MD-11 Flight Crew Operations Manual
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FMS -
Description and Operation
FMS.10.4
Functions
Navigation
Position and velocity data from the GNS and the Inertial Reference Units (IRU)
are combined with range and bearing data from VOR/DME stations to form an
accurate and stable estimate of the aircraft position and groundspeed. The primary
mode of operation is to combine range from two DME stations, and position and
groundspeed data from a weighted average of the 2 GNSs and the three IRUs. If
two available navaids do not form an angle between 30 degrees and 150 degrees,
range and bearing from a single VOR/DME may be used with the GNS and IRU
data. As the aircraft progresses along the route, the FMC uses current estimate of
the aircraft position and the stored navaids in the database to tune the VOR/DME
receivers to the stations that will yield the most accurate estimate of position.
The database contains data on the class and figure of merit of the available
navaids. The class of a navaid is defined as VOR, DME, VOR/DME, VORTAC
or LOC. The figure of merit (reception quality) is based primarily on usable range
of the station relative to the aircraft.
Each FMC independently computes the FMC position as a weighted average of
all three IRUs and GNSs. Below 78 degrees latitude, if any latitude or longitude
data from one IRU differs from the previous average by 36 NM or more, that IRU
will not be used in the averaging process until the output of that IRU is within 36
NM of the previous average. More complex but similar calculations occur above
78 degrees latitude. The defective IRU will also be eliminated if north/south or
east/west velocities differ more than 20 KTS. When only two valid IRUs are
available, each FMC will use one valid IRU for its independent calculation of the
aircraft position.
Performance
The FMS performance modes optimize the aircraft's vertical profile. Those
performance or speed modes include economy (ECON), POLICY, EDIT, MAX
climb (CLB), MAX endurance (END), and MAX descent (DES). Speed targets
associated with these modes are:
ECON - The ECON climb, cruise, and descent phase speed/mach targets are
calculated to obtain the minimum operating cost per mile traveled enroute, based
on the entered cost index. Some factors considered in these calculations include
cruise FL, GW, temperature, and current or predicted winds.
POLICY - Same as the ECON speed mode, except with a crew-selected (CAS)
(intended to suit a specific airline descent policy).
EDIT - Pilot entered CAS/MACH for CLB, cruise (CRZ), and DES phases of
flight subject to flight envelope limits.
October 02, 2006
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