Why is the LOC RWY 26 approach at Innsbruck classified as LOC instead of ILS or IGS?

Question

After reading about Localiser Type Directional Aid (LDA) and Instrument Guidance System (IGS) on SKYbrary, my understanding of classifying these approaches is this:

Course Offset	With GP	Without GP
up to 5°:	ILS approach	LOC approach
more than 5°:	IGS approach	LDA approach

^{(In the US (TERPS), the limit is only 3° course offset, but ICAO Doc 8168 allows up to 5°)}

However, the LOC R Rwy 26 approach at Innsbruck (LOWI) is classified as LOC, even though it has a GP (and a course offset of 3°). AFAIK, the approach chart does not indicate at all that a GP is available (correct me, if I'm wrong):

But when digging into the AIP, one finds the GP information:

^{(Austrian eAIP: LOWI)}

The GP frequency of 331.7 MHz is paired to the LOC frequency of 111.1 MHz (example source). So this LOC approach has a paired GP. Then, why is it not classified as ILS?

I suspect that it might have something to do with the LOC antenna location, which is not on the runway centerline:

^{(Austrian eAIP: LOWI ground chart)}

However, if this offset no longer satisfies the conditions for an ILS, then shouldn't the approach be classified as IGS instead (because it has a GP)?

Answer 1

Having transmitters for lateral and vertical guidance doesn't mean they satisfy the required tolerances for an ILS/IGS, as defined in ICAO Annex 10, volume 1 (3.1.3 and 3.1.5). An example can be found in: What are the reasons for autopilot restrictions on instrument approaches?

In the case of LOWI, the likely reason for the glidepath (GP) navaid not meeting the standard is the approach path goes down in the steep-sided Inn valley:

^{LOWI airport, seen from West, source Shutterstock}

This might be due to reflection on the mountains around (there are also other suggestions like temperature and snow affecting the radio signal).

The GP guidance is thus provided only for the purpose of monitoring, but it can't be used to follow the glidepath with the autopilot. Pilots must use the DME and the altimeter. Extract from AIP:

A localizer (OEV 111,10 MHZ, LOC course 255° MAG) on the aerodrome but 3,5° offset from the runway centre line and a collocated DME are providing course guidance and distance information during the instrument phase of the descent procedure and in case of a missed approach. A glidepath which is frequency-paired with LOC OEV is available coinciding with the DME descent gradient of 3,77° between D-19 OEV and the threshold.

Answer 2

There could be a few reasons for no ILS Approach being published despite the existence of GP:

• steep GP of 3.77deg
• GP not calibrated/'calibratable'.
• NO operational benefit (eg ILS minimums worse than LOC/VOR/RNAV minimums due terrain for different obstacle surface). VOCL in India had an ILS 28 DA of around 900ft compared to VOR MDA of 700 odd ft. This is no longer true, I'm not sure of the technicalities that allowed the change, but for many years it was.
• An ILS can result in an autoland and this is taboo for an offset LOC. An approach flown in IAN/VNAV (RNAV/VOR/LOC) has no autoland capability and would alert/warn the pilot very clearly if the Autopilot was still engaged as the airplane approached the RW threshold.

(Note that there do exist ILS approaches with offset-LOC, KJFK ILS 22R comes to mind.)