used. 2) The manual trim switches, mounted on the control wheel, are not activated. 3) The alternate electric trim switches, located on the aisle stand, are not activated. 4) Airplane is in the air and airspeed is below 220 Knots (above 0.86 Mach for Mach trim). 5) The flap position is less than 25°.
(b)
The Mach/Speed Trim System consists of two independent channels, either of which can perform the Mach/speed trim function. Each channel consists of two digital Air Data Computers (DADC), one SRM, one Stabilizer Trim Control Module (STCM), one stabilizer position sensor (RVDT), one Flap Control Unit (FCU) and various sensors.
(c)
During Mach/speed trim operation, the stabilizer position is related to Mc/Vc. The SRM determines the desired stabilizer position command that powers one of the hydraulic motors. The motor output is transmitted via a differential gearbox to drive the stabilizer jackscrew.
(d)
Half-speed trim (0.1 or 0.25 degree/sec) is provided by either SRM. The controlling SRM is randomly selected at power-up to avoid latent faults. The left SRM provides half-speed stabilizer control in response to variations in Mc/Vc from the left or right DADC. Priority is given to the left (primary) DADC as a source of Mc/Vc for the left SRM. Similarly, the right SRM provides half-speed stabilizer control in response to variation in Mc/Vc from the right or left DADC. Priority is given to the right (primary) DADC as a source of Mc/Vc for the right SRM.
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LEFT
STABILIZER
TRIM/RUDDER
RATIO
MODULE
RIGHT
STABILIZER
TRIM/RUDDER
RATIO
MODULE
LEFT
STABILIZER
TRIM CONTROL
MODULE
RIGHT
STABILIZER
TRIM CONTROL
MODULE
Mach/Speed Trim System
Figure 1
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C83110
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A = BRAKE RELEASED B = MOV OPEN, CUTOUT
1
FLAP POSITION
Mach/Speed Trim Block Diagram Figure 2
H20015
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(e) The SRM uses the following inputs to determine arm and control output commands to the STCM. 1) Airspeed data from DADC. 2) Stabilizer position from the Rotary Variable Differential
Transformer Transducers (RVDTs) via the FCUs.
(2)
Mach/Speed Trim Control Law Schedule (Fig. 3)
(a) The Mach/speed trim control law is synchronized to the stabilizer position as the Mach/speed trim mode is engaged, and the incremental control of the stabilizer position is provided as a function of Mc/Vc. Speed trim is inhibited when Vc is greater than 220 Knots. Mach trim is inhibited when Mc is less than 0.86 Mach. The allowable trim threshold is less than or equal to /.075/ (/0.035/ for Mach trim) degrees. After trimming, the final stabilizer position change produced is approximately equal to the position change corresponding to the ideal characteristic shown in Fig.3. The speed trim mode is disengaged on ground and remains disengaged until 20 seconds following a ground to air transition.
(3)
Mach/Speed Trim Schematic (Fig. 4)
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