DME multiple stations scan

[Xavier]

Hi,

Another question here that arised after going through Aerowinx and Boeing manuals :
From what I understand, the 744 is equipped with 2 DME boxes, but each one of them is able to, at the same time, scan and then follow 5 different stations.

Channels 1 & 2 are said to be autotuned for FMC updating and 3 & 4 dedicated notably for manual tuning.
Furthermore, Boeing's FCOM stipulates that "Manual DME tuning does not inhibit FMC DME tuning".

Does this mean that if I select manually 2 DME stations that are not optimum for FMC position updating (or even not reachable at the time) the system will be able to track other stations more relevant for its updating while still searching for the ones I manually entered ?

I try in the sim to manually set 2 out-of-range stations ; when I checked page POS REF 2/3 it shows no updating stations. If I enter reachable stations manually, the Nav Stations line always shows those stations (or no station) even if they are not located ideally for pos update, but never another station (except ILS-DME, ie channel 5, when relevant).
What did I misunderstood here ?

Thanks in advance, enjoy your flights !

Xav

[John H Watson]

Does this mean that if I select manually 2 DME stations that are not optimum for FMC position updating (or even not reachable at the time) the system will be able to track other stations more relevant for its updating while still searching for the ones I manually entered ?

I believe you are correct. On the 767 PIP/Pegasus FMC, manual tuning of both VORs basically forced the FMC to attempt to use the manually tuned stations. On the 744 & 777, freescanning mode is always available.

I assume the non-manually tuned stations you were using met the updating criteria? VOR-DME range must be less than 40 when flying towards a station (and 25 when flying away), but not over the cone of confusion. DME-DME range must be between 9 and 300nm, but require specific angles between the two DME stations. On later versions 1~9nm was also accepted, but the reduced accuracy was factored into the suitability equation.

Rgds
JHW

[Xavier]

Hi John,

Well that confirms what I had understood from the manuals. But the stations selection criterias are really interesting to know, thanks !

As for the tests I made I had chosen VOR/DME stations more than 1000NM away so sadly I do think there might be a little logic mistake in PSX right now. Surprising though as Hardy stipulated precisely the 5 independant channels in the Aerowinx manual very clearly. Please note that the 5th channel seems to already work independantly (one is able in the sim to get DME distance from 2 VOR/DME + 2 ILS)...

Xav

[Hardy Heinlin]

Hi Xav,

I'll investigate this display function (3R) a bit closer for the next update.


Regards,

|-|ardy

[Xavier]

Hi Hardy,

Many thanks for your dedication ! And sorry for harassing you with such "complaints" every times... but you gave us such a perfect tool to dig with pleasure deep into the systems logics that we can't help but to try everything. Furthermore I must admit that I'm at the moment quite reluctant to manually tune VORs as I'm afraid it may downgrade my FMC updating to more or less GPS only (and thus lower ANP).

Btw reading your answer I tend to understand that you mean that the FMC is already managing its DME stations for pos update independantly from manual tuned stations, but that the issue is only with the 3R on POS REF 2 indication ?

Merry Xmas to all the 744 enthusiasts on the forum !

Xavier

[Hardy Heinlin]

GPS is the best you can get.

Watch the FMC radio update status on the ND (see page 303). Your manually tuned VORs may be out of range, but the FMC radio update status on the ND may still indicate DD. To get rid of that, pull both DME CBs on P7.

To get rid of GPS updating, select the GPS INHIB> key on the CDU.

Then purge the bias. And only then you will use pure IRS updating. Check the POS stars on the ND: when mixed IRS updating is no longer possible, the left FMC will move to IRU L, and FMC R to IRU R.

(Set the captain's ND to FMC L, F/O's ND to FMC R, using each pilot's source select panel.)


Cheers,

|-|ardy