9) The DME/DME pair is geometrically valid, the crossing angle between the two DMEs must not exceed more than 150 degrees or be less than 30 degrees. The optimal crossing angle is 90 degress (Fig. 32).
(b) The VOR is considered short-term valid if: 1) Raw data passes the I/O process validation. 2) The tuned frequency is equal to the received frequency. 3) The airplane is not within the zone of confusion (an
inverted cone with an apex of 60 degrees, or ground range
is less than 1.5 nm) for the respective ground station.
4) The airplane is flying away from the VOR station at a range no greater than 25 nm, or flying towards the station at a range no greater than 40 nm.
5) The noise amplitude on the VOR signal does not exceed 2.5 degrees.
(5) Long-term validation of both DMEs and VORs are the basis for autoselection and NAV MODE selection. If either become long-term invalid, they will not be selected as navigational candidates. If either become long-term valid, they will then again become candidates for navigation.
(a) Both the DMEs and the VORs will be declared long-term invalid if 50 percent or less of the last 20 data samples were considered short-term valid. If 70 percent or more of the last 20 data samples were short-term valid, then the DMEs or VORS are considered valid. The 4 sample dead band provides some hysteresis between selection of long-term valid and long-term invalid. Both the DME and VOR are initially long-term valid except when they are procedure specified as long-term invalid.
I. Polar Navigation
(1)
When the airplane is greater than 85 degrees north or south latitude, it is considered, by the FMC, to be in the polar region. While in the polar region, each FMC will automatically use only one individual IRU position instead of the mixed position of all three IRUs for airplane positioning .
(2)
The left FMC will select either the left or center IRU and the right FMC will select either the right or center IRU. The selection proccess will be based on the two IRUs having the least distance between them following a comparison of each IRU with the other two.
(3)
After reaching the polar region, each FMC's mixed IRU position will gradually change to equal the selected single IRU position. This gradual change will minimize airplane maneuvering and prevent any sudden movement of the ND's display.
(4)
If the FMCs used single IRU positions before the polar region was entered, they will continue to use the same IRU for position data. However, the FMC function which gradually makes equal the IRS position and the FMC position will automatically become active.
(5)
When exiting from the polar region (less than 83.5 degrees north or south) the FMCs will automatically resume using the mixed IRS position, if the mixed IRS position is considered valid. There will be no instantaneous change in either FMC position when returnig to the mixed IRS position.
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DME 2
DME/DME RADIO POSITIONING IS VALID WHEN THE CROSSING ANGLE IS LESS THAN 150` AND GREATER THAN 30`. DESIRED CROSSING ANGLE IS 90`
DME Slant Range Crossing Angle
Figure 32
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J. Navaid Blackballing and Inhibiting
(1)
Blackballing or inhibiting of navaids can be accomplished in several ways.
(a)
Automatically
(b)
Pilot entry
(c)
Through the navigational data base.
(d)
When a navaid is inhibited automatically through long-term validation criteria, this navaid cannot be used for radio updating in the autoselection mode. However, they can be tuned through manual, procedure or route tuning and can be used for radio updating when in one of these modes.
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