DME Autotuning

[John H Watson]

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.

Cheers
JHW

[United744]

I think by "displayed", it means anything in channel 1 on the left side is shown as tuned, and anything in channel 4 on the right side is shown as tuned.

Ergo, anything else could be tuned, but not displayed on either RAD NAV page at 1L/R or on the ND.

I presume if nothing is tuned, it shows dashes.

Maybe the 12500 ft rule was removed or not implemented in the version in the photo? Also, the aircraft appears to be on the ground, so does this change the behavior?

[John H Watson]

Maybe the 12500 ft rule was removed or not implemented in the version in the photo? Also, the aircraft appears to be on the ground, so does this change the behavior?

Possibly.  I'm glad the pilots didn't notice and logged it. I wouldn't have known if it was right or wrong.

I think by "displayed", it means anything in channel 1 on the left side is shown as tuned, and anything in channel 4 on the right side is shown as tuned.

It seems odd to me that (on the POS REF page of the Captain's CDU) only one of a Left DME station pair is shown in the left station position and that in the right station position, the Right DME Channel 4 is shown (which isn't one of a primary pair)... Unless I'm misunderstanding the text. Does the Captain's ND display the primary pair or the 1 and 4 channel stations?

In the following situation, the stations TFE and SGC are shown on the ND as autotuned and on POS REF page.

http://www.members.iinet.net.au/~b744er@ozemail.com.au/PSX/Autotuning_Test_1.zip

If I inhibit SGC, only TFE is in green on the ND. On the POS REF page, this is shown:

DME DME
TFE  TFE

I don't understand this. If there is only one station, shouldn't the POS REF page show:

VOR-DME
TFE

Neither the Left or Right DME have a primary or a secondary tuning pair.

Screenshots

Thanks,
Cheers
JHW

[mgeiss]

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?

Note that it says "closest navaids" not "closest DME". So as I understand it, channels 1 and 2 will use the 2 available DME stations, while channels 3 and 4 could show just the closest VORs even if they don't have DME. It could also mean "the closest stations which aren't already used in channels 1 or 2"?

I think there might also be different situations where a VOR/DME station is...
1. technically close but not yet receivable (on the ground or at low altitudes or because of terrain obstacles)
2. VOR is received but no DME indication yet (far away from the VOR/DME)
3. VOR and DME received

[John H Watson]

So as I understand it, channels 1 and 2 will use the 2 available DME stations, while channels 3 and 4 could show just the closest VORs even if they don't have DME. It could also mean "the closest stations which aren't already used in channels 1 or 2"?

All good points, thanks.

As Lewis Carroll might say... "curiouser and curiouser" (or should I say muddier and muddier)

Cheers
JHW