10
$\begingroup$

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?

Improve this question
$\endgroup$
12
  • $\begingroup$ 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. $\endgroup$ – Jan Hudec Jan 6 at 10:18
  • 1
    $\begingroup$ 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? $\endgroup$ – John K Jan 6 at 13:48
  • $\begingroup$ @JohnK Bingo. Would you care to add that as an actual answer? $\endgroup$ – expeditedescent Jan 6 at 14:47
  • 1
    $\begingroup$ 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. $\endgroup$ – Jamiec♦ Jan 6 at 16:25
  • $\begingroup$ @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? $\endgroup$ – Ralph J Jan 6 at 17:11
5
$\begingroup$

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.

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.