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MAINTENANCE MANUAL
2) Inner loop data is from the Inertial Reference System (IRS) and is used for autopilot stability. These signals are: a) Pitch, pitch rate, roll, and roll rate during cruise.
3) Inner loop data is considered critical and always used in Signal Selection Fault Detection (SSFD) voting. The SSFD provides high-integrity input monitoring.
4) Outer loop data is from the following units:
a) Instrument Landing System (ILS)
b) Radio Altimeter (RA)
5) The above units provide the following signals: a) Angle of attack, altitude rate, altitude, airspeed,
mach, and impact pressure (ADC). b) VNAV and LNAV control laws (FMC). c) Localizer and glideslope deviation and runway course
(ILS).
d) Altitude above terrain (RA).
(b) Signal Selection 1) Triple Sensors a) For single-channel engage or F/D modes, local outer loop data is used by the FCC. An invalid outer loop signal required for an autopilot engage mode causes the A/P to hold attitude and display mode fail and caution annunciations (Ref 22-14-00). An invalid outer loop signal does not disengage the A/P. If the invalid condition corrects before a timeout, the mode is automatically re-entered. An invalid local signal required for any F/D mode removes the F/D command bars from the EADI. b) During multi-channel CMD, SSFD voted ILS and RA data is used by the FCC. The SSFD voting sets invalid signals to zero or the last voted value and selects the mid value signal of the three inputs. The first invalid signal required for multi-channel operation changes the ASA display (Ref 22-14-00). A second invalid input of the same signal disengages all A/P channels, inhibits engagement in channels with local failures, changes the ASA display, and display warning annunciations (Ref 22-14-00).
c) Signal selection while on the ground is inhibited. Local sensor data will be used for computation within each FCC.
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2) Dual Sensors
a) Signal selection for ADC data allows selection of one of the two outer loop ADC data sources. For a F/D mode, left ADC data is used by all FCC's if the captain's F/D switch is ON and right ADC data is used if only the first officer's F/D switch is ON. If the autopilot is engaged, left ADC data is used if the left or center channel is engaged and right ADC data is used if the right channel is engaged. If any of the data being used becomes invalid, the FCC will automatically switch over and use the opposite ADC data.
b) For single-channel engage or F/D modes, failure of both ADC signals causes the autopilot to show mode fail and caution annunciations and to disengage. During multi-channel engage, failure causes no change until after touchdown and disengage.
(4) Flight Director Engage
(a)
The two F/D ON inputs to the FCCs originate at the two F/D switches at each end of the MCP. Either input will generate an F/D SELECT signal. This signal is sent to FCC mode logic and to the EFIS symbol generators for mode and F/D bar annunciation on the EADIs.
(b)
AIRPLANES WITH CONTROL WHEEL STEERING; when the airplane is in the air, the F/D SELECT signal is also used to generate a MODE ON output. This is used to prepare the mode logic to activate the CWS, CMD, or FD modes. The MODE ON output will also result from the following conditions.
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