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According to EASA the preferred Glide Path Angle is 3°: EASA Easy Access Rules - Page 22 - 4.1.2.1.

There are some airports (e.g. Larnaca 2.75°) which are using a lower Glide Path Angle than 3°. Using a steeper angle (e.g. London City 5.5°) makes sense due to obstacle clearance. But what's the reason for using a flatter angle than recommended?

Pondlife
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  • I thought it may be to deconflict the approach with another, but there are no other airports around Larnaca, so at least in that case it won't be the reason. – Jan Hudec Jan 06 '21 at 10:18
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    Shallow glideslopes were usually to accommodate military aircraft that needed to have higher thrust settings on approach and so benefit from the shallower path. Does Larnaca see significant military traffic? – John K Jan 06 '21 at 13:48
  • @JohnK Bingo. Would you care to add that as an actual answer? – 60levelchange Jan 06 '21 at 14:47
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    Larnaca has never had much military traffic. It was used by the British as an airbase but long before it became an international airport (and I suspect long before "high thrust" aircraft). Unless John's theoretical answer can be corroborated, I don't think it makes a very good one. – Jamiec Jan 06 '21 at 16:25
  • @JohnK That line of reasoning breaks down at KTUL, which has a 2.75 degree GS angle landing south, but a standard 3.0 degree GS landing north. Neither is a recent change, as far as I can remember. Most military bases have standard 3-degree ILS approaches; shouldn't we expect to see a number of hold-over shallower approaches at those, if that's who originally needed them? – Ralph J Jan 06 '21 at 17:11
  • @expeditedescent I don't have any source info; it's only a dim recollection from the past. I found some commentary on PPrune that supports it, but not sufficient for an answer. – John K Jan 06 '21 at 17:12
  • @Jamiec The bit about military using shallow slopes was an artifact of earlier slow-to-spool engines that could be a problem if the glide path requires you to approach closer to idle. – John K Jan 06 '21 at 17:15
  • @JanHudec Akrotiri just along the coast to the west with conflicting runways – Arkhem Jan 06 '21 at 19:22
  • @Arkhem. Akrotiri isn't ‘just’ along the coast, it is over 30 nautical miles, and at the point the extended centrelines cross, Akrotiri is much closer, so the approaches would be at different height there if the point wasn't moot because glideslope does not extend over 10 miles anyway. – Jan Hudec Jan 06 '21 at 21:27
  • @Arkhem, before I wrote the first comment, I did pull out skyvector and did check the distances (the parallels are drawn 30' apart, so they are 30 nm apart—that's how you get scale on maps that don't preserve scale globally, as navigation maps often don't, and the reason why nautical miles are used). – Jan Hudec Jan 06 '21 at 21:34
  • @JanHudec I hadn’t checked the actual distance. However I often fly into Akrotiri and when the winds are light or N/S ATC has to deconflict the traffic and I believe the holds etc are non-standard. – Arkhem Jan 06 '21 at 21:52
  • @Arkhem, do they deconflict it on final though? The complete arrival procedures are much longer than 30 nm and surely some combinations of them conflict, but it shouldn't be on final where the glideslope angle matters. – Jan Hudec Jan 06 '21 at 21:58

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I won't pretend that I understand the physics of this, but one possible explanation is in FAA Order 6750.16E - Siting Criteria for Instrument Landing Systems. Section 3 discusses terrain effects on the ILS signal, and it notes that approaches that are partially over water may have signal quality issues. One way to address that is using a lower glide angle, per this section on page 3-4 (emphasis mine):

Where the Fresnel zone is smooth but not consistent, as where the approach path is partially over water, a change may be encountered in the received signal as the coefficient of reflection changes abruptly. This effect is unavoidable; however, it can be minimized by establishing the lowest possible glide angle (and, therefore, the lowest possible grazing angle) and/or locating the glide slope antennas within the specified criteria, so that the change in the received signal does not occur within a critical part of the approach.

I don't know exactly which procedure you're looking at, but Larnaca is a coastal airport and it looks like approaches to 04/22 are at least partially over water.

I have no idea if the same issue occurs with other types of terrain. Someone mentioned in comments that the ILS at KTUL (Tulsa, Oklahoma) has a 2.75° glideslope landing south, but 3° landing north. KTUL isn't on the coast and there are no large bodies of water directly north of it. Maybe the same signal issue occurs there for different reasons, or maybe there's a completely different reason.

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