It is also a further 'safety check' in that we are where we think we are.
Not a very dumb idea actually... The following is an extract from an Irish AIC taken from an AAIU serious incident report, a very "interesting" story if someone wants to read the full report:
http://www.aaiu.ie/upload/general/12...2010_012-0.PDF
"The use of a localiser outside these areas can lead to false course and reverse sense indications being received and such use should not be attempted." The AIC also states,
"Certain combinations of localiser beam characteristics and modern receiver/autopilot combinations can cause premature localiser capture; flight crews should be alert to this possibility. Flight deck procedures should be designed to reduce the risk of premature capture by not allowing Flight Director/Autopilot capture modes to be armed too early. Flight crews are advised to confirm the validity of ILS capture by cross-checking with other sources of navigational information when available."
A gross altimeter setting error can also be detected. Happened once to me, ATIS U/S and weather received from controller as 30.02 inHg. Read back (not objected to) as 30.22. Interestingly enough the DME which defined the crossing altitude fix was also U/S (ATIS sent out on same VOR/DME) and the fix was now a RADAR FIX. But ATC did not call out our position as we passed. I asked them for it but at the time they did we were already past descending on the G/S. The difference between .02 and .22 is about 200 ft. I guess VMC saved us that day. Since we had just passed over a mountain ridge, which tend to change the QNH significantly, both 30.02 and 30.22 seemed reasonable. Lesson learnt that the published crossing altitude can be very valuable. The holes in the Swiss cheese comes to mind again....