Smiths Aerospace
Electronic Systems – Malvern
DOC NO.0134ED
MAINTENANCE MANUAL SUPPLEMENT
BOEING B737-100/200/300/400/500
28-41-04
FUEL QUANTITY INDICATING SYSTEM - DESCRIPTION AND OPERATION
1. General (
Fig
. 1)
A.
The
fuel quantity indicating system measures the weight of usable fuel in the fuel tanks. Fuel quantity
indicators on the pilots' instrument panel and refuel quantity indicators at the refuel station on the wing
supply visual indication of the fuel weight in each fuel tank. In addition, a fuel summation unit found
on the first officer's auxiliary panel supplies a continuous summation signal input to an on-board
computer where it is converted to an on-demand display of combined tank total remaining fuel or is
used by the computer in various performance parameters. In the No. 1 and No. 2 tanks, five fuel-
measuring sticks are supplied to find the fuel quantity of the applicable fuel tank by mechanical
means.
B. The overall system is composed of a subsystem for each fuel tank, with components found in the
control cabin, in the fuel tank, and at the refuel station on the wing.
(1) Components in the control cabin include a fuel quantity indicator, digital calibration trim unit
(DCTU), and capacitance trimmer (where applicable) for each fuel tank, an indicator test switch
found above the fuel quantity indicators, and a fuel summation unit or V-Reference totalizer
indicator.
(2) Components at the refuel station on the wing include a refuel quantity indicator for each fuel tank
and an indicator test switch. The refuel quantity indicators repeat the fuel quantity shown by the
fuel quantity indicators in the control cabin. The refuel quantity indicators operate only when the
access door for the refuel station is open.
(3) Each fuel or refuel quantity indicator is "universal" - It can be used to supply indication for any fuel
tank; however, a refuel quantity indicator from the refuel panel on the wing cannot be installed in
the control cabin. Likewise, a fuel quantity indicator from the control cabin cannot be installed in
the refuel panel on the wing.
(4) Components In the fuel tanks include a compensator unit and a number of tank units for each fuel
tank. Fuel measuring sticks are installed In the No. 1 and No. 2 tanks. The tank units in each
fuel tank are connected in parallel and positioned so that only usable fuel is measured.
Capacitance values of the tank units in each fuel tank are added to give a single capacitance
value, which is transmitted to fuel quantity indicator.
(5) Components for electrical protection include the Transient Suppression Units (TSU).
The Transient Suppression Unit (TSU), which incorporates a suppression circuit, is designed to
prevent unsafe electrical signals, either current or voltage from entering the fuel tank.
The TSU is inserted in the Fuel Quantity Indicating System wiring between the in-tank
components and the indicator in the cockpit. The suppression unit is mounted near the specific
tank to be protected.
The suppression circuit is placed in series with the fuel measurement signal leads entering the
tank. The protective circuit of the suppression circuit and the Hi-Z shield are grounded to the
aircraft structure via grounding straps and the attaching hardware at its physical location.
Page 1
Apr 19/02
Airplanes with Smiths 2307 Model Fuel Quantity
Indicators with TSU
EFFECTIVITY
