ATT mode. In the ATT mode, the computer is turned off
and the INS will only provide attitude signals.
Standby mode. The characteristics of the
standby mode vary, depending upon whether the INS is
being turned on or is being downmoded from NAV,
ALIGN, or ATT. During standby mode the alignment
state number is 9.
Standby mode (INS being turned on).
If the INS is being turned on by setting the mode selector
to STBY, the navigation unit is brought up to operating
temperature by fast warm-up heaters and the gyro wheels
are brought up to speed. The INS platform is caged to
the aircraft axes and all instrument warning lamps
controlled by the INS indicate warning.
downmoded). If the INS is being downmoded from a
higher mode by setting the mode selector switch to
STBY, the INS platform retains its alignment with local
horizontal, and all instrument flags controlled by the INS
except ATTITUDE and PLATFORM HEADING flags, will
coordinates, and TACAN station data
can be inserted into the INS while in
the standby mode.
Alignment mode. In the alignment (ALIGN)
mode the INS computer progresses through a series of
submodes called alignment states.
Alignment state 8. During alignment
state 8, the INS platform is aligned to local horizontal and
the INS battery is tested. Attitude warnings are removed
at the beginning of state 8, but all other instrument flags
con-trolled by the INS continue to indicate warnings. The
battery unit is tested during the first 12 seconds of this
state. The INS spends a minimum of 51 seconds in state
During the transition of state 8 to state 7, the heading
is set to the stored value. If this value compares
favorably with magnetic heading plus magnetic variation,
the status will remain unchanged. If the comparison is
unfavorable, the status will be set invalid, and the system
must be upmoded to NAV before a heading may be
Present position must be loaded
before the system will progress from
state 8 to state 7.
Alignment states 7 and 6. The
primary function of states 7 and 6 is to refine the
relationship between the INS platform and true north.
This function continues unchanged throughout both states
and 7. In state 6 the INS compares the latitude of the
loaded present position with the latitude it has computed
as part of the alignment process, and it uses the results of
this comparison (with other information) to update certain
self-calibration data. During these states it is possible to
enter the navigation mode if the stored heading is valid.
Entry to NAV will immediately follow the setting of the
mode selector to NAV. The alignment will be entirely
dependent upon the stored values regardless of the
length of time in states 7 or 6.
Alignment states 5 through 0. Entry
into alignment state 5 indicates that normal or standard
accuracy alignment is complete. The NAV mode may be
selected at any time after entry into this mode, but for
precision inertial accuracies, the alignment should be
allowed to progress until the READY NAV lamp is
illuminated, and preferably to alignment state 2 or 1. A
standard accuracy alignment takes about 20 minutes,
while a high accuracy alignment requires about 45
Alignment states 4 through 0 indicate continuing
operating of the self calibration process begun in
alignment states 6 and 5. In addition, automatic
calibration data for the azimuth gyro is now available. If
the INS is left in ALIGN, the self-calibration data will
continue to be refined and the alignment state will
decrease toward 0.
Navigation mode. In the navigation (NAV)
mode, all navigation data is computed and available for
display. All warning flags controlled by the INS will
disappear from view. The INS depends on its own inertial
instruments for all position and velocity data. The present
position computed by the INS can be updated by TACAN,
GPS, data link, or manually when the aircraft passes over
a known position reference.
Attitude mode. In the attitude (ATT) mode,
the computer is shut down and no navigation information
is available. The ATT mode is entered at the discretion of
the pilot if navigation and/or steering data signals become
unreliable. Attitude signals and platform heading outputs
continue to be available. The NAV mode can only be
reentered from the ATT mode after the system has been
realigned. Alignment can take place only on the ground.
steering provides a box or straight line pattern which may
be used to fly the aircraft in a specific pattern. The
pattern will originate at the current TO waypoint. The
length of the base leg, true bearing of the pattern, offset
distance of the return leg, and direction of the turns are
loaded into the computer via the ASE/ACS. Should the
operator specify a zero offset distance for the return leg,
the direction of the turns will alternate at each end of the