KLM MD-11 - Degraded Navigation; General; IRS Degraded

MD-11 Flight Crew Operations Manual
k
FMS -
Controls and Displays
FMS.30.109
Degraded Navigation
General
Due to inherent drift rates in the IRUs or noisy radio inputs, there are times when
the IRS position and the FMC computed radio position are different. This
condition may indicate faulty IRUs or radio inputs to FMC position computation.
When the radio position is different from the IRS position by more than 12 NM,
the following will occur automatically:
• If possible, different radios are selected and tuned and the new radio
position is compared to the IRS. If the difference is less than 12 NM no
further corrective action will occur.
• If the position difference remains 12 NM or more, the MCDU message
VERIFY A/C POSITION is displayed.
The pilot must now resolve the discrepancy between the two positions. If it is not
possible to select different navaids and resolve the discrepancy, the position
update mode changes from radio/inertial to inertial. The VERIFY A/C POSITION
MCDU message is automatically cleared if the difference between IRS and mean
radio position falls below 8 NM. The pilot may also clear the message.
IRS Degraded
Inertial position is normally computed in the FMC at a 5-per-second rate by
combining three independent IRU positions. Each IRU position is normally
compared to the previous position and after passing the comparison test is
combined into the new inertial position using a weighted average. For the
comparison test, errors more than 36 NM result in a failed test. If any IRU fails
the comparison test or is operated in other than NORMAL mode, each FMC will
revert to and use a single IRU for position inputs. The IRU failure and usage
combinations are as follows:
• If IRU1 fails, FMC1 uses IRU3 and FMC2 uses IRU2.
• If IRU2 fails, FMC1 uses IRU1 and FMC2 uses IRU3.
• If IRU3 fails, FMC1 uses IRU1 and FMC2 uses IRU2.
Inertial groundspeed is also computed in the FMC by a simple average after a
comparison test. If the comparison test is failed or any IRU is operated in other
than normal mode, the FMC receives a single groundspeed input per the previous
table.
NOTE: The SENSOR FAIL MCDU message is displayed for this
degraded operation. IRS failures can be found on the SENSOR
STATUS PAGE.
October 02, 2006