NO NAV INTERCEPT
 

So, we were diverting around weather the other night and set an offset of 30Nm right from our planned route through the FMGC in our A320. After inserting the offset the PF used HDG mode to cut through the middle of our planned route and the set offset and armed NAV later on. We got a NO NAV INTCPT message on the scratchpad, knew what it meant but ignored it. At a later point we noticed the lateral deviation from our planned route was greater than 30Nm yet we were still inside the two lines of the offset and planned route. At one point we thought we had deviated more than 30Nm but looking at the ND pictorial that wasn’t the case. What also made it more confusing is that even when turned left to re intercept planned route the lateral displacement was still counting up? Any ideas or knowledge as to what happened here?

When you enter an OFFSET and subsequently you fly in HDG, the flightplan will not automatically sequence, unless you pass the wpt within 5NM. Before arming NAV check your ‘TO-waypoint’.

Once in heading mode with the offset already inserted in the box, clear the first waypoint created by the offset, then manage nav. No nav intercept problem sorted. It's a quick way to fix the the 5nm issue mentioned by the previous poster.

If you still need the waypoint info that you deleted, create it in the fix info page with abeam function.

So the To Waypoint not sequencing was the reason for the excessive indicated lateral displacement?

Was it a GPS equipped plane? Was the incorrect offset displayed in both ND's? Nav accuracy high on both sides? Possible map shift? ADIRS aligned or within limits before takeoff? Can be many things that cause that, you would have to troubleshoot a bit if it happens again.

Yes it was GPS equipped, the correct offset was displayed on both ND’s, ADIRS were within limits but not sure about the NAV Accuracy, didn’t check that. We deleted the offset and re inserted it and that fixed the issue.

That's interesting, I have had it before when one ND display does not match the other, eg. One showing a cross track error but the other is correct. Maybe even a possible coding error in the FM database, computer Voodoo!

The excessive xtk error was most likely caused by the non-sequencing of the the flightplan. I.e. XTK error based on a flightplan leg well behind you!

Thanks for your input guys, if it happens again, I shall update you all.