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MAINTENANCE MANUAL
(b) Mach Trim Control Law
1) When the Mach trim mode engages, the Mach trim control law synchronizes to stabilizer position. When the flaps are retracted, the control law provides incremental stabilizer position changes dependent upon computed Mach (Mc). The ADCs provide digital Mc inputs. Half-speed trim commands are provided by the selected SAM. The Mach trim mode is disengaged on-ground and until 20 seconds into the in-air mode.
2) After each trim event the stabilizer position change is equal to or above the Mach trim control law (more negative) for increasing Mach and is equal to or below the control law (more positive) for decreasing Mach. The threshold of Mach change for the initial trim event following deviation from trim speed is nominally 0.0l Mach. The threshold of Mach change for subsequent trim events is approximately
0.005 Mach, based on system overtrim characteristics.
(3) Mach Trim/Speed Stability Schematic (Fig. 4)
(a) The Stabilizer Trim and Elevator Asymmerty Limit Module (SAM) uses the following inputs to determine Mach trim and speed stability commands. 1) The Air Data Computers (ADCs) provide digital airspeed and
Mach inputs to both SAMs (Ref 34-12-00/001, Air Data
Computing System).
2) The Flap/Slat Electronic Units (FSEUs) and Flap/Slat Accessory Module (FSAM) (if installed) input a flap retracted signal to both SAMs (Ref 27-51-00/001, Trailing Edge Flap System). The left and right SPMs input stabilizer position to the corresponding SAM (Ref 27-48-00/001, Stabilizer Trim Positions Indicating System).
3) The air/ground relays provide airplane air/ground status. Each SAM receives three air/ground inputs from four relays (Ref 32-09-00/001, Landing Gear Multiple Use System/Components).
(b) Each SAM contains ARM and CONTROL microprocessors to form a dual redundant system. Each processor receives digital inputs from the left and right ADCs. The inputs are Mach (Mc) and computed airspeed (Vc). Continuous monitoring is provided for both primary and secondary ADC inputs. The following input checks are provided: 1) If a parity error is detected, the value is discarded and
the previous valid value is used. Four or more parity
errors within eight successive samples set an ADC fault.
2) Data which is identified as FAILED by the ARINC 429 sign status matrix is discarded and the previous valid value is used. Four or more FAILED values of a parameter within eight successive samples sets an ADC fault.
3) The Mc data identified as No Computed Data (NCD) by the sign status matrix is discarded. The value is replaced by
0.1 Mach. The Vc data identified as NCD is discarded and replaced by 30 knots.
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DATA BUS 1 C (SH 2)
MACH/SPEED TRIM
DATA BUS 2
L&R EICAS
COMPUTER
RIGHT AIR DATACOMPUTER
DATA BUS 1
DATA BUS 4 B (SH 2)
LEFT AIR DATACOMPUTER AA
MD&T L (SH 2) STAB. POSNMACH/SPD TRIM IND
AUX ANN. PANELLEFT STAB.
H (SH 2)
POSITION G (SH 2)
INTERNALS
OTHER CSEU MODULES
FLAP RETRACTED
SAME AS
RIGHT
STABILIZER
FLAP/SLAT ELECT
AIR AIRUNIT 2
TRIM/ELEVATOR
ASYMMETRY
J (SH 2)
LIMIT
AIR
MODULE GND GND
(SH 2)
AIR/GND AIR/GND
GND
RELAY 1 RELAY 2 SYS 1 AIR/GND RLY
OTHER CSEUK (SH 2)
MODULES
AIR
N
GND M E (SH 2)
(SH 2)
R
FLAP RETRACTED
P
STAB. POSN
SYS 2 AIR/GND RLY
D (SH 2)
FLAP/SLAT ELECT
UNIT 1
RIGHT STAB. POSITION F (SH 2) FLAP RETRACTED
FLAP/SLAT ELECT LEFT STAB. TRIM/ELEVATORUNIT 3 ASYMMETRY
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