Smiths Aerospace
Electronic Systems – Malvern
DOC NO.0134ED
MAINTENANCE MANUAL SUPPLEMENT
BOEING B737-100/200/300/400/500
28-41-04
11. TSU (Figure 5)
A. A TSU is located on the inboard side of the rib at leading edge station 118 for the left and right wing
and station LBL 55 for the center tank. The TSU is placed in series with the connector of the existing
fuel indicator harness and the fuel tank. The auxiliary tank (if equipped) has its TSU mounted at rib
station 727D.
12. Operation
A. The fuel quantity indicating system operates on 115v ac power from the standby bus or external
power bus.
B. The tank units function as variable capacitors, their capacitance value is determined by fuel level in
the fuel tank.
C. The fuel quantity indicator does a capacitance measurement with a sine wave. The fuel quantity
Indicator microcomputer feeds a sine wave to the tank units and a sine wave that is 180 degrees out
of phase with the tank unit sine wave to the compensator unit. The microcomputer measures the
capacitance of the tank units first against an internal virtual compensator. This is done by adjusting
the drive voltage to the internal compensator to achieve a null on the return Hi-Z signal. The tank
units are then measured against the in-tank compensator unit. From these two measurements the
microcomputer is able to determine the tank unit and compensator capacitance. With these two
values, it calculates and shows the fuel mass.
D. The microcomputer monitors the fuel quantity Indicating system and supplies an error indication and
code to help in troubleshooting.
Page 16
Dec 18/02
Airplanes with Smiths 2307 Model Fuel Quantity
Indicators with TSU
EFFECTIVITY
