FMC VOR Auto/Route Tuning Behavior

[cavaricooper]

Peter, Jon and others Typed in the 744-

I am currently engaged in conversation about the 744 and FMC auto-tuning behavior.  My position is (and I believe confirmed by the behavior exhibited in PSX) that the FMC selects various VORs appropriate for the routing and then selects them based on angle and distance.  I believe that the FMC tries to select two DIFFERENT VORs for use/display, in much the same way as manual operation does.  I am told that this is not the case and the FMCs frequently/almost always select and display the SAME VOR on both the left and right radio.

My feeble brain, having had the rudimentary concepts of triangulation drummed into it long ago, refuses to accept this "position".  Could you please comment based on your observations/experience?

Additionally, John W. if you have any documentation describing the actual VOR selection process I would be most obliged.

Best- C

[G-CIVA]

Carl,

I am told ...  the FMCs frequently/almost always select and display the SAME VOR on both the left and right radio.

I bet you $5 I know just the people who are telling you.

Have a read of this ... from the MX Training Manual of a current operator of the B744 in both the PAX & F variant in the Asia Pacifc region ... it might shed some light on the subject ....

[Hardy Heinlin]

Also note that there is a pair on the ND and another pair on the FMC's POS REF 2/3 page at 3R.

For example, if you load this situation file ...

03 Approach 025 - New York Kennedy - ILS31L via CHANT.situ

... you will see JFK and CRI on the ND (JFK being P-tuned and CRI autotuned), and you will see COL and CRI on the POS REF page, with DME DME in the line title; those are two DMEs used for FMC position updating. DME is prefered over VOR. And for the DME method you always need at least two different DMEs.

It could be that P tuning should set both VOR L and R radios to the same procedural VOR so that both pilots automatically get the same VOR on their onside EFIS HSI* display (the display mode between APP and MAP). But I haven't seen any documentation on that particular detail yet. Anyway, there's always just one recommended navaid stored in a procedure leg.

* The left seat HSI is linked with VOR L, the right seat with VOR R. -- On the other hand, if you want to use the HSI, you also want to enter an HSI course for the CDI reference. This is only possible when dashes are displayed in line 2 on the NAV RAD page. Dashes are only displayed in manual tuning mode. So, disregard that point.


|-|ardy

[G-CIVA]

Just to add to what Hardy has explained here is a tabulated format of how the process works out the priorities ....

[John H Watson]

Additionally, John W. if you have any documentation describing the actual VOR selection process I would be most obliged.

I gave up trying to understand autotuning a long time ago    I've seen all kinds of combinations on the Nav Rad page on the ground (I have no experience in the air).

For example (on the Nav Rad page):

Procedure Tune + Route Tune
Procedure + Manual Tune
A manually tuned Tacan station (which is DME-only for civilian aircraft) producing a Nav Radio VOR display of, for example "110.7MRIC". This was entered using the ident "RIC".
Manual  + blank

Behind the scenes, it gets very complicated.

I am told that this is not the case and the FMCs frequently/almost always select and display the SAME VOR on both the left and right radio.

I've never seen this even when only one VOR-DME station has been in range.

[Britjet]

Carl,

The FMC selects all kinds of different VORs. En-route in particular they are usually different. As Jon says it us difficult to predict. I think your source is talking bo**ox.
Peter.

[cavaricooper]

Gentlemen-

I appreciate the input greatly!  I have unsuccessfully argued this issue for YEARS now.  It helps to know I'm not imagining things.  Having yet to set foot on our beloved's Flight Deck- I can only rely on you all for verification.  This place IS MY ORACLE AT DELPHI  .

MOST APPRECIATED!

Ta- C

[Hardy Heinlin]

What is the source of those with the opposite opinion?

The final truth may be a composition of multiple (arti-)facts.

[Dirk Schepmann]

I think the PMDG guys might have a different opinion on this. 

The PMDG 747-400 is always tuning VOR L and VOR R to the same VOR. That's one of the first things I noticed when I did my first flight with the PMDG 747-400. In real life and also in PSX the frequent "sweep" of the VOR needles in different directions is quite characteristic on the 747-400 and can also be clearly seen in different videos.

The DME pair is correctly tuning 2 different DMEs, though.

I'm no longer pointing out these things on the PMDG forum, they tend to be quite harsh and arrogant in their replies.

Best regards.
Dirk

[G-CIVA]

Dirk,

To be honest, when I decided to reply I did not know the answer either ... I had a fair idea ... but I went searching amongst the material I am lucky enough to have access to - because I was interested in finding out the answer too.  I always learn something new when I stop by here.

That is the great thing about this small community ... information is shared & regardless of who you are.

In other 'places' information is 'power'.

'They' who run said places have clearly stated unless 'you' are a 'unicorn' (& in that case YOU must prove your credentials to them) 'they' will then take from you information behind closed doors in private.

'They' will not accept information from mere 'mortals' from open sources since they might get made to look silly on occasion.

[Hardy Heinlin]

Anyway, ...

... the above quoted Boeing document (34-61-00) clearly states:

"When autotuning mode is active, the function does a best pair tuning of VORs."

This is supported by countless real-deck observations. My own eyes have seen it too.

If there's any higher evidence of the opposite or of undiscovered special conditions, I'll have, as usual, no problem to accept it and to think about modifications. Good aviators have no problem to admit misunderstandings. Admitting a mistake is not silly. Hiding a mistake is silly.


Regards,

|-|ardy

[John H Watson]

I am told that this is not the case and the FMCs frequently/almost always select and display the SAME VOR on both the left and right radio.

