaporative
blower fan. The evaporative blower switch is used to control the blower speed as HIGH or LOW.
The Sanden SD-508 compressor has five reciprocating pistons driven by a rotating wobble-
plate. It uses reed-type valves to control flow at the suction and pressure ports. It is powered by an
engine drive pad. Gears in the engine pad drive a splined quill shaft that turns a small poly “V”
groove pulley. This small pulley drives the larger flywheel pulley with a poly V-belt. This larger
drive pulley is connected, through an electromagnetic clutch, to the compressor shaft.
In Drawing 60749, hot, high pressure, gaseous refrigerant leaves the pressure port of the
compressor and goes to the upper inlet of the condenser coil. While in the condenser coil, the
refrigerant is cooled at high pressure by the condenser fans. The temperature drop in the
condenser coil causes the gas to liquify. The refrigerant emerges from the bottom of the condenser
coil as a warm liquid under high pressure.
Next, the warm liquid passes to the receiver/dryer. The receiver/dryer acts as an accumulator
to dampen pressure pulses from the compressor. It also removes moisture from the system,
preventing internal freeze-up of the expansion valve.

Description Page 14 Air Tractor, Inc.
March 1, 2018 AT-402A/402B
From the receiver/dryer, the warm liquid goes to the inlet of the expansion valve. The
expansion valve throttles the flow of warm liquid, dropping its pressure and temperature,
transforming it to a cold gas.
The cold gas goes from the expansion valve to the inlet of the evaporator coil where it is
warmed by removing heat from the air drawn through the coil by evaporator blower. This cooled air
is ducted beneath the cockpit floor to an outlet in the cockpit floor. This outlet has a throttling valve
at its mouth to allow the pilot further to regulate the air flow into the cockpit.
The refrigerant leaves the evaporator coil as a warm gas. This warm gas flows from the
evaporator coil back through the outlet port of the block expansion valve and then returns to the
suction port of the compressor where the compressor raises the pressure and temperature. The
hot gas leaves the pressure port of the compressor and the process starts over.
The wiring schematic for the air-conditioning system is shown in Drawing 60910. The air
conditioner fans, blowers, and compressor clutch are powered by the airplane bus. Mechanical
power to drive the compressor is taken from an engine accessory drive pad.
The air conditioner master switch (A/C Switch) enables all functions of the air conditioner
system. This switch may be used to select air conditioning with blower-forced air or blower-forced
air circulation without air conditioning.
The Blower Switch controls the speed of the evaporative blower. The blower is ON any time
that the A/C Switch is not OFF. Its speed is controlled HIGH or LOW with the Blower Switch.
The Trinary Switch, located atop the receiver/dryer, does two things to control electrical power
to the compressor clutch. A set of contacts in the Trinary Switch protects the compressor by
opening the Compressor Clutch circuit any time the system’s high pressure is less than 25 psi. This
same set of contacts opens when the system’s high pressure exceeds 350 psi.
DISPERSAL SYSTEM
The standard spray system includes two-inch stainless plumbing and streamlined extruded
aluminum booms. Thirty-eight nozzles are incorporated although the boom is drilled and tapped for
an additional 36 nozzles if desired. Quick-couplers are used in order to remove the booms and
pump from the aircraft in minutes. The spray pump is an Agrinautics p/n 65219 two-inch capacity
pump. The fan is a Micronair five blade adjustable pitch fan. The control valve is a Transland p/n
20384 and the strainer is an Air Tractor p/n 80392-10. The bottom-load valve is a Transland two
and a half inch p/n 22395.
All hoses used in the spray system have a special lining which is impervious to nearly all
chemicals. The hoses are double clamped at all connections with stainless worm-drive clamps. All
plumbing parts have beaded ends to provide secure hose attachments.
However, high line pressures and water-hammer effects can blow fittings loose and it is a
good practice to wire all Quick-couplers together with a loop or two of safety wire. In addition,
stainless steel straps are used to safety the plumbing on the discharge side of the pump to the top
of the pump, and at the spray valve.
Spray nozzles used are Spraying Systems Inc. p/n 4666 diaphragm check valve, p/n 1/8 Q JJ-
V body, p/n QU-4010 Spray tip. The aircraft is equipped with -4010 spray tips which are for
medium volume applications.
The boom pressure gauge is glycerin-filled for long gauge life and to prevent needle
fluctuations. Copper tubing is used to connect the boom pressure gauge to a bulkhead fitting in the
lower fuselage skin. From the bulkhead fitting to the center boom assembly a Stratoflex hose is
used.
The fan brake lever is an industrial over-center type lever which has considerable leverage so
that the fan can be stopped instantly. The brake cable tension can be adjusted in the cockpit by
rotating the brake lever handle.

