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MAINTENANCE MANUAL
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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MAINTENANCE MANUAL
4) If either Mc or Vc is not received within 150 milliseconds for 2 consecutive frames, an ADC Fault is set. Old data is used when new data is not received within one frame.
Cross comparison between primary and secondary Mc and Vc is provided. An ADC comparison fault is set if: 1) Primary and secondary Mc differ by more than 0.05 Mach for
more than five seconds. 2) Primary and secondary Vc differ by more than 10 knots for
more than five seconds.The cross comparison is inhibited if either ADC is invalid or in the TEST mode. An ADC fault is cleared if valid data is received continuously for 30 seconds and no more than four resets have occured.The flaps retract and air/ground monitor are majority vote (two out of three) monitors. Flaps-retract failures set the FSPM/SPM faultball. Air/ground failures set the AIR/GND fault ball.The stabilizer position is monitored and input to synchronize the control laws. A input level monitor checks for signal inputs less than +1.0 vdc or an open circuit. A detected failure sets the FSPM/SPM faultball. A stabilizer fault is cleared in the air if a valid input is received for five seconds and no more than four resets have occured.Mach or speed commands are generated in the control law. The command outputs depend on the following SAM modes and inputs: 1) Flaps retracted for Mach command outputs. 2) Flaps not retracted for speed command outputs 3) Neither SAM in autotrim or manual trim modes (Ref
27-41-00/001, Horizontal Stabilizer Trim Control System).Stabilizer movement of more than 0.3 degree in either direction without a valid SAM command is detected by the Mach loop or speed loop monitor. An UNSCD TRIM discrete is output from the controlling SAM. The UNSCD TRIM discrete turns on the UNSCHED STAB TRIM light and causes the message UNSCHD STAB TRIM to appear on the upper EICAS display unit. The Mach loop or speed loop monitor is also active when the airplane is on ground.The internal monitors in each SAM determine a MACH/SPEED fault. The monitors detect the following conditions. 1) The stabilizer does not move in the correct direction
within 3.5 seconds of a valid SAM command. (The loop fault
latch resets after 5 seconds of no fault conditions). 2) Input stabilizer signal level is outside preset limits. 3) The control loop error for the CONTROL channel exceeds 0.3
degree or control loop error for the ARM channel exceeds
0.5 degree for longer than 3.5 seconds with a valid SAM command. (The loop fault latch resets after 5 seconds of no fault conditions).
4) Release of a hydraulic brake without a valid SAM output trim command. 5) A hydrualic brake not released within 2 seconds of a valid SAM output trim command.
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757 MAINTENANCE MANUAL
6) Faults within ADCs or wiring to SAM's that cause air data to be monitored as invalid.
7) Following a second fault in either flap retracted or air/ground discrete signals majority vote.
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Switching to the other SAM occurs when a MACH/SPEED fault or SAM fault is detected within controlling SAM. If a MACH/SPEED fault or SAM fault is detected within both SAMs, the MACH/SPD TRIM light on panel P5 is illuminated and the message MACH/SPD TRIM is displayed on the upper EICAS display unit. This indicates a complete loss of the Mach/speed function.
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A SAM is able to reset itself if it is not in control (other SAM is selected for Mach/speed mode) and a SAM fault occurs. A SAM is also able to reset itself if a SAM fault has occured and the other SAM picks up control (Mach/speed mode select goes high on other SAM). If a SAM fault occurs again within one minute of an autoreset the SAM will not be able to perform another autoreset. (All monitors in the SAM are less than one minute in length.) The SAM fault ball will reset itself if an autoreset is successful and no other SAM faults occur within one minute.
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