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maintained in the horizontal component of the earth magnetic field. The air data functions process the
detected magnetic heading and develop the magnetic error compensation signals.
25.2.4 Backup Attitude and Navigation System Controls and Indicators. The controls and indicators
for the backup system are on the MPCD and the standby attitude reference indicator.
25.2.4.1 MPCD. The MPCD provides horizontal situation and steering control displays.
25.2.4.2 Standby Attitude Reference Indicator Controls and Indicators. The controls and indicators
on the standby attitude reference indicator (figure 25-1) are described as follows:
1. OFF flag. This flag is in view when power is removed or the pull to cage knob is pulled out.
2. Miniature aircraft. This represents the nose and wings of the aircraft and indicates pitch and roll
attitude relative to the horizon. The miniature aircraft is adjustable through ±5° of pitch trim by
rotating the pull to cage knob clockwise or counterclockwise when the knob is pushed in.
3. Pull to cage knob. The knob is pulled out and held to orient the gyro spin axis to the ARI case
(pitch position). When the knob is pulled out and rotated clockwise to engage detent the ARI
becomes caged.
Do not lock the gyro in the caged position with the pull to cage knob if
the gyro is spinning. Damage to the gyro might occur if the indicator or
the aircraft is moved while the gyro is spinning and caged. If the knob is
in the locked position, it must be pulled out to clear the detent before it
can be turned counterclockwise.
4. Slip indicator (inclinometer). This mechanical indicator displays sideslip.
5. Rate of turn needle. This needle displays the rate of turn. One needle width of deflection indicates
a 90° per minute rate of turn.
6. Test switch. The vertical and horizontal pointers and rate of turn needle are deflected when the
test switch is pressed.
7. Pointer shield. This shield conceals the vertical pointer in the stowed position.
8. Elevation deviation bar. This bar provides direction information for pitch steering.
9. Bank scale. The scale rotates with the aircraft to provide measurement of angular displacement
by the bank angle index during maneuvers.
10. Azimuth deviation bar. This bar provides direction information for azimuth steering.
A1-F18EA-NFM-000
VII-25-2 ORIGINAL
11. Sky pointer. The pointer rotates with the aircraft to indicate vertical (sky) in any roll attitude.
12. Bank angle index. The index indicates vertical in any roll attitude.
13. Display sphere. The sphere is directly coupled to the gyro gimbals to provide a direct reading of
pitch and roll. The sphere is marked at each 5° in pitch.
25.3 NAVIGATION BACKUP
Position keeping for aircraft navigation requires available sources of heading information, attitude
information, and velocity. The INS normally provides position keeping for the aircraft. Under various
failure conditions alternate sources of heading, attitude, and/or velocity information may be used for
position keeping. The backup heading modes are discussed in a following paragraph. The attitude
reference indicator is the alternate source of attitude information. The alternate sources of velocity
information are air data (true airspeed and angle of attack) or radar doppler velocities. Air data vertical
velocity is an alternate velocity source if only the vertical component of INS velocity is invalid. The
flight control set is an alternate source for angle of attack information.
The system automatically reverts to alternate data sources under failure conditions. For example, air
data position keeping is automatically selected in case of an INS failure. Air data position keeping uses
true airspeed, and angle of attack from the air data functions, and the last computed wind or the wind
inserted by the pilot on the UFCD. The wind can also be updated during air data position keeping by
performing a velocity update. A new wind is calculated when the velocity update is accepted. The
velocity vector on the HUD is flashed at a slow rate (on for 0.8 seconds and off for 0.8 seconds) during
air data position keeping. A POS/ADC caution is displayed on the DDI along with a master caution
light and tone when the INS reverts to ADC position keeping.
The radar doppler velocities are automatically used by the mission computer if available and no
other velocity sources are available (INS, GPS and FCC air data failed). This applies whether or not
TACAN position keeping is selected. Doppler velocities are available when the radar is operating in a
Figure 25-1. Standby Attitude Reference Indicator
A1-F18EA-NFM-000
VII-25-3 ORIGINAL
doppler beam sharpened (DBS) mode or PVU mode. The MC automatically uses the doppler velocities
　
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