M. Takeoff (T/O) Mode Capabilities
(1)
Flight Director takeoff guidance is provided in pitch and roll. The T/O mode is activated by pressing the GA switch while on the ground. The pitch T/O mode controls basically to the speed selected on the MCP while the roll T/O mode controls heading as selected on the MCP.
(2)
The transiting out of the F/D T/O mode is identical to that for the G/A mode except that cruise HDG SEL is automatically established at 400 feet in T/O to prevent roll bar bias when exiting pitch T/O.
(3)
An A/T Takeoff capability is also provided. If the PDC is engaged in a Takeoff page and the A/T is armed, pressing a GA switch will engage the A/T Takeoff EPR mode. The A/T will drive the thrust levers to the Takeoff EPR, go to throttle hold for 17.5 seconds and when radio altitude is above 400 feet will then go the ARM mode. From this time on, the integrated system control function can control the auto throttle. Therefore, engagement of any A/P and/or F/D mode (except LVL CHG), after the A/T has come out of Throttle Hold, will cause the A/T and PDC control to operate as described under Cruise Capabilities. If A/P and/or F/D mode is engaged prior to the A/T coming out of Throttle Hold, the A/T will not be automatically commanded to the compatible mode. Subsequent pilot selection of the compatible A/T mode after the A/T has come out of Throttle Hold will cause engagement of the compatible A/T mode and proper moding of the PDC.
N. Autopilot Stabilizer Trim
(1)
The automatic stabilizer trim operation (Fig. 12) is controlled from signals within the elevator command and output amplifier. Input signals are summed from the neutral shift sensor and elevator position sensor. The signal amplitude varies directly with elevator displacement. Prior to AP engagement the input signal is synchronized. After engagement the signal goes thru a stabilizer trim pot which attenuates the signal according to stabilizer position. The signal is then amplified and monitored for polarity and amplitude. When the signal exceeds a specific threshold, trim up or trim down control signals result. These signals thru system logic cause the stabilizer ac motor to reposition the stabilizer and reduce the elevator displacement signal.
(2)
Stabilizer trim is activated when the threshold is exceeded for 3 seconds except when in the A/P GA mode. With flaps up the stabilizer trim at a rate of 1 unit in 10-13 seconds; with the flaps down, the rate is 1 unit in 4-7 seconds. The automatic stabilizer trim can trim the stabilizer between the nose down limit of 2 units and the nose up limit of 14 units.
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22-11-01 Page 54 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. May 20/82
H28601
580 Automatic Stabilizer Trim SchematicJun 20/84 Figure 12 22-11-01 Page 55/56BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
(3)
The automatic stabilizer trim logic monitors the system for valid operation, changes trim delay depending on mode and operated a warning for sustained operation. The trim request delays differ, depending on G/A engagement. Logic at (1), (2), and (4) determine the required delay. The output of (3) along with engage logic and no CWS input provide a high output from (5) to (6) and (7) which results in a high output from either (6) or (7). Whenever both (6) and (7) simultaneously provide high outputs, logic at (8) disengages the A/P system. The outputs of (6) or (7) are connected to the same point on Channel B, then routed thru switches to operate the applicable relay in the stabilizer trim servo. Either relay determines applicable drive direction and energizes the drive clutch to move the stabilizer. A speed change relay in the servo, changes motor speed from low to high when flaps are lowered. Another output from (1) starts a 12 second timer which illuminates a warning light if trim requirement remains after the elapsed time.
(4)
At 400 feet radio altitude, a trim bias is inserted. This bias drives the stabilizer nose up to achieve elevators slightly mistrimmed in the nose down direction. The bias effectively shifts the stab trim threshold 3 degrees in the nose up direction. This forms the “flare spring.”
(5)
When the stabilizer drives, bias voltages are switched into the pitch computation to move the elevators toward neutral. These bias voltages, called stab negator signals, improve autopilot performance in cruise and approach.
(6)
Nose up stabilizer trim is initially inhibited in AP G/A mode for 6 seconds. The trim time delay is reduced to 0.5 second and the trim bias is inhibited or removed. This helps alleviate nose down loss of autopilot authority during the G/A maneuver. Positioning the aisle stand STAB TRIM switch to cutout or pilot operation of control wheel stab trim switch will disconnect the autopilot.
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