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selecting the STD HDG option would not reduce alignment time; or, during a CV alignment, when the
MAN option is selected.
Selecting STD HDG option results in an alignment based on the stored heading when the INS was
shutdown. Stored heading alignment is entered by setting the parking brake, placing the INS mode
select knob to CV/GND (CV/GND alignment display appears on the MPCD, see figure 24-4), and
selecting the STD HDG option. When stored heading alignment is entered the alignment progresses
the same as a normal ground alignment.
24.2.5.4 IFA (In-flight Alignment) Mode. IFA alignments include a complete IFA, CV/GND alignment
completion, and gyro recovery. With GPS selected and operating an inflight alignment is
performed using GPS position and velocity. A complete IFA may be performed when the INS
experiences a total shutdown. An in-flight alignment may be performed to complete a partial CV/GND
alignment. A gyro recovery may be performed when the INS completely shuts down and radar or FCC
air data is not available for a complete IFA, see figure 24-5.
24.2.5.4.1 Complete IFA. A complete IFA may be initiated after a total INS shutdown. During IFA,
the ADC must be available to provide magnetic heading information, and the radar must be capable
of providing continuous precision velocity update (CONT PVU) information. Once the INS has
shutdown, place the ATT select switch to STBY to verify the accuracy of HUD attitude data by cross
A1-F18EA-NFM-000
VII-24-15 ORIGINAL
Figure 24-4. INS Ground Align
A1-F18EA-NFM-000
VII-24-16 ORIGINAL
checking the standby instruments. Select NAV master mode and radar altitude to HUD. Then place
the INS mode select knob to OFF for 5 seconds.
Bring up the A/C data sublevel display on the HSI/DATA/(A/C) to check magnetic variation,
present position, and winds aloft. If A/C data is incorrect, refer to A/C Programming (this chapter), to
enter correct aircraft data. Fly straight and level, unaccelerated flight for 20 seconds, then place the
INS mode select knob to IFA. When IFA has been selected, the radar may be commanded to the PVU
mode and the alignment display appears on the HSI. If the PVU mode is initialized, CONT PVU and
SEA options will be boxed (default selection). Verify the proper radar PVU mode is selected (LAND
or SEA) and observe that the TIME display begins to increment. During the first 10 to 15 seconds of
the alignment while the INS platform is being leveled, NO ATT is displayed to the right of the QUAL
on the In-Flight Align display on the HSI. Wait for 20 seconds of align time for the platform to level,
then place the ATT select switch to AUTO or INS. NO ATT is then replaced with the quality number
(which is an estimate of present position accuracy) and the INS ATT caution clears. Selecting INS or
AUTO with the ATT select switch replaces standby attitude reference data with INS attitude data (the
waterline symbol on the HUD is replaced with a slowly flashing velocity vector which symbolizes a
degraded vertical velocity). When horizontal position becomes valid, the POS/ADC caution clears. The
velocity vector continues to slowly flash until vertical velocity becomes valid (at approximately align
quality of 5.0).
PVU mode may be overridden by deselecting the CONT PVU option but IFA quality will be
affected. Deselecting CONT PVU commands PVU for 10 seconds of each minute, alternating with the
last selected radar mode. When air-to-ground ranging (AGR) mode is selected (e.g., by HUD
designation) CONT PVU is deselected, 20 seconds of AGR is commanded, and PVU is commanded for
10 seconds of each minute alternating with AGR. In PVU mode, the radar provides Doppler velocities
for INS alignment. The radar look-down angles are optimized for land or sea return by selection of
LAND or SEA option at the bottom of the display. PVU is inhibited for IFA if the aircraft is not in the
NAV master mode.
If the radar is not operating or is inhibited from operating in PVU the time-in-alignment display
flashes and the CONT PVU option is not displayed because continuous PVU cannot be commanded.
Present position can be provided for the alignment by performing a position update using the UPDT
option or by entering the aircraft data on the UFCD using the DATA option. Another very good
technique is to select TACAN position keeping if a stationary TACAN is available. Velocity updates
cannot be performed during IFA. The VEL update option is displayed but returns to the In-Flight
Align display top level when selected.
24.2.5.4.2 If IFA GPS Indicated. Good satellite data for the IFA is indicated by IFA GPS on the HSI
format. If GPS acquires good satellite data, perform one gentle 90° S-turn (less than 20° AOB and
±10° pitch) to facilitate GPS IFA. Then maintain straight-and-level as much as practical. GPS IFA
　
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