FMC radio update purge

[Hardy Heinlin]

Good morning,

has anyone of the 7?7 pilots on the forum ever used the PURGE feature on the POS REF 2/3 page?

I understand it inhibits radio updating and replaces the FMC position by the mixed IRS position (if GPS is not used). It stays in this mode only momentarily or until both CDUs have no POS page on the screen.

I wonder how long "momentarily" is. Some seconds? Some minutes?

Also: Will the ND directly jump to the IRS reference or is it the usual incremental washout movement? I guess in this case it will jump.


Thank you!


Cheers,

|-|ardy



 
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[Jeroen D]

Hi Hardy,
check Bill Bulfer's FMC user guide, chapter 4.7

The way I read it is that GPS updating will be inhibited for 2 minutes.

Jeroen

[Jeroen D]

Having said that I just realised that the Bulfer FMC user guide is 10 years old. Not sure what modern FMC's do.
Still nice little book. I also have the official Honyewell Boeing 747-400 Flight Management System Pilot's guide. (1996 version). That hardly mentions the purge function.

I like the Bulfer guide much better.

Jeroen

[Hardy Heinlin]

Hi Jeroen,

page 4.7 is PIP. (And the 2 minutes refer to GPS, I think.)

747 is on page 4.9. No details there either.


Cheers,

|-|ardy

[Mariano]

Hi,

Not a Boeing guy, but maybe my 'grain of sand contribution" can help a bit. I was taught that the main reason for having the "GPS NAV INHIBIT" (or "GPS DESEL" on our airplanes) is mainly in case you fly inside a non-WGS84 country. If you leave the GPS NAV uninhibited and decide to do an LNAV approach (or a non-precision approach flown through LNAV), you may suddenly encounter some Culmuls Garnite or Durtus Startus. If you are just transiting the country through the airways, you will end up off course at some point (the amount of deviation of course depends on how much that country's datum differs from WGS84's datum.) In non-WGS84 countries, you need the FMC to be "anchored" to those countries' ground-based navigation beacons through DME-DME, LOC-DME, or VOR-DME updating. Since GPS conforms to WGS84 standards, its position and the ground-base-derived position won't match. The FMC will tend to always update its position first through GPS (if available or not inhibited.)

To make a long story short, it is my opinion (and experience with non-Honeywell FMSs) that the GPS should stay inhibited for as long as the pilot desires.

Thanks for yet another fantastic screen-shot, Hardy!

Mariano

[Richard McDonald Woods]

Hi H,

For what it's worth, the PMDG 744X FMC User's Manual states:

"Pressing the
Is it true to say that GPS and ILS lat/lon will only differ in PSX when:

- ILS drift occurs
- the instructor sets a different GPS position after ILS alignment?

[Hardy Heinlin]

Hi all,

GPS inhibit is something else, and NAVAID INHIBIT is something else, too.

On the 744, after radio purge is confirmed, the FMC position runs momentarily without radio updating; it automatically resumes radio updating when a valid radio is detected. Problem: If no navaid has been manually inhibited on the INHIBIT page, how does the FMC know what's valid and invalid? I assume there's a simple time delay in the 744 software, similar to the GPS inhibit delay in the PIP software. Perhaps also 2 minutes.

GPS and IRS differ when IRS drifts (PSX and real life). GPS is normally within 100 meters. IRS may drift more than 30 NM away and will then be ignored by the FMC. On the PSX Instructor page there are some malfunctions for GPS dropouts (random and persistent) and for excessive IRS drift.


Cheers,

|-|ardy

[stekeller]

OK, This is from Tim Gleason (ex 744 driver) again...

"I'm not sure exactly what would happen.  JAL feared that button so much that they would rather ditch in the North Atlantic than push the button.    We were never allowed to push it in the sim because it messed with the sim programming somehow.   I assume that the reference position would "jump" to the IRS position, but I have no idea."

Not that helpful I guess.

- Stefan Keller
KORD

[Hardy Heinlin]

Nevertheless, highly interesting, Stefan :-) Thanks!

It makes the purge button even more mysterious (in an entertaining way) :-)

Mariano, the datum code (e.g. WGS84) of each navaid position is stored in the ARINC 424 nav database, hence it's also carried over to the 744 FMC nav database, I assume. So the FMC is in most cases probably aware of what geometric calculation is to be used. -- GPS, on the other hand, seems independent of that. The GPS inhibit button on your aircraft type is certainly the same as the GPS inhibit key on the 744, on the opposite side of the radio purge key; and the 744, too, may certainly encounter the WGS84 problem you mentioned (that's why it has the GPS inhibit key. I think it's even an ICAO standard together with of all that RNP/ANP stuff).


Cheers,

|-|ardy

[Jeroen D]

Slightly off topic to the start of this topic, but it triggered another question for me.
I'm aware of the concepts of mapdatums, WGS84 and how that works with GPS.

And I understand that if you're going to touch down in a non WGS84 country you're going to have issues if your GPS is working based on WGS84.

