VOR/DME Autotuning logic

[JRBarrett 3,038]

I have noted on both the PMDG 777 and 747 V3, that when the number 1 and 2 NAV radios are set to auto tune, they always both tune to the same VOR/DME frequency.

I'm not trying to say "this is definitely wrong", as it may be correct on the 777 and later model 747s, but it was not always the case. 

On the 757, early 767 and early 747, GPS was not standard equipment. The IRS system was the primary source of lateral NAV guidance, and due to the tendency for the IRS position to drift with time, the NAV radio auto tune logic was set to select two different navaids whenever possible - with preference being given to DME/DME cross fixes to update the IRS position enroute.

On the older systems, priority was given to selecting two VOR/DMEs located at a sufficient angle to each other to give a true "cross" bearing - even choosing standalone terminal or ILS DME stations if no standard VOR/DME was in range.

If course, there are some locations,(oceanic, arctic or remote continental areas), where there simply may not be multiple navaids to choose from.

On modern aircraft, the GPS provides primary lateral navigation, (and IRS updating), with DME/DME updating as a fallback - so auto tuning separate VOR/DME stations may no longer be a priority.

Perhaps the estimable Mr. John Watson may have some insights regarding how more recent iterations of the 747-400 avionics handle auto tuning?

 

 

[downscc 1,327]

18 hours ago, JRBarrett said:

I have noted on both the PMDG 777 and 747 V3, that when the number 1 and 2 NAV radios are set to auto tune, they always both tune to the same VOR/DME frequency.

Last time this came up, which it does occasionally, the answer is that for positioning purposes the aircraft is not using two of the same navaid.  Have a look at POS REF pg 2/3 and you'll note that it has two different DME stations tuned for that purpose, regardless of the VORs tuned, and it is the DME DME information that provides the "highest quality" of radio positioning.

[JRBarrett 3,038]

36 minutes ago, downscc said:

Last time this came up, which it does occasionally, the answer is that for positioning purposes the aircraft is not using two of the same navaid.  Have a look at POS REF pg 2/3 and you'll note that it has two different DME stations tuned for that purpose, regardless of the VORs tuned, and it is the DME DME information that provides the "highest quality" of radio positioning.

I assume later model 747s have dual frequency DME transceivers (as does the 737NG).

Though the auto tuning is normally always running in the background - it is not actually used for IRS updating unless the GPS signal is lost.

[Captain_Al 66]

1 hour ago, JRBarrett said:

I assume later model 747s have dual frequency DME transceivers (as does the 737NG).

Though the auto tuning is normally always running in the background - it is not actually used for IRS updating unless the GPS signal is lost.

There are normally 5 frequencies that are being tuned in the background, 2 for the radios used on the NAV RAD page. These are usually either Auto Tuned (A), Route Tuned (R), or Procedure Tuned (P) automatically and of course you can Manually Tune (M). If you have a SID selected, you will probably have a P and/or A, or R. Logic determines what it wants to use. The other 2 frequencies are tuned for use on POS REF page 2 for radio updating if GPS is not available. The 5th is tuned as a backup or standby.

It is probably just your terminology, but the IRU's are never updated, it is the FMC Inertial position that is updated. Once you give the IRU's their position, they are on their own to wander, especially over a long period of time, though still highly accurate.

[JRBarrett 3,038]

14 minutes ago, Captain_Al said:

 

It is probably just your terminology, but the IRU's are never updated, it is the FMC Inertial position that is updated. Once you give the IRU's their position, they are on their own to wander, especially over a long period of time, though still highly accurate.

Yes. Even though they are only simulated, I do check the total drift of the 3 IRUs at the end of each flight. Usually one of the three is farther out than the other two.

I believe that outright IRU failures are modeled if one chooses to use the PMDG service or probability-based failure options.

 

 

[Captain_Al 66]

3 minutes ago, JRBarrett said:

Yes. Even though they are only simulated, I do check the total drift of the 3 IRUs at the end of each flight. Usually one of the three is farther out than the other two.

I believe that outright IRU failures are modeled if one chooses to use the PMDG service or probability-based failure options.

 

 

How do you check those Jim? We used to check them from the INIT/REF index, then MAINT, then IRS Monitor, but PMDG does not simulate that. Do you do it on one of the POS pages?

[JRBarrett 3,038]

6 minutes ago, Captain_Al said:

How do you check those Jim? We used to check them from the INIT/REF index, then MAINT, then IRS Monitor, but PMDG does not simulate that. Do you do it on one of the POS pages?

From the status menu of the pos init pages. (Second page I believe - I'm not at my FS computer). It shows the current calculated lat/lon of each IRU, but does not show as much info as I imagine you could see on the IRU monitor page.

