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):

LOWI: LOC R Rwy 26 approach chart

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

AIP 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:

LOWI ground chart
(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)?


2 Answers 2


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

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.

  • $\begingroup$ @Bianfable. Yes, I misread your comment. I'm not knowledgeable in SOPs, but I guess as long as they double check with DME/Altitude it's ok. $\endgroup$
    – mins
    Commented Aug 24, 2021 at 12:55
  • 1
    $\begingroup$ Can someone expand what GP stands for? $\endgroup$
    – TomMcW
    Commented Aug 24, 2021 at 18:19
  • 1
    $\begingroup$ @TomMcW GP is GlidePath, also called glide slope (G/S). $\endgroup$
    – Bianfable
    Commented Aug 24, 2021 at 18:51
  • $\begingroup$ GlideSlope (G/S) refers specifically to ILS style vertical guidance from an ILS GlideSlope transmitter. It will display the GS diamond on the vertical G/S scale and the Autopilot lateral mode can be coupled using LOC or ILS mode. GlidePath refers to a Final Approach path constructed by the FMC and flown using VNAV or equivalent FMC mode. An ILS can have a GlidePath which the Autopilot can couple using VNAV, but a GP can't always have a G/S. A point to notice is that the grey + white LOC symbol is not depicted on the 'plan' mode chart, and neither is the GS symbol used in the profile diagram. $\endgroup$
    – skipper44
    Commented Aug 24, 2021 at 22:43
  • 1
    $\begingroup$ @skipper44: I used GP because this is the wording of ICAO related to ILS in annex 10. GP and GS are used interchangeably to refer to the transmitters and beams used to create the guidance signals. Strictly, GS is the slope of the glidepath, when the GP is a straight segment. $\endgroup$
    – mins
    Commented Aug 24, 2021 at 22:59

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.)

  • $\begingroup$ Thanks for the additional info! I don't think the steepness is the reason though (London City has 5.5° and is classified as ILS). Autoland only applies to Cat II or III, so that should not restrict Cat I ILS either. About ILS 22R at JFK: is that only an offset course ILS (of which there are many in the world), or actually an antenna offset from the runway centerline (I cannot tell from the chart). $\endgroup$
    – Bianfable
    Commented Aug 25, 2021 at 19:50
  • $\begingroup$ I've listed factors that might prevent the approach being classified as an ILS, one, or more, or a combination could be limiting. Besides, there's no restriction preventing an autoland during CAT1 or even CAVOK conditions - not having an ILS approach removes an opportunity for something to go wrong. The offset course is achieved by having the transmitter antenna off centre line/off RWY. $\endgroup$
    – skipper44
    Commented Aug 26, 2021 at 10:59

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