(a)
The SRM uses the following inputs to determine Mach/speed trim commands. 1) The DADCs provide input to both SRMs. 2) The left and right FCUs provide a stabilizer position input
to the corresponding SRM. 3) The air/ground relays provide the status of airplane in the air or on the ground.
(b)
Mach/speed Trim Operation
1) Each SRM contains ARM and CONTROL microprocessors to form a dual system. Each microprocessor receives airspeed inputs from the left and right DADCs. Continuous monitoring is provided for both primary and secondary DADC inputs. The following input checks are provided. a) If a parity error is detected, the new value is
discarded and the previous valid value is used. If four or more parity errors occur within eight successive samples, the failure is set as DADC fault.
b) Data which is identified as "failed" by the status matrix is discarded and the previous valid value is used. If four or more invalid values of a parameter are received within eight successive samples, the failure is set as a DADC fault.
c) The Vc data which is identified as "No Computed Data" by the status matrix is discarded and replaced by 30 knots. The Mc data which is identified as "No Computed Data" by the status matrix is discarded and replaced by
0.1 Mach.
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STABILIZER POSITION (DEGREES) STABILIZER POSITION (DEGREES)
-3.00 -2.75 -2.50 -2.25 -2.00 -1.75 -1.50 -1.25 -1.00 -0.75 -0.50 -0.25 0
-1.25 -1.00 -0.75 -0.50 -0.25 0
0.86 0.88
0.90
MACH NUMBER
Mach/Speed Trim Control Law Schedule Figure 3
C83762
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R SRM
Mach/Speed Trim Simplified Schematic Figure 4
H20014
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2)
3)
4) 5)
6)
7)
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d) If DADC data is not received within 0.5 second when
required by the SRM, the loss is set as watchdog fault.
Last valid data is used when new data is not received.Cross comparison between primary and secondary computed airspeed is provided. If primary and secondary computed airspeed differ by more than 50 knots (primary and secondary Mc differ by more than 0.05 Mach) for more than 1 second, a DADC comparison fault is latched. The cross comparison is inhibited if either DADC is invalid or in "test" mode, or the airplane is on the ground. A DADC fault set by cross comparison monitor is cleared if valid data is continuously received for at least 30 seconds.The inputs to the control law are computed airspeed , Mach number and stabilizer position. The control law output is a discrete command to the Stabilizer Trim Control Module (STCM) to trim up, trim down or not to trim.The DADC source selection logic directs the primary DADC data, left DADC for left SRM and right DADC for right SRM, to the Mach/speed trim control law.A random selection of left and right SRM is made at power-up. The selected SRM will inform the other SRM that it is "active" and "valid" for Mach/speed trim mode after power up. The "active" SRM will operate in the Mach/speed trim mode as long as no faults are detected. As the Mach/speed trim control law generates a trim command, ARM and CONTROL channel signals are transmitted to the STCM which directs hydraulic fluid to release the stabilizer secondary brake and drives the stabilizer motor. Each STCM can drive the stabilizer at half the full rate. A position sensor provides stabilizer position feedback via the FCU to the SRM.When stabilizer movement of more than 1.0 degree in either direction without a corresponding trim command is detected by unscheduled trim monitor, an unscheduled trim fault is set. Unscheduled trim fault detected by either SRM will cause the STAB TRIM 2 or STAB TRIM 3 message to appear on the EICAS display. If the unscheduled trim fault clears, the message goes away 30 seconds later.When a valid trim output command is present and the magnitude of the Mach/speed trim servo loop error is greater than 0.3 degree for a continuous period of 15 seconds or more, a speed control loop error is set. The monitor is inhibited if unscheduled trim fault has been detected. The fault is reset after 12 seconds of no fault condition.
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