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attempt to cover every combination of messages possible for the
particular mix of sensors installed on an aircraft, included in the
following paragraphs are the factors and general thought processes
which should be considered in an assessment of the situation.
The FMS integrates information from all available navigation sensors
in order to derive a "Best Computed Position." Because each type
of Nav sensor has different characteristics affecting their ability to
provide accurate latitude/longitude information, the FMS computer
must assign a priority to each sensor for use in the position
calculations. A conservative approach has been used for automatic
sensor deselection to eliminate errant sensors; however, the pilot
must still monitor sensor input and evaluate sensor deselection or
the need for such deselection. While every effort is made to make
these priorities in the smartest way, certain conventions are made.
Messages concerning navigation sensor inaccuracies are especially
noteworthy on over water flights where DME-DME position is not
available to determine which sensor is inaccurate. Proper action by
the crew in manually deselecting the faulty sensor and possibly
updating the FMS is necessary to assure accurate tracking. This
discussion deals with how to determine which sensor is faulty.
A sensor watchdog gives position difference warnings with FMS
messages for OSS, GPS and LCS sensors and for a second FMS in
dual installations, at 6, 12, 18, and 24 NM from the FMS Best
Computed Position. For IRSs, the sensor watchdog compares the
rate of change of the IRS position against that of the FMS Best
Computed Position. A decision on the part of the pilot to update or to
deselect a sensor must take into account the position error involved,
the error growth rate, and the stations being received.
For highly divergent sensors, based on the average position of all
the other sensor positions, an individual sensor will be automatically
deselected if that sensor's position differs from the average position
by more than 50 NM, or if the sensor position is changing from that
position at a rate exceeding 50 kts.
Section V
Operations
5-132 2423sv602
Change 1
Always check the latitude/longitude position of all sensors whenever
any navigation sensor's accuracy message becomes active. Action
should be taken only after the following examination.
Factors to consider include:
• Which sensor(s) was more accurate upon leaving DME
coverage?
• Are the coordinates of more than one sensor in agreement?
• How long has the IRS(s) been in NAV?
• What is the Q of each sensor?
• How many stations are the OSS or LCS working with?
• Which sensor has done better in the past?
• How does the navigation information compare with the flight
plan?
Two examples of actual situations which could arise, and a scenario
for solution follows;
• Loran C is assigned second priority, contingent on its having a
good quality factor and being inside the LOA (Loran Operating
Area), or outside an LOA after flying there from inside an LOA
with continuous signal reception.
• Omega/VLF is assigned third priority, contingent again on its
quality factor.
• The IRSs provide smoothing of velocity calculations when
available. With a single IRS, the position derived from inertial
inputs is assigned fourth priority for long term positioning, but
a higher priority for short term position keeping. It is normal
for the long term IRS position accuracy to drift as a function of
time. Inputs from dual and triple IRSs are given more weight;
thus the potential error induced by other sensors is minimized
due to extremely accurate IRS outputs from multiple IRSs.
Section V
Operations
2423sv602 5-133
Change 1
Example 1: While flying in a non-DME environment in an aircraft
equipped with three IRSs and one OSS, the following messages all
become active simultaneously:
IRS 1 DIFF WARNING
IRS 2 DIFF WARNING
IRS 3 DIFF WARNING
The possibility of three like sensors all drifting off simultaneously is
extremely remote. It is more likely in this instance that the single
OSS sensor is, at least temporarily, contaminating the position data
from sensors which are lower in priority. It is important to note that
the only sensor, which does not have an active message pertaining
to its accuracy, is the suspected "bad apple."
The proper course of action in this case would begin by looking at
the latitude/longitude positions of the three IRSs, on the FMS
POSITION page (DATA Page 3) to determine if they are close to
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