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navigation modes will provide most of the outputs availablei
n the prbnsq ands econdarym odesb ut with considerably
degraded accuracy. Unlike the primary and
secondarym odes,t he availablen avigationp arametersa re
generatewd ithin the MCS, with the exceptiono f attitude
information, which is t7om the sensor selected. Best
available system velocity is used to generatep osition
information, and CADC inputs are used for vertical velocity.
In backup navigation modes, certain data entries
may be made and will be accepteda s describedb elow.
20.3.5.3.1 IMU Backup Mode. The IMU backup
navigation mode will be automatically selected when
both the primary (INS inertial) and the secondary
(SAHRS normal) modes are not available, but the INS
can still provide attitude and platform heading information
and some form of system velocity is available. It
may also be manually selected by positioning the NAV
MODE switch to A’IT with some form of system velocity
available. The OWN AK MFD format for this mode
(Figure 20-26) will show a boxed INS legend in the
upper right portion and an IMU or IMU/SAHRS legend
will appear in the center, depending on SAHRS availability.
If this mode was manually selected, an M will
appear next to the boxed INS legend as shown in Figure
20-26. For this mode, the system will compute a bestavailable
initial true heading. This parameter,h owever
may not be accuratea nd it may be necessaryt o enter,a t
least one time, some estimate ofthe aircraft true heading
via the DEU OWN A/C format.
20-39 ORIGINAL
NAVAIR 01.F14AAD.1
SAHRS NORMAL, SYSTEM VELOCITY
NAV SYSTEM AIDFigure
20-25. SAHRS Velocity Reference Selection
Note
Once ATT has been selected manually,
the INS will degrade from an inertial mode
to an attitude reference mode, and reversion
to viable INS mode may be difficult or
impossible.
20.3.5.3.2 SAHRS Backup Modes. A SAHRS
backupm odew ill bea utomaticallys electedw hent he INS
has failed or is not available, the SAHRS normal mode is
not available, and some form of system velocity is available.
The SAHRS backup modes include three submodes
based on SAHRS operating as an attitude reference system.
These include the following in the order ofpreference:
1. Slaved - Magnetic heading.
2. DG - Directional gyro.
3. EC - Emergency compass.
In the slaved and DG modes, the system will provide
typical navigation outputs to the displays and other systerns,
as long as some form of system velocity is available.
In addition to automatic selection, SAHRS submode
operation may be manually selected via the NAV
SYSTEM AID MFD format shown in Figure 20-27.
This is doneb y first selecting SAHRS by depressingth e
SAHRS pushbutton on the upper right portion of the
NAV SYSTEM AID MFD format and verifying that
SAHRS is boxed. A SAHRS submode may be selected
by depressing the SAHR MODE pushtile on the lower
let? of the same MFD format until the desired submode
is boxed.
pii-
Do not attempt this on deck. A weight-onwheels
interlock for the in-flight restart will
freeze the SAHRS in the restart mode until
weight off wheels. There will be no attitude
information available from the SAHRS
when this situation occurs.
ORIGINAL 20-40
NAVAIR Ol-Fl4AAD-1
Figure 20-26. IMU Backup Navigation Mode Selection
Note
Although it is possible to cycle through the
SLV, DG, EC submodes, reversion to
NORM requires an in-flight restart. Inflight
restart will automatically be initiated
when the selection pushbutton is depressed
to roll from EC to NORM or can be accomplished
by depressing the in-flight RST
pushbutton.
Considerable degradation in accuracy from the primary
and secondary and even the IMU backup modes
can be expected when the SLV and DG modes are selected.
The SAHRS EC submode will not provide navigation
parameters to the system although some air data
parametersw ill be available.
Selection of the slaved submode will result in magnetic
heading information derived from the magnetic
aximuth detector (flux valve) and true heading computed
fWm this source plus magnetic variation. Attitude
information is derived from SAT-IRS using first-order
leveling and, therefore, may be subject to certain dynamic
errors.
Selection of the DG submode will allow enuy of a
desired grid heading via the DEU, using the DEU NAV
AID-M; HDG format (Figure 20-26). This entered parameter
will be the initial heading reference until a new
DG heading entry is made. In the system, it is treated as
　
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