Smiths Aerospace
Electronic Systems – Malvern
DOC NO.0134ED
MAINTENANCE MANUAL SUPPLEMENT
BOEING B737-100/200/300/400/500
28-41-04
A. The tank units are cylindrical capacitors which are installed throughout each fuel tank to sense fuel
level variations and, as a result of capacitance change, transmit electrical signals to the fuel quantity
indicators. The tank units have two coaxial cylinders which are rigidly spaced at intervals throughout
their length by insulating centering spacers. The inner electrode is a variable diameter nickel alloy
tube and is connected to the Hi-Z input. The outer electrode is an aluminum alloy tube of constant
diameter and is connected to the Lo-Z voltage excitation lead. The diameter of the inner electrode is
profiled to supply an output signal which is linear with respect to the volume of the fuel in the tank.
Openings at the ends of the tank units let fuel assume the same level between the two cylinders as
that in the fuel tank.
B. Changes in airplane attitude can cause errors in the fuel quantity measurement. The tank units are
installed in locations in the fuel tank that keep these errors to a minimum. The large number of tank
units in different locations in the fuel tank also keeps these errors to a minimum.
C. Tank units are attached to wing ribs in the No. 1 and No. 2 tanks and to spanwise beams in the center
tank. Access to the tank units is through fuel tank access openings. Electrical connections to the tank
units are made at the tank unit terminals. Tank unit leads in a given fuel tank, form a harness that
terminates at a receptacle. The receptacles for the No. 1 and No. 2 tanks are found in the front spar
of the applicable wing, and for the center tank in the forward bulkhead of the wheel well for the main
landing gear on the left side. Connected to each receptacle is a bussing plug assembly. The bussing
plug assembly connects the tank units in parallel.
3. Compensator Unit (Fig. 3)
A. The compensator unit in each fuel tank acts as a sensing element to maintain a linear relationship
between fuel dielectric properties and fuel density. Each compensator unit has a low impedance plate
and two high impedance plates, which are protected by a mechanical shield. The compensators are
attached to the same wing structure as the tank units and are installed so that they are completely
submerged in fuel at all times. Access to the compensator units is through the fuel tank access
openings. Leads from the compensator units are contained in the same harness used for the tank
units.
4.
Page 12
Apr 19/02
Airplanes with Smiths 2307 Model Fuel Quantity
Indicators with TSU
EFFECTIVITY
