Map shift and ILS approaches. Classic 737
All EFIS classic drivers flying into a remote strip with a u/s VOR but serviceable ILS know the FMS updates it’s “lateral” position once on the localiser. The map shift can be a few miles.
Once upon a time….. we had a false localiser capture a full three miles off the centreline. I picked it up because
1. It happened sooner than I expected (after manoeuvring around weather tru the localiser and back towards it expecting to become visual at 3500 AGL.)
2. The track that was 212 instead of the required 222 and the NDB showed us off track as well
3. On our request, Radar confirmed the three miles right of centreline.
As we began to “go raw data” on the NDB to correct we became visual, the FO corrected visually whilst I had a close look at the EFIS.
Except for the giveaways above, all indications on the EFIS (on 10 mile scale) were that we were on centreline with the runway ahead. Only a very close look showed the white track line on 212 missing the runway
I selected IRS 1 on the position page and entered, the map adjusted, putting the runway where it actually was, to the left. As I watched the map updated itself on the false localiser and put the runway dead ahead again!.
As we approached the real centreline, the localiser needle twitched, went full scale and then centred as we intercepted the real centreline. At this stage the “map” runway was now off to the right. A few seconds later the FMS updated itself on this new localiser and put the map in front again.
I recount this little tale to explain why I always check all available raw data before and whilst using the wonderful map mode.
If at any time the localiser pointer moves off the ADI expanded scale I immediately select VOR/ILS on the EHSI.
I now fly both classic and the NG which of course has GPS and map shift does not happen on the NG.