As long as we're quoting manuals;
The Fuel Quantity indication system shows total fuel quantity with differing annunciations regarding fuel level (i.e. usable, configuration, overfill, etc.) The gauges do not "hide" unusable fuel. The original intention was that the CONFIG message would show and the indication would turn yellow if there was usable fuel in the center tank with the pumps turned off. This occured with 1600 or more pounds in the tank. With the AD's that have been issued the wording has changed but the indication remains the same. A lot of people still think the system doesn't show unusable fuel. That's not the case.
From the 737-800 SDS
28-41-00-006
"FUEL INDICATING - PROCESSOR - FUNCTIONAL DESCRIPTION
General
The fuel quantity processor unit (FQPU) has three signal conditioner circuit cards (SCCC) and a BITE display card (BDC).
Fuel quantity shows on the common display system (CDS).
Signal Conditioner Circuit Cards
There is one SCCC for each fuel tank. Each SCCC has these functions:
Sends a low Z signal to the tank units and compensator
Reads the high Z return signal from the tank units and compensators
Calculates total fuel weight for its tank
Changes analog signals to ARINC 429 signals
Sends tank fuel weight data to the DEUs Sends real time fault data to the BDC.
The tank 2 SCCC also sends total fuel tank weight to the FMC. BITE Display Card
The BDC has these functions:
Keep fault data in non-volatile memory
Send a signal to each SCCC to start a real time test for fault isolation
Send and receives fault data with the flight management computer for fault isolation."
28-41-00-007
"FUEL INDICATING - FUEL QUANTITY INDICATIONS
General Individual fuel tank quantity shows on the CDS and the refuel panel.
Fuel quantity shows in pounds.
Fuel Configuration Messages
Fuel configuration messages show on the CDS. These messages show a problem with the configuration of the fuel system.
The LOW message shows that either main tank 1 or main tank 2 has less than 2,000 lb (907 kg) of fuel. This message goes away when there is more than 2,500 lb (1133 kg) of fuel in that tank. The low fuel condition must exist for 30 seconds before the LOW message shows.
The IMBAL message shows when there is a difference of 1,000 lb (453 kg) between main tank 1 and main tank 2. The message goes away when the difference between tanks is 200 lb (90 kg) or less. The IMBAL message only shows when the airplane is in the air. The IMBAL message does not show when the LOW message shows. The imbalance condition must exist for 60 seconds before the IMBAL message shows.
The CONFIG message shows when one of these conditions exist:
1,600 lb (725 kg) or more of fuel in the center tank
Both center tank boost pumps off
Either engine is in operation.
After the CONFIG message shows, it stays on until all of these conditions are true:
800 lb (360 kg) or less of fuel in the center tank
A minimum of one center tank pump is producing high pressure The two engines are not in operation."