A really high-workload IRS emulation for FS is the CIVA implementation of the old Delco Carousel 9-waypoint IRU of the 1970s. It has all the quirks of the real thing, including random failures, the necessity to  enter all waypoints as lat/lon coordinates AND the coordinates and elevations of all DME stations that would be used for enroute position updating. 

[Qavion2 39]

Quote

I assume later model 747s have dual frequency DME transceivers (as does the 737NG).

 

Ugh.... Autotuning of DMEs on the 744 is a crazy complex process and even though I've read all the available notes and manuals on this many times over the last 20 years, I won't even pretend to know what happens  I've also spent hours experimenting with the real aircraft on the ground trying to make sense of it. I don't think even big sims seem to match what I've seen at my local airport.

 

3 hours ago, Captain_Al said:

There are normally 5 frequencies that are being tuned in the background, 2 for the radios used on the NAV RAD page. These are usually either Auto Tuned (A), Route Tuned (R), or Procedure Tuned (P) automatically and of course you can Manually Tune (M).

Captain Al, what do you mean by "the radios used on the NAV RAD page"?

My 747-400 books say (paraphrased):

Channel 1 and 2: A primary DME pair are autotuned by the FMC

Channel 3 and 4: Normally reserved for Procedure, Route and Manual tuning (but can be used for "regular" autotuning). A position comparison is made by the FMC between the primary pair and with Channel 3 & 4 position calculations.

Channel 5: For ILS-DME

Normally, the Left FMC uses the Left DME Interrogator information, but will use the Right DME Interrogator if the Left DME fails. Similarly, the Right FMC uses the Right DME information. However, the Master FMC does the tuning of the Interrogators.

The DME interrogators constantly scan 252 frequencies for (within-range) DME stations and compute the distance to these stations. The FMC uses this information to select the best stations for for position updating (based on the stations' relative positions and distances).

So what is shown on the Nav Rad page, the POS REF page 2/3, the ND (in the lower corners) and on the display itself? There are complex flow charts in the Maintenance Manual which are supposed to tell you what is "displayed" on the CDU, but they don't say where it is displayed.

e.g. If there are two DME pairs available, "channel 1 will be displayed" on the CDU. A footnote says that the flow chart applies only to the Left DME and VOR. The Right DME and VOR operate the same, except Channel 4 will contain the displayed information.

During autotuning (not the other types), does the Left CDU POS REF page 2/3 show the two primary stations tuned by the Left FMC/Left DME? e.g. if the display shows SFO SJC (is this only Left DME channel info?)

Below 12500', according to one FCOM supplement, the closest stations are chosen (and geometry is ignored), but at my local airport, on the ground, in the real world, the nearest station was ignored (and I had to pull circuit breakers and inhibit navaids to force the FMC to use the closest station despite the station coming in loud and clear).

Note that below 12,500', according to some documents, the DME tuning selection process changes. The FMC simply tunes the closest pair of DME navaids.  I couldn't replicate this on the ground at my local station, however.

Here's a photo of the Right CDU Nav Rad page in Sydney:

Autotune Strangeness

The Left VOR is tuned to a VOR-DME station (out of range), the Right VOR is tuned to a TACAN station (out of range) and no Route has been entered. Neither represent the closest station "SY" (which used to be a VOR-DME station). I recall BIK and RIC were shown on the ND in green.

Thanks

Cheers

JHW

 

 

 

 

 

 

 

[Qavion2 39]

P.S. Sorry... too much editing.. I repeated myself a few times

[Captain_Al 66]

Yes, I mean exactly what you looked up John regarding the 5 channels.

Boy, that is a lot of detailed information there.

When it gets too deep for us pilots, we usually revert to standard pilot phraseology like, "it's magic" or "it's system logic" meaning it "works great, lasts a long time", which is another pilot saying.

I kind of put this information into that category since this is happening all behind the scenes and most of the time we just need to know how it operationally works and not the nitty gritty details, because for most pilots, their heads would explode with trying to cope with TMIO, too much information overload and go into brain dump mode   ; )  

[Captain_Al 66]

19 hours ago, JRBarrett said:

From the status menu of the pos init pages. (Second page I believe - I'm not at my FS computer). It shows the current calculated lat/lon of each IRU, but does not show as much info as I imagine you could see on the IRU monitor page.

A really high-workload IRS emulation for FS is the CIVA implementation of the old Delco Carousel 9-waypoint IRU of the 1970s. It has all the quirks of the real thing, including random failures, the necessity to  enter all waypoints as lat/lon coordinates AND the coordinates and elevations of all DME stations that would be used for enroute position updating. 

Yes, if I remember correctly the IRS Monitor page only gave you the L, C, and R drift errors in nm/hour and we used to record those in the logbook after flight, so it might show like .2, .3 and .1 miles for the L, C, and R IRU's, maintenance would track these numbers...