28-41-00
 ú CONFIG ú 09 Page 33 ú May 28/99

A
757
MAINTENANCE MANUAL

 b)  Both center tank override switches in the off position with over 1200 lbs or 500 kgs of fuel remaining in the center fuel tank. The message and light are reset when the fuel remaining in the center tank drops below 1000 lbs or 400 kgs.
 (b)  The FQPU sends a maintenance required signal to EICAS when the FQPU detects a fault. 1) The subsequent messages are shown:
 a)  FUEL QTY BITE (maintenance message) - Shown when there are any faults recorded in the FQIS processor NVM (non-volatile memory). To show this message the ECS MSG switch on the EICAS MAINT module (P61 panel) must be pushed.
 1.  Faults can be recorded into NVM during any of the
_
 subsequent:

 a.  When the airplane is in the ground mode and the
_ fueling station door is open.
 b.  After the airplane has been in the air mode for
_ more than 8 minutes.
 c.  While the FQIS processor unit does a VERIFY
_ test.
 b)  FUEL QTY CHANNEL (status message) - Automatically shown when the FQIS processor has a fault in either channel with valid digital data. This message is inhibited by the FUEL QTY BITE status message.
 c)  FUEL QTY IND (status message) - Automatically shown when there is unknown accuracy in one or two fuel tank quantity indicators with valid digital data. This message is inhibited by the FUEL QTY BITE status message.
 d)  FUEL QTY BITE (status message) - Automatically shown when there is unknown accuracy in all three fuel tank quantity indicators, or any FQIS fault with loss of valid digital data.
 (4)  
Densitometer Signals

 (a)  
All three densitometers operate continuously. Each densitometer outputs two pulse signal trains to the FQPU for processing. Should one densitometer malfunction the FQPU uses data from one of the other two densitometers.

 (b)  
At the FQPU, the microcomputer counts the pulses, and calculates the density of the fuel. Fuel density is proportional to the natural logarithmic value of the ratio of the two pulse outputs.

 (5)  
FQIS Time Sharing

 (a)  
The FQIS uses a major time cycle of 1024 milliseconds to read, compute and test all functions. The FQPU divides the time cycle equally among the three fuel tanks. Further time breakdown is as follows; 17 MSEC for each tank unit and compensator, 52 MSEC for load select and 17 MSEC for data transmission.
　
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