A single VOR with a co-located DME as a navigation reference has the lowest priority of all the navigation solutions. Why would the FMC want to choose this? A VOR without a co-located DME cannot be used as a solution (Even two VORs without DMEs are not used as a solution).

Perhaps we need to determine when these same VOR stations are being displayed. e.g. If you are flying to a remote island where there is only one VOR-DME station. Will both VOR radios be tuned to the same VOR-DME station? Are these stations being autotuned, route tuned or procedure tuned?

For example, I have entered a Departure routine (I recall) for London, Heathrow with the aircraft on the ground in Sydney, and the stations displayed on the Nav Rad page were for London (even though these Procedure/Route-Tuned stations could not possibly be used). In some circumstances, I think the FMC would rather tune stations it cannot use than tune the same local stations.

[cagarini]

Thinking about it I should consider it's given by some auto-tuning priority.

.) If no other sources are available, stations will be picked based on signal quality and adequation for positioning;
.) With all sources available, the criteria could be the procedure being flown and the closeness of those stations, so, the example given above by John is something I would never expect to happen .... but it does, so, my theory starts wrong from the very beginning;

I fetched the FAA Navigation manuals and am starting to read across it - it's a long time since I last devoted any time to this kind of technical stuff ( I would say since PS1, which was the first flight simulator that really put me to ( try to ) learn stuff...

[John H Watson]

Here's a (somewhat blurry) extract from the Maintenance Manual showing that same stations are possible*:

Autotuning Logic


See below...

[cagarini]

Thx John !

[Hardy Heinlin]

Isn't this 34-61-00 flow diagram just about DME channels? The red box says "VOR and DME" though. But the flow diagram just talks about DME channel management. No word about VOR tuning management.

When the system has only two useable DME channels due to failures or lack of stations, it still uses two different DME stations. The VOR and DME indication on the EFIS on one side is always co-located; a certain frequency or morse ID always refers to the same VOR and DME. So the EFIS can't display one particular VOR station on both sides while the left and right DMEs refer to different DME stations. And when more than two DME channels are available, which is normally the case, there is even more variety possible.

When the left display uses channel 1, and the right display uses channel 4, as indicated in the text, can't we assume that channel 1 (odd number) is different to channel 4 (even number)?
I mean, if the "3 & 4" pair is a copy of the "1 & 2" pair, I assume 1=3 and 2=4, not 1=4 and 2=3.


|-|ardy

[John H Watson]

Isn't this 34-61-00 flow diagram just about DME channels?

The text in the boxes does include a reference to (CDU) "displayed frequencies" (either DME channel 1 or 4). Since the Nav Rad page doesn't show actual (UHF) DME frequencies*, then what is shown is the co-located or paired (VHF) VOR frequencies. DME and VOR frequencies are paired (even if the station is a DME-only station). Manually tuning a VOR station will lock in a specific DME paired frequency (even if the VOR station is VOR-only and there is no usable DME information).

I think I misunderstood the expression "simultaneous displays". I earlier thought they meant identical displays.  I wonder, though, if there are some unusual situations (due to say a temporary power loss) where one DME interrogator might power up later than the other with the aircraft in a different position... and the tuning algorithm begin at a different time causing a different solution... which may lead to identical displayed VOR frequencies (from channels 1 and 4).

I'm having problems understanding the flow chart. Does the statement "only one M, P, R" refer to the FMC's decision to use only one? Or there is only one of these stations available?

When the left display uses channel 1, and the right display uses channel 4, as indicated in the text, can't we assume that channel 1 (odd number) is different to channel 4 (even number)?

Sounds logical.

This stuff gets complicated because I've read that the stations being used for navigation are not always the stations shown. e.g. A manually tuned VOR-DME station will not stop the FMC using two other separate DMEs.

*DME frequencies are in the  960Mhz - 1215 MHz band. VOR frequencies are in the 108~117.95MHz frequency band.

[Hardy Heinlin]

I just meant to say that if channels 1 and 4 are different, which is usually the case, and if 1 and 4 are displayed on the ND, then 1 and 4 cannot be co-located with the same VOR. No VOR station in the world has two co-located DMEs.

Does the statement "only one M, P, R" refer to the FMC's decision to use only one?

My interpretation: Of the two radios on the NAV RAD page, just one is M, P, or R tuned. M is the pilot's decision, P and R are the FMC's decision. I guess it doesn't matter whose decision it is.


|-|ardy

[John H Watson]

I just meant to say that if channels 1 and 4 are different, which is usually the case,

I keep forgetting that there is only one FMC doing the tuning, so you would think the FMC would pick different stations.

This closest station logic is a confusion factor, though...

Procedure and Autotuned Stations

[Hardy Heinlin]

... is the Right DME providing the data for the right slot?

34-61-00 says "there can be two simultaneous displays on the CDU, one from the left side and one from the right." The chart refers to the left DME and VOR. "The right DME and VOR operate the same except channel 4 will contain the displayed information".

So, yes, the CDU shows two things simultaneously: Channel 3 stuff and channel 4 stuff. It does not show just one channel, i.e. not just the left system which is described on the chart. The right system, for which there is no chart, is also displayed on the CDU -- through channel 4.

If there is just one DME pair, channel 3 gets DME AAA, and channel 4 gets DME BBB.

AAA and BBB are simultaneously displayed.

There may be a language barrier, but for me the word "simultaneous" doesn't mean the same as "identical".

Two simultaneous displays are not necessarily two identical displays.

"Simultaneous" refers to time. "Identical" refers to content.


|-|ardy