NO NAV INTERCEPT

[STLTH]

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?

[OPEN DES]

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’.

[STLTH]

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

[STLTH]

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.

[OPEN DES]

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!

[STLTH]

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