I 4. B. NZ-920 Navigation Computer (Figure 263)
(1) Avionics Standard Communication Bus (ASCB) Interface
The navigation computer transmits flight plan data, display data,
guidance commands, dual NZ-920 sync data, and navigation and custom
databases during normal operation of the bus.
The navigation computer
receives dynamic information about the
aircraft’s present state via
the ASCB. This information consists of
attitude, headinq, velocities,
accelerations, altitude and soeeds.
Also, dual NZ-926’sync data is received from-the ASCB. ‘
(a) NZ-920 Navigation Computer ASCB Inputs
~ FZ-820 Flight Guidance Computer (FGC) ASCB Inputs
The NZ-920 inputs both left and right FGC data. It
processes and uses data from the priority FGC only. -
priority FGC is established by reading WSP 1, bit 11,
mode/engage priority,
of both FGC’S transmissions. A
one setting of this bit identifies the priority FGC.
priority bits
for both FGCS are set or neither set
simultaneously, the data from each
FGC is considered .
and is not processed
for use by the NZ-920. Only one
he
logic
If the
rivalid
FGC
can be
priority at a time in order for the NZ-920 to use its
data.
~ AZ-81O Digital Air Data Computer (DADC)
ASCB Inputs
The NZ-920 inputs both left- and right-side DADC data from
the ASCB. Selection of DADC data is performed by the
navigation and guidance (LNAV and VNAV) subsystems of the
NZ-920.
The navigation subsystem selects and uses its on-side DADC
data if valid. Otherwise, it will switch to the off-side
DADC and use that data if the on-side DADC is invalid. If
both DADCS are invalid, the NZ-920 continues to search
between the two DADCS until a valid sensor is found.
If
both DADCS are valid, the navigation subsystem in each
NZ-920 of a dual system configuration have independent
sources of air data
for input into the position computation.
The guidance subsystem selects and uses the same DADC from
which the high priority autopilot gets its data for use in
its control laws for steering the airplane. By using this
DADC, guidance is using the same altitudes and speeds as the
autopilot, except during the approach mode. During an
approach, the autopilot will average the DADCS and guidance
will use the previously AFCS selected DADC state to
determine its DADC usage.
22-14-00
Page 298.152
Aug 15/91
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