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AT-402A/402B March 1, 2018
Micronair rotary atomizers may be installed if desired. The factory supplies a special boom for
8 atomizers.
For dry materials a Transland p/n 22358 extra high-volume spreader is used. When the
spreader is installed the strainer should be rotated as required for clearance. For rapid changeover
from spray to dry materials remove only the pump and booms. For extended fertilizer application it
is suggested that the center boom assembly and control valve be removed to prevent fertilizer from
getting into the valve assembly. This can be accomplished in only a few minutes by removing the
stainless “T” pins which support the center boom assembly and removing the two bolts that attach
the valve to the stainless bracket.
The Air Tractor features a near leak-proof hopper lid which allows full liquid loads to be
carried. The leak-proof feature is made possible by a sturdy over-center latch assembly and a
curved hopper top which allows the hopper lid to be pulled solidly into place. Medium density nitrile
3/8 x 1’ wide is used for a hopper lid gasket.
The hopper vent tube is welded 3” diameter stainless tubing inside the hopper. A 3” stainless
ball is used to seal off the overboard vent when the liquid level reaches the top of the hopper. The
vent tube protruding from the lower fuselage skin is made of aluminum and can be rotated to point
forward to provide positive pressure in the hopper for dry material application. This slight
pressurization in the hopper requires a little less gate opening for a given poundage which reduces
the blockage effects of the door opening into the slipstream. When the vent tube is rotated to point
aft, a slight negative pressure is created inside the hopper which prevents any fumes from
escaping around the hopper lid gasket.
Optional dropped booms may be ordered from the factory which lower the spray booms 8-
inches and reduce the number of fine spray particles by as much as 50%. In areas where off-
target drift is to be avoided, the dropped booms should be installed.
Optional dispersal equipment includes a 3-inch spray system which utilizes the Agrinautics p/n
65715-1 3-inch pump, 3-inch plumbing to the pump, and 2 1/2-inch plumbing to the booms.
Later models feature a trigger in the hopper vent tube that prevents the round stainless ball
from floating up and closing off the vent before the hopper is completely filled. When the bottom
load valve is closed, a cable which is attached to the valve handle releases the round ball and
allows it to seal off the vent tube to minimize overflow out the vent system during takeoff.
Right hand boom shut off is also available as an option from the factory. This feature gives
the pilot greater control of swath placement.
An optional spreader is the Transland 54401 NorCal Swathmaster which requires the p/n
24274 adapter box and the 25-inch gate box.
DIGITAL PAYLOAD HOPPER QUANTITY INDICATION SYSTEM
The Digital Payload Hopper Quantity Indication system and Spray Boom Pressure Indication
system consists of a magnetostrictive sensor, pressure sensor and a digital cockpit display, with an
optional external display. The displays indicate the quantity of the hopper (gallons or liters) and
pressure of the spray boom (PSI).

Description Page 16 Air Tractor, Inc.
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The magnetostrictive probe and floating magnet measures payload fluid level and the cockpit
display uses look up tables to convert the probe output to the quantity of fluid in the tank. There
are 2 look up tables, one for ground attitude and one for flight attitude, and is selected using an
input from the aircraft hour meter torque pressure switch. The display also performs data
averaging to reduce sloshing effects, thus increasing the accuracy of the system. If a remote
display indicator is installed, an RS232 signal is sent from the cockpit display to the remote display,
both displays will indicate the same quantity. A pressure sensor is installed in the left boom
pressure line, and is wired to the display. The display indicates the pressure in the boom line, and
also alerts the pilot if negative pressure is present by illuminating a light on the display indication
“suck back”. This indicates when the aircraft recirculating ball valve feature is working. The
cockpit display can be dimmed using the dim button.
Your Payload Hopper Quantity Indication System comes pre-calibrated from the factory and
should never require adjustment. Periodic cleaning and preventive maintenance will ensure that
your system remains in good working order. See inspection section of this manual for
recommended inspection items and intervals.
HOPPER RINSE SYSTEM
A hopper rinse tank is located forward of the firewall and may be filled with tap water for
hopper rinse. To rinse the hopper after spray chemicals are expended turn the rinse switch ON for
a sufficient time for water to clean the hopper walls, then OFF.
The pump will empty the 18 gal. tank in approximately 1 1/2 minutes. Be sure to open the
drain valves during cold weather so that the system is not damaged by freezing. The drain valves
are located at the pump and at the check valve.
COCKPIT HEATER
An optional cockpit heater is available for the AT-402A/-402B. The heater system taps bleed
air off the compressor section of the engine and routes it to an ON-OFF valve on the left-hand side
of the fuselage just forward of the cockpit. This valve may be actuated by pulling on a pushrod
marked HEAT located below the left side of the pilot’s seat and routed through the cockpit floor.
On the aft side of the valve a line is routed to the diffuser which is located on the cockpit floor
at the forward end. The amount of heat desired may be regulated by pulling the valve partially
open or fully open.
To limit power loss in the case of line failure to the valve, the Bleed Valve Outlet Fitting that
attaches to the compressor case has a “Choke” welded to the mounting plate. The “Choke” limits
the diameter of the compressor tap to .250 inches.
The cockpit heater is very effective under even the coldest conditions and is a requirement for
those operators who have winter work.