SYSTEM DESCRIPTION (cont)
g. Auxiliary Fuel System.
The purpose of the auxiliary fuel system is to transfer additional fuel from auxiliary tanks to
internal fuel cells.
(2) System Operation (fig. 1026).
(a) The cap assembly provides fueling access to the gravity filler port. When the pilots FUEL panel EXT
TK switch is placed to ON, the auxiliary fuel tank air shutoff valve and auxiliary tank fuel shutoff valves are
opened. PAS air pressurizes the fuel, forcing fuel flow through auxiliary tank fuel shutoff valves into the internal
fuel cells. When four external fuel tanks are used, the PAS air is connected only to the outboard auxiliary fuel
tanks. The fuel is forced from the outboard fuel tanks, through the inboard fuel tanks, into the internal fuel cells.
When the internal fuel cells are full, automatic fuel shutoff occurs, terminating fuel flow. The fuel empty switches
complete a circuit when the fuel level is below the float, lighting the EXT EMP indicator on the pilot
caution/warning panel. Disconnect couplings are used for jettison capabilities of the external fuel tanks.
Auxiliary Fuel Tank Transfer System Interface Diagram
(b) Electrical power (fig. 1027) is supplied through FUEL TRANS circuit breaker (CB56) to the external
EXT TK switch on the pilot FUEL panel. When the EXT TK switch is in the OFF position, 28 VDC is applied
through contacts 2 and 1 to close the auxiliary tank shutoff valves. When the EXT TK switch is in the ON position,
28 VDC is applied through contacts 2 and 3 to open the auxiliary tank shutoff valves and the auxiliary tank air
shutoff valve. Contacts 5 and 6 complete a ground path for the EXT EMP indicator on the pilots caution/warning
panel. The EXT EMP indicator will light only when both fuel empty switches are below the float level. When four
external fuel tanks are used, the fuel empty switches are connected only to the inboard auxiliary fuel tanks.