How does it work if you're just flying over a non WGS84 county, following a particular airway. Are airways also datum dependent?

By the way, many marine GPS receivers can handle multiple datums, as marine charts come with different datums. Atlhough these days most is WGS84, you should always check and set the GPS accordingly or risk running aground.

Jeroen

[Mariano]

Jeroen,

If the airways are ground-based, yes, then I believe you would indeed not be following them with GPS un-inhibited on a non-WGS84 country. We are told to be careful when flying near (bordering) a non-WGS84 country or even before you enter one. You should be vigilant because at even as far as 299NM your FMS may pick up and try to update its position with a VOR-DME inside that country. If so, you could "deselect" that navaid from the solution until you enter that country. Of course, this only applies if your GPS receivers are not working (very rare) or in case you loose RAIM (after which we are taught to downgrade to navaid updating until RAIM is re-established.)

Mariano

[ray]

As far as I know, the enroute section from each country needs to be in WGS84 or 'equivalent'.  Some AIPs still publish their own datums in the airport section.  GIS databases can transform these into the WGS coordinates but not many data suppliers will do that to protect their own interests.  So you get a mix.

ie: Russia, uses the PZ90, which in their statement, says 'practically identical to WGS84'. So if u use navaids along the airways for posn updating, i think that shud be ok, unless u get a company order not to do so.  In that case, GPS is god-send.

ray

[Hardy Heinlin]

I think the correction to WGS84 datum could be done when the binary FMC database is being processed from the ARINC 424 database (which has the datum code for each navaid). The conversion is done by the database supplier or by the airline. Perhaps some database engineers correct it, some don't.


|-|

[ray]

Hi All,

revisiting this topic ... By the way Happy new year to ALL.
Okay, There are RNAV procedures that do not allow radio updates from any ground aid to mess around with its GPS/IRS position,  In this case .. is there a page to inhibit ALL dme/dme?

Tks, Ray

[Hardy Heinlin]

Hi Ray,

as far as I know, the combination GPS + localizer has the highest priority. If we assume that there is no localizer available on a GPS approach, the GPS alone is automatically used. DME/DME updating is only used when GPS is unavailable. There is no GPS/DME combo, only GPS/LOC. So with GPS active, there's no need to manually inhibit the DME/DME updating, I think.


Cheers,

|-|ardy

[ray]

Hardy,

Tks for info.  I was reading some RNP APCH, RNP AR procedures and so came the questions.  I beliv the manufacturers are required to clearly state the setup in their systems with regards to this position updates, but of course the aircraft mentioned is to be approved for such procedures.  Thanks

Ray

[Arnout van Maanen]

Found this below.
I think related to the last 2 messages of the subject

Regards,

Arnout

The FMC position updating using IRS and navigation sensor positions occurs
in the following priority order:
• LOC and GPS
• LOC and DME-DME
• LOC and collocated VOR/DME
• IRS and LOC
• IRS and GPS
• IRS and DME-DME
• IRS and collocated VOR/DME
• IRS

read the below as a table ( ||| separates the collums)

Primary FMC Position Update Source |||POS REF page 2/3|||ND Annunciation
LOC, GPS valid* ||| LOC-GPS ||| LOC GPS
LOC, DME DME valid;GPS invalid*||| LOC-RADIO ||| LOC DD
LOC, VOR DME valid; GPS invalid* |||LOC-RADIO ||| LOC VD
LOC valid; GPS, DME, VOR invalid* |||LOC|||LOC
GPS valid, LOC invalid |||GPS L, GPS R |||GPS
DME valid; GPS invalid|||RADIO|||DD
VOR DME valid; GPS invalid|||RADIO|||VD
GPS, VOR, DME invalid|||INERTIAL|||IRS(X)

* The FMC changes to LOC updating when:
• The tuned localizer is associated with the destination runway
• The airplane is less than 6,000 feet above the localizer navaid
elevation
• The airplane is less than 20 nm from the localizer navaid for a
front course approach or less than 12 nm for a back course
approach
• The airplane is within a 25° sector of the inbound localizer course
• The difference between airplane track and the localizer course is
less than a 45° intercept angle

[ray]

Thanks for the references guys..

Ray

[Michael Stanley]

Good Evening Gentleman,

I know this is coming a fare while after Hardy asked the initial  FMC purge button question, and reading through the numerous replies I'm still not sure if the question has been definitely answered, none the less, a number of very  interesting 'tit bits' have been gleaned over the course of this discussion. In any case, when I put this same 'mysterious' purge button question to my resident (he rents one of my houses) 744ERF captain ; His answer (following in quotes " ") though not quite definitive, describes anecdotally a rather interesting use of this feather."
       
"As far as I know, "momentarily" will be the time enough to erase the radio information received until that time and search and tune new radios.
One of the Lufthansa instructors told me that they use to do that when flying over the former USSR. The radio information received was soooooo bad that the pilots used this function when leaving the border into European airspace and put the FMCs back on track.

"Every time  I have seen a position update in-flight, the ND has "jumped", and the autopilot LNAV function will make the corrections."


Michael