"Unexplained Discrepancy" ILS

[jpilotj]

Gday,

I was wondering if someone knew the tolerance value in the ILS altimeter checks over a marker that are referred to be in the below paragraph from the AIP.

7.3 Altimeter Checks
7.3.1 The final approach segment contains a fix at which the glide path/
altimeter relationship should be verified. If the check indicates an unexplained discrepancy, the ILS approach should be discon- tinned.

I have heard it is 70 feet. Is this correct?

Cheers

[BOAC]

I believe the check is there to determine a false GP and I cannot remember the angular spacing of the false lobes, but I suspect it would show a lot more than 70' at around 1000'. 70' !!! You want precision??

[Tmbstory]

Altimeter Checks

jpilotj
BOAC

Here are three pages from the Australian AIP of a Supplement H13/92 about Altimeter Accuracy Checks. It was published a long time ago, however it may help.

Tmb






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[Sygyzy]

This may help

Errrr...No....RTFQ.

[BOAC]

A little time to Giggle and the answer is that false GP lobes (ooh nurse!) are at multiples of GP angle, so at least 5 degrees, which would put you at around 7-800 ft high on a 2.5 and around 1000' high on a 3 degree, and this is one of the things I believe we are checking for - not 70'. It is also a further 'safety check' in that we are where we think we are.

[172_driver]

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....