In my idle retirement hours (days, weeks, months), I decided to try to figure out how the FMC chooses and displays the DME (and VOR) stations on the CDU and Nav displays during autotuning. Every time I opened a manual, I seemed to find more and more things I didn't know about (or had forgotten).
The 747-400 Left and Right DME Interrogators (transceivers) scan through 252 stations constantly (even when the transceivers are manually tuned) and measure the distances to the stations which reply. This data is sent to the FMCs and the FMCs decide which stations to use. Normally the stations are chosen based on distance and geometry, but I just read that below 12,500', the tuned stations are the closest ones (That doesn't seem to agree with my real world tests, but... ).
Normally, data from the Left DME is sent to the Left FMC computer, and data from the Right DME is sent to the Right FMC computer. i.e. unless the DME breaks. However, the selected Master FMC (Left or Right) tunes both DME's to the stations it thinks are the best. 5 stations are chosen for the purpose of position updating. These stations are assigned to 5 "channels" in each DME Interrogator.
Channels 1 and 2: These are known as the primary pair and are reserved for "Autotuning" (i.e. not manual tuning, Route tuning or Procedure tuning)
Channels 3 & 4: These are for manually tuned, routed tuned or procedure tuned stations... but if there are none of these, these channels will be for a secondary pair of autotuned stations.
Channel 5: This is reserved for an ILS-DME station.
Here's a flowchart I'm trying to understand...Flow Chart
If there are only 2 DME stations in range, and there is no Route, Procedure or manual tuning in progress, according to the chart....
"The stations which create the valid DME/DME pair will have their data stored in Channels 1 & 2 for position updating. Channel 3 and 4 will contain the closest navaid's distance and frequency. Channel 1 will be displayed
However, as there are no other stations, will Channel 3 and 4 be tuned to the same stations as 1 and 2 or will they just be empty?
Where will Channel 1 be displayed? On the ND as a green station? On the Left CDU on POS REF page 2/3? (i.e the first of the two stations displayed there?).
If Channel 1's station is a DME-only station (no VOR), will the Left VOR slot on the NAV RAD page be blank or will the FMC be free to tune a VOR station ((non co-located)
? How will this look on the ND?
What is happening on the Right CDU? The footnote on the flowchart says the Right DME and VOR are operating the same way, but "Channel 4 will contain the displayed information". Again, what do they mean by displayed information? Does this mean that Channel 4 of the Right DME will be displayed on both CDU's or only on the Left CDU (NAV RAD or POS REF 2/3)? Is the station pair shown on the POS REF page 2/3 actually data from the Left and Right DME data or data from just one DME?
This is the supposedly "easy stuff".... I can't wait to get into modes other than autotuning
Here's a photo of an aircraft not
obeying the 12,500' rule which says the closest stations are tuned..Autotune Strangeness
The closest station is SY (112.1). BIK is a VOR-DME. RIC (on the ND) is a TACAN. RIC is closer than BIK. Even though the SY data was good, the FMC chose two out-of-range stations. Note that no route was programmed.