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.
