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under these conditions if a DBS mode is selected or if velocity update is selected without accepting or
rejecting the update.
25.3.1 Navigation Controls and Indicators.
25.3.1.1 HSI Display. The symbols and digital readouts that normally appear on the HSI during
backup system operation are the same as in the INS operation except for the POS option display.
25.3.1.2 POS Option Display. Pressing the POS/ADC pushbutton on the HSI display commands the
mission computer to use air data function true air speed, MAD heading, and wind speed and direction
to compute aircraft latitude and longitude in waypoint steering computations.
25.3.2 Backup Heading Mode Control. If the INS computer fails or if the INS switch is rotated to
the GYRO position the INS reverts to the gyro mode and true heading is no longer available from the
INS. The mission computer slaves the INS platform heading with the MAD to provide damped
magnetic heading. The slaving of the INS platform to the MAD occurs in straight and level flight.
During maneuvers, when roll is greater than ±5° or pitch is greater than ±10°, the MAD output is not
used. Upon reversion to the Slaved heading mode, the bottom row of option selections on the HSI are
HDG/SLV, SYNC, and ERECT. The SYNC option can be used to quickly synchronize the heading
with the MAD output if a heading error exists. The MC automatically synchronizes the heading with
the MAD output when the heading error is greater than 11.25° during level flight. Pressing the ERECT
option commands the INS to increase the gains in the INS erection loops, fast-leveling the platform.
SYNC and ERECT are momentary options and should be used only in straight and level flight.
If it is desired to change the backup heading mode, the heading status option pushbutton should be
pressed (HDG/SLV in this case) and the available heading options (SLV, DG, and COMP) are
presented on the HSI. Selecting any of these options commands the MC to display the selected option
and the necessary controls for that option. For failure conditions the next best available source is
automatically selected.
Selection of the DG heading mode or failure of the MAD while in the HDG/SLV mode causes the
bottom pushbuttons on the HSI to display HDG/DG, HDG (arrow left), HDG (arrow right), and
ERECT. The MC computes aircraft heading using the INS platform heading as a smoothed heading
source to compensate for the difference between true north, platform heading (wander angle), and
earth rate. The aircrew may correct the aircraft heading by using the HDG slew option buttons.
Following the initial setting of heading, the MC provides heading compensation for earth rate but
changes in magnetic variation must be entered using the UFCD because the aircraft uses the last
known value of magnetic variation.
If the aircrew selects the compass heading mode, if the INS is turned off, or if the INS fails
completely the mission computer uses the MAD output, damped with body rate data from the flight
control system. During maneuvers, when roll is greater than ±5° or pitch is greater than ±10°, the flight
control system body rates are used to determine heading. In the compass heading (HDG/COMP) mode,
the bottom buttons are labeled HDG/COMP and ERECT. The ERECT option is displayed when the
INS is operating in the AHRS mode. The MC uses the last known magnetic variation to compute true
heading.
25.4 BACKUP FREQUENCY CONTROL
Backup frequency control for the radios in case of an UFCD or MPCD malfunction is provided by
the multiplex bus and a DDI display. The UFC backup (UFC BU) display can be selected from the
A1-F18EA-NFM-000
VII-25-4 ORIGINAL
SUPT menu. Both comm 1 and comm 2 can be controlled from the UFC BU display. When the COM1
or COM2 button is pressed, the frequency on which that radio is operating is displayed below the
legend COM1 or COM2, as appropriate. A new frequency is selected by selecting OVRD and using the
numerical buttons along the sides of the display and the ENT button at the bottom. Frequencies can
be entered to 5 KHz of resolution. As the new frequency is entered, it is displayed in the scratch pad
above the COM1 or COM2 legend. When the ENT button is pressed, the frequency displayed in the
scratchpad is stored in the mission computer as the preset frequency for the radio. Pressing the CLR
button clears the scratch pad if an error is made when entering a backup frequency. Selecting the
OVRD option in the upper right corner of the display boxes the option and the radio operates on the
preset frequency stored in the mission computer, overriding the normal frequency control from the
UFCD. Frequency control reverts to the UFCD when OVRD is deselected. The frequency displayed
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NATOPS Flight Manual 飞行手册 2(115)
