(b)
After a short-term power interrupt, the radio corrections (velocity and position) sent to the IRS, will retain their values, previous to the power interrupt.
(c)
After a long term power interrupt the radio corrections to the IRS velocity are set to zero. The position radio corrections will be set to zero if the offside FMC position is invalid. If not, the position radio correction to the IRS will be an adjusted position in which the onside FMC position equals the offside FMC position. The softfault will be reset.
C. Position and Velocity Determination
(1) IRS position and velocity processing
(a)
The IRS provides the basic position and velocity which the FMC uses for navigation. These IRS outputs are used in conjunction with radio positioning and will be used alone if radio updating is not possible. Due to the importance of the IRS outputs, processing of these signals are quite extensive.
(b)
Each individual IRU will go through a signal validation process. This process will validate both the position and velocity signals. If either of these signals are not valid the corresponding IRU will not be used.
(c)
If any of the IRUs are invalid each FMC will choose only one IRU for position and velocity updating. The FMC will choose the closest IRU for source data. For example the L-FMC will choose either the left or center IRU depending upon their validity.
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(d)
If all three IRUs are valid then their positon and velocity will be mixed and averaged, respectively. The individual IRU positions and velocities will be compared to the mixed and averaged values. If the difference between the individual values and the mixed and averaged values differ by more than predetermined value, then a softfault failure will occur for the respective IRU.
(e)
If a softfault failure exists for an IRU, that IRU will not be used for navigational purposes by the FMC. All or none of the IRUs may be used for navigation by the FMC. The IRU's data will be routed in a manner corresponding to the number of failed IRUs as follows: 1) IRS (3) indicates all three IRUs are valid and each FMC
will use the mixed IRS position and the averaged velocity. In this case both FMCs will contain the same data.
2) IRS (2) indicates only two valid IRUs . In this case, the FMCs will use the closest IRU. The left FMC will use either the left IRU or center IRU. The right FMC will use either the right IRU or the center IRU. The FMCs will contain different data and the master FMCs data will be used for navigation.
3) IRS (1) indicates one valid IRU. In this case the valid IRU's data will be used by both FMCs for navigational purposes.
4) IRS (0) means there are no valid IRUs and the FMCs must revert to VHF NAV radio updating for navigational purposes.
(2) Radio position processing
(a)
The FMC selects the best VHF NAV radio positioning NAVMODE to be used. These NAVMODES, listed in order of priority, are ILS/DME, DME/DME and VOR/DME. The FMC bases its selection of these NAVMODES on the type of navaids available and the geometry of the navaids with respect to the airplane. The ILS/DME NAVMODE will only be used during airplane approach.
(b)
ILS/DME positioning is derived from calculating the airplane deviation from the ILS center beam as well as using the IRS estimated position. Both FMCs will use data from a selected ILS sensor to prevent divergence of the two navigation solutions. If one of the FMCs fail, the operating FMC will only use the onside ILS for positioning.
(c)
The DME/DME NAVMODE calculates airplane position by using the slant range distance from two DME stations. The DME has the capability of storing distance and frequency information for 252 separate channels. There are five high priority channels or memory slots which are used by the FMC. The FMC controls the allocation of DME data into these channels. The allocation will be as follows: 1) Channels 1 and 2 will be used for the best tuned DME/DME
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