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7)
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747-400 MAINTENANCE MANUAL
b) the tuning word sign/status matrix indicates an invalid condition, or
c) the word parity is incorrect.
In FREE SCAN mode, the DME interrogator cycles through all
but the UHF paired DME channels. There are 200 channels
within this frequency range of 108.00 MHz and 117.95 MHz.
The DME provides distance information for all stations that
are within this range. The DME can independently scans all
available channels which form a foreground loop and a
background loop.
a) The foreground loop consists of five stations, any or all of which can be designated by the FMC. If less than five stations are designated, the FMC fills these empty slot(s) with the closest DME paired station(s). The foreground loop stations are given priority service over the background loop stations. The display of the foreground stations are identical to that of the DIRECTED SCAN in that if a foreground station is not selected for display, the DME will generate no computed data and the DME will generate NCD BCD distance word for that station and provide normal frequency and bnr distance words with non-foreground stations selected.
b) The remaining 195 stations make up the background loop. The DME interrogator scans the frequency band searching for stations that respond with 450 squitter pp/s or more. These stations are then interrogated, distance information is calculated and information is sent to the FMC. Distance information is up-dated every 2 seconds.
The five foreground channels in FREE SCAN are restricted in
use by the FMC as follows:
a) Channels 1 and 2 are reserved for the best DME-DME pair available and is used in radio updating for the FMC. The best pair criteria is based on the geometric relationship between the airplane and DME station. The criteria indicates that lines drawn from each station to the airplane intersect at an angle between 30 and 150 degrees.
b) Channels 3 and 4 are reserved for navaids mannually (M) entered on by the CDU. These channels are also used in autoselected route tuned stations (R), autoselected procedure tuned stations (P), or in autoselected stations (A) that are not M, R, or P. whenever manually selected stations have not be chosen.
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747-400
MAINTENANCE MANUAL
c) Channel 5 is reserved exclusively for an ILS/DME or MLS/DME paired station. The channel normally becomes active whenever the airplane is within 25 NM (direct distance) of the runway or within 10 NM of an approach leg. The FMC will autotune to a station for this channel only when the airplane is 40 NM or less from the runway. The FMC parks this channel whenever the above criteria is not met.
8) The system operates in the MEMORY mode when station reply pulses are lost. The airplane range is then extrapolated from the most recent range data stored in memory. This occurs for approximately 10 seconds or, until sufficient signal strength is regained.
9) The DME enters the FAULT mode whenever it detects a fault. DME monitoring circuits continuously check the system and alert the flight crew if a fault has been detected by blanking DME displays.
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