MD-11 Flight Crew Operations Manual
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FMS -
Controls and Displays
FMS.30.110
When two IRU platforms are operated in other than normal modes the FMCs both
use the remaining IRS or position and groundspeed inputs. In the enroute flight
phase the message IRS ONLY NAVIGATION is displayed when the position
update mode has been inertial only for 10 minutes. The message automatically
clears when a radio update is obtained. When in transition to descent or approach,
the message is immediately displayed when the FMC goes IRS ONLY
NAVIGATION. The loss of all IRU inputs to the FMC will result in RADIO
ONLY NAVIGATION being displayed on the MCDU if it can be calculated. This
message is automatically cleared when the FMC transitions out of RADIO ONLY
mode.
Radio Degraded
Radio navigation is accomplished by either DME/DME or DME/bearing
calculations of aircraft position. Position errors of up to.35 NM for DME/DME
and a calculated error based once-per-second confidence check is accomplished
on new radio data to verify radio position data. Radio deviation outside the
allowable range declares the radio position invalid and reverts navigation to
inertial, or in its absence, to no navigation. The message IRS ONLY
NAVIGATION will be displayed on the MCDU. No navigation is declared two
minutes after a radio and inertial position calculation cannot be made. During the
two minutes before no navigation is declared, the FMC position on the IRS
STATUS page does not change. If no navigation is declared or a radio position is
recalculated, an FMC position update is required before FMC position can be
established.
CAUTION: No navigation entry into this condition indicates a
complete failure to engage in any navigation mode.
While airborne, the aircraft position is considered valid for the first two minutes,
and during this time, a resumption of radio data and/or the return of IRU data
would cause the engagement of a navigation mode consistent with sensor
availability. After two minutes the aircraft position is considered invalid and at
least one valid IRU position is required to terminate the no navigation condition.
In the no navigation condition, the FMC position on the IRS STATUS page
becomes dashed and MCDU scratchpad message A/C POSITION INVALID is
displayed.
October 02, 2006
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