(8)
Dual Channel Localizer Capture
(a) Localizer capture occurs single channel with the A/P channel first engaged and is exactly as described for single channel.
(9)
Dual Channel LOC O/C
(a) Discussion of this mode is as described for single channel
(10)
Dual Channel Approach on Course (AOC)
(a)
The single channel description remains applicable for dual approach except that at AOC the second up engaged roll autopilot channel is enabled. When the second autopilot actuator is pressurized hydraulically the system is dualized for roll.
(b)
Roll equalization begins and the Roll Confidence Test (RTC) is initiated in each channel when the second roll channel is on line. The automatic confidence test checks the lateral accelerometer circuitry. When the test passes, the Roll Monitor Confidence Test (RMTC) for that channel is allowed to start at AOC + 23 seconds. The RMTC in each channel verifies the integrity of the dual redundant roll fault monitors and they are enabled when the tests pass. When both the PMCT and RMCT in both channels have passes, FLARE ARM is annunciated to enable the pitch and roll monitors. SINGLE CH annunciation ceases at this point.
(c)
Radio altitude less than 1500 feet initiates a programmed reduction in G/S and localizer beam gains to compensate for beam convergence. Equalization gain is also reduced by radio altitude programming to maintain constant monitor sensitivity during beam convergence. Gain programming is ratcheted so that the gains can only decrease and not increase.
(11)
Dual Channel G/S Capture and Track
(a)
The single channel discussion is also applicable for dual operation. G/S capture occurs in the single "first up" channel during dual operation.
(b)
At G/S Engage an Alpha and Flaps Confidence Test is initiated in each channel whether single or dual channel has been selected. Successful completion is required for passing of the upcoming Pitch Confidence Test. (The Alpha test runs during single channel operation but because the PTC does not run, failure of the Alpha test does not have any consequences.)
(c)
At G/S + 10 seconds, the second up pitch autopilot actuator is hydraulically
pressurized and approximately 2 seconds later, dual operation begins.
(d)
At dual pitch control, equalization between channels begins and the Pitch Confidence Test is initiated in each channel to check go around, normal acceleration, flare and trim bias circuitry. At successful test completion, the Pitch Monitor Confidence Test (PMCT) for that channel is initiated which checks the integrity of the dual redundant pitch fault monitors. When the tests pass, the pitch monitors are unbypassed enabling pitch failure detection. When both PMCT's and RMCT's in both channels have passes, FLARE ARM is annunciated indicating that all fault monitoring is enabled.
(12)
Dual Channel 800-Foot Milestone
(a)
The system must be in dual channel (i.e. second paddle engaged in CMD) at 800 feet. This point is chosen to allow the system enough time to stabilize and the automatic tests to run. If the second paddles have not been engaged by 800 feet, it is locked down.
(b)
At 800 feet in APP mode with the monitors enabled, the AP Disengage Warning lights are used as additional stab out-of-trim annunciators. These lights when tripped illuminate steady red to indicate that the stabilizer is in a mistrimmed condition and that the autopilot should be disengaged.
(13)
Dual Channel Trim Bias and Monitor Sensitivity Change
(a)
A bias is added to the elevator position input to the trim detector at 400 feet. This shifts the threshold for auto stabilizer trim to drive the stabilizer and position the elevators slightly trailing edge down. This provides a "flare spring" so at low altitudes a failure which causes an automatic autopilot disconnect allows a nose up command when the elevators return to the neutral position.
(b)
The fault monitor time delay above 60 feet is 2.5 seconds for pitch and 10 seconds and 3 seconds respectively for single and dual roll. At 60-foot radio altitude, the monitor sensitivity is decreased to 1 second for pitch and 1.7 seconds for roll approach and dual-channel roll go-around. The monitor time delay below 60 feet is 10 seconds for single channel go-around.
(14)
Flare
(a)
Automatic flare is intended only for dual channel Autoland operation (Category IIIa); however, since it is an integral part of AFCS design it is functional but not annunciated during single channel operation.
(b)
Flare engages at 42 feet of radio altitude and terminates in a sink rate at touchdown of 2.5 fps. The flare command is developed by programming altitude rate as a function of radio altitude, which results in an exponential flight path to touchdown.
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