Fuel System -Cont. - P-307

CHAPTER FOUR

T-34C AIRCRAFT SYSTEMS FAMILIARIZATION

WORKBOOK

The fuel system consists of an interconnected bladder type 40-gallon fuel tank and 25-gallon fuel

tank in each wing. The aircraft is gravity fueled through the filler neck incorporated on top of

each 25-gallon tank with a tab lock type red cap. An anti-siphoning flapper valve is located in

each filler neck to retain fuel if the fuel cap is lost in flight.

The fuel gravity-feeds from the 25-gallon tank into the 40-gallon tank, then through one-way

check valves into a 1.5-gallon fuel sump tank in the fuselage center section. Each 40-gallon tank

is equipped with a baffle and flapper valve at the inboard end to prevent the fuel from being

drawn away from the outlets during hard turns or other maneuvers.

The sump tank is a rigid metal container which provides a common pickup point for the fuel

flowing from both wing tanks. Check valves are incorporated to prevent fuel from flowing back

to the wings and a gravity-operated vent valve prevents fuel from entering the vent system when

inverted. Since the sump tank is the lowest point of the fuel system, a defueling valve is located

on the bottom of the tank to allow the system to be drained.

All five tanks are vented to the atmosphere to prevent over or under-pressure situations. There

are "siphon break" valves located in each wing vent line to prevent fuel siphoning action if fuel

expands due to heat. To allow for draining sediment and water, there is a snap type drain at the

low point of each tank.

The weight of the fuel in the wing tanks is measured by capacitance type fuel quantity sensors.

These sensors send the information to the fuel quantity indicators in both cockpits to indicate

usable fuel in pounds. The face of each indicator has a yellow range (no takeoff) and a red range

(fuel is critical for flight). The power for the quantity indicating system is 28-Volt Direct

Current (VDC).

There are two yellow lights on the annunciator panels, L FUEL LOW and R FUEL LOW. These

visually indicate approximately 90 pounds of usable fuel remaining in that tank upon initial

illumination. The lights receive their input from a thermistor located in the baffled portion of

each 40-gallon tank. When the thermistor is covered with fuel, it remains cool and there is no

current flow to the light. Once it is exposed to the air, there is a 28-VDC current flow and

illumination of the fuel low lights.

Once the fuel leaves the sump tank, it will be under pressure throughout the remaining part of the

system. There is an electrically powered (28V DC) standby boost pump mounted forward of the

fuel sump tank which delivers fuel under pressure to the primary pump in the event that the

engine-driven boost pump fails. This standby pump is controlled by a two-position circuit

breaker toggle switch located on the right console of the front cockpit only.

The fuel passes next through a fuel shutoff valve located aft of the firewall. It secures the flow

of fuel for emergencies and maintenance functions. This valve is manually operated by pulling a

yellow and black striped T-handle, located at the aft end of the left console in the front cockpit.

When pulled on the ground it shall be reset by maintenance personnel only.

4-4 FUEL SYSTEM