"The aim of this site is discussion, and not to apportion blame."
I agree 100%.
If you are looking at your workstation (radar scope or Nav Display map), and it shows you need to make an input to correct an action, it is tempting to do so without referring to back-up data.
I look at the FMS as my primary source of navigation nearly 100% of the time, backing up my localizer intercepts with raw data. Then one time the FMS shows me going through the localizer before the autopilot intercepts the localzer, and with the raw data localizer needle still offscale. Is the needle stuck? Do I have a gage problem? Is there an autopilot/autoflight irregularity?
Maybe, but the FMS display of the localizer course is not a true localizer display, but rather a course generated on a map based on FMS position (which is subject to error on rare occasions). Everything I normally look at (with the exception of the raw data localizer info) says I am going through the loc.
If the jet's FMS map showed a pilot was blowing through the localizer, he could:
1) turn inbound and then verify with raw data if the Map was in error (could be due to map shift or IRS drift);
2) hold the heading and risk going way beyond the localizer course (if there was no map error) while checking raw data;
3) always keep an eye on raw data to prevent conflicting data.
This is a reminder for me to keep up with raw data, and to share this event with others when the subject comes up.