EPU in Airbus gone crazy
 

So flying along in my Airbus the other day with both FMS in IRS/DME/DME due GPS jamming. But picking up plenty of rad navs and position is updating nicely, both FMS have EPU under 0.5nm. All of a sudden FMS 1 goes crazy and EPU shoots up to 16nm in around 30 seconds. ND shows us 6nm left of track. Luckily AP2 is engaged so it's following the good side. Around 2 minutes later FMS1 settles down and goes back to 0.5nm.

Has anyone seen this before? Any ideas why this happens? A software bug or is there another explanation? Could a malfunctioning Navaid cause it?

A few years ago, in a F70 (non-GPS aircraft that ran on IRS/VORDME), there used to be a problem similar to what You describe. In the area of Ankara (LTAC), there always was a massive mapshift introduced into the system. This caused some scratching of heads and a checking of the nav database.

What was found was that a VOR slightly west of the field was physically not at the coordinates at which the database expected it to be. Whenever the systems did their calculations based on that VORDMEs signals, the results were consequently off by several miles. Thereafter, it became an obligatory part of flight preparations to deselect the VOR in the FMS, and henceforth, no such troubles were encountered. Whether this error was present in the Turkish AIP or somehow snuck in on the way to the FMSs database I do not remember.

That error affected both FMSs at the same time, not just one. But, coming back to Your flight, if one FMS happens to come across such a misplaced VOR on its "tour de VORs" while the other side receives other stations that do not suffer such an error, the described error may be the result.

Could You name the general area You were flying around in?

It was southern Iraq.

Yeah I was wondering, do the FMSs choose different VORs for radio updates?

Where did that happen?

We have had these issues in the recent past in Iraq and the reason was a rogue VOR/DME in Iran that wasn’t where it was charted, similar to Tu.114’s experiences.

It sounds like you experienced spoofing where one or more of your GPSs changes totally changes location. Only way to verify this is the difference between GPS coordinates and IRS coordinates. One way I fo7nd to make sure I was where I thought I was was to look at the coordinates of waypoint on the airways in Jepps and then when we fly over it, check coordinates of IRS 1 2 and 3. Then check coordinates of GPSs if you have any showing. It’s at times like this that you may get GPWS Terrain Terrain etc etc.

Normally, the multiple FMGCs do their own individual position calculations and, being separate units, may well use different VORDMEs for their calculations.

On the SELECTED NAVAIDS page, the MCDU will show what navaids are tuned by its onside FMGC at the momenent, so if one decided to have both MCDUs display said page for a while inflight, it would be possible to compare the two sides automatic selections.

Back then, it was the Baglum (BAG) VOR in Turkey that caused the mapshifts. Is the identifier of the troublesome Iranian station known as well?

Thank You for sharing this experience. I will definitely be more aware the VORs being tuned.

what airplane were you in ? Was it a biz jet by chance ? ….EPU must be a G6000 or 7500 ?

Read the title of the thread

my bad 😜😜😜😜 ….i thought I read Airbus somewhere.

If you have the ident or freq or anything related to that VOR it would probably be useful (and contribute to safety of flight) if you could provide it here... seeing as it does appear to be having a problematic effect on others...

I was about to ask if the clocks were synced OK. We've noticed this on other types that during GPS spoofing/jamming the clocks can be out by as much as 5 minutes if the GPS do not recover.

If the clocks are out by more than one second it can cause issues with ADSB etc per the FCOM.