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Pilotdude3407

INS only navigation question

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So I am simulating a non-GPS aircraft by failing the left and right GPS's and disabling GPS position updates.  I have some funny things happening with the tracking of LNAV.  With the radios in autotune, on the ND it just shows INS(3).  When I go to either FMS and select POS INIT 2/3, then the NDs show DD(dme/dme) and INS(3) below it and the aircraft seems to be about 5nm off course and the aircraft turns to correct that.  As soon as the FMC page leaves POS INIT 2/3 (either 1/3, 3/3, or another page altogether), the DD leaves the ND and the aircraft turns back to the original course.  When the GPS is operating, GPS is always displayed on the ND.  Is this a bug or does the real aircraft actually act like this?  The manuals don't seem to speak to this in too much detail other than confirming DD,GPS, or nothing on the ND depending on what the primary update source is.  Ideas?

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Do you mean IRS(3)?

It sounds like it might be normal. If you fail both GPS receivers, you revert to the next higher level, which would be DME DME (DD). It will do this provided it can find 2 stations that are suitable. The distance and geometry have to be right to use this.

If it cannot find 2 stations, then it will revert to VOR Radial/DME off of 1 navaid (VD). But, it has to be pretty close to the station to do this. If it can't find that, it will use the FMC inertial position, which is the 3 IRS's, combined into 1 position via the average weighing, or the triple mix position.

The difference between the FMC Inertial position and any updated radio position is called the bias and that bias or difference is maintained throughout the flight. If you lose DD, the jet will keep the bias that existed prior to losing the radio update. Once it gets updating back, it will re-establish a new bias and you may see the map shift. Map shifts were common before GPS days, so it sounds like this could be normal.

For example, when we left the US for Japan, at some point you lose radio updating in a non-GPS jet. The bias is maintained all the way across the water with the triple mix position. When you pick up radio updates again as you approach Japan, the map would shift and the jet would correct because the position was updated and a new bias was created.

Many times in a non GPS jet, when capturing the LOC, it will update off that, and you would notice the map change position or shift as a result of the update to a more accurate position.

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Yes, IRS.

I am not debating that map shifts are possible based on whether or not it has a radio source vs the IRS, but I am saying that I don't think that it is required to have one of your FMC's on POS INIT 2/3 for it to happen.  It will only show DD IRS(3) when I am on that page.  When I go away from that page, it reverts back to just IRS(3).  I think this would be an automated feature.  That's my issue.

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You'd better submit a ticket. Looks like a legit bug to me.

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6 hours ago, Pilotdude3407 said:

When I go to either FMS and select POS INIT 2/3, then the NDs show DD(dme/dme) and INS(3) below it and the aircraft seems to be about 5nm off course and the aircraft turns to correct that.  As soon as the FMC page leaves POS INIT 2/3 (either 1/3, 3/3, or another page altogether), the DD leaves the ND and the aircraft turns back to the original course.

Does the GPS annunciation return when you leave the POS pages?

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1 hour ago, Qavion2 said:

Does the GPS annunciation return when you leave the POS pages?

 Yes, it stays up whenever the GPS is enabled.

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Only the PURGE RADIO function should return the radio updating to normal (after two minutes), not the INHIBIT> GPS.

As Fabo (Peter) says, it's probably best if you submit a ticket.

Cheers

JHW

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On 3/30/2017 at 0:07 PM, Pilotdude3407 said:

Yes, IRS.

I am not debating that map shifts are possible based on whether or not it has a radio source vs the IRS, but I am saying that I don't think that it is required to have one of your FMC's on POS INIT 2/3 for it to happen.  It will only show DD IRS(3) when I am on that page.  When I go away from that page, it reverts back to just IRS(3).  I think this would be an automated feature.  That's my issue.

Sorry, I misunderstood the problem of staying on the page vs when you leave it and it reverts to IRS(3), but sounds like you decided on a course of action based on it doesn't make sense...

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