ICAO Perf Engineer Question

[Boeingdriver999]

Hi all,

Just looking at some plates and I notice that LEPA VOR 24R has an IF at 9.2nm/2,300ft. However it also has a FAF at 4.2nm/1,600ft. The issue is that the IF does not give a 5.8% profile as suggested by the profile - only the FAF point does. So in this age of NOT diving and driving; why is that IF at that particular location?

Many thanks,

BD
https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/995x1420/screen_shot_2023_01_23_at_8_13_13_pm_74023eca1b7c88ed315dd0f 152246afe9f86cf4e.png
999

[KingAir1978]

Not a performance engineer, but wouldn't this be related to obstacle clearance between the IF and FAF? There's nothing that prevents you from overflying the IF at a higher altitude to fly a 5.8% profile.

[TeeS]

Hi Boeing
All 2d approaches are designed as dive and drive, that doesn't stop the actual approach being flown at a constant descent angle. Where there is a descent on the intermediate approach it must be designed so that the altitude can be lost using a gradient of less than 5.2% and also provide a level segment of at least 1.5NM (Cat C and D) or 1NM (Cat A and B) prior to the FAF to allow an aircraft to decelerate and carry out configuration changes.
Hope that helps.
TeeS

[Boeingdriver999]

Ok thanks guys. I'm just surprised at how it's visually represented. I can see how pilot's would be led to believe that they should commence descent at 9.2nm. It seems like a trap that could be designed out of the plate? Other approaches are represented better visually so why not LEPA?

BD

[TeeS]

It does look as if you need to roll inverted and pull once passing 9.2NM but I have a feeling they were just trying to get the descent drawn without getting in the way of the text reading 'R-058 JOA', with the excuse being the statement of 'Not to Scale' :)

Cheers
TeeS

[AerocatS2A]

Old school dive and driving would have you diving and driving after the FAF as well as before. These days the charts will show a CDA from the FAF at the latest. Interestingly (and mildly infuriatingly) the A320 still flies level segments at various places prior to the FAF depending on the coding of the approach.

[vilas]

Dive and drive term denotes descent from FAF to minima. In this particular approach there's altitude restriction at 9.2D which is the IF, after that it is unrestricted descent to FAF altitude with a level segment to configure the aircraft. From FAF it's 5.8% or 3.3° FPA CDA to minimum. I don't see any problem with the chart.

[john_tullamarine]

Just as a side FYI, this work has naught to do with Performance Engineers, rather it is a procedures designer problem.

[TeeS]

Dive and drive term denotes descent from FAF to minima. In this particular approach there's altitude restriction at 9.2D which is the IF, after that it is unrestricted descent to FAF altitude with a level segment to configure the aircraft. From FAF it's 5.8% or 3.3° FPA CDA to minimum. I don't see any problem with the chart.

Hi Vilas
It is not an entirely unrestricted descent to FAF, the flown descent gradient must not be more than 15%.
Cheers
TeeS

[FlightDetent]

BoeingDriver These 4-5 NM FAFs with preceding IF or other stepped altitudes make a complete mockery of the whole CDFA rule. Your confusion is understandable.
a) vilas is correct (but that does not fix the problem)
b) as Aerocat complains the vertical guidance by FMS (depends on the coding standard, a bit) is a mess and is not much help if you try to fly it like a reasonable person from about 9 NM/ 3000 ft.
c) why the IF restriction is so high is unclear, probably compensation for FAF being too close.

Check Constanta and Varna (BG) RNAV approach charts, those used to be a great headache too - and completely uncecessary.

[Boeingdriver999]

15% gradient sounds like an Airbus non-approved procedure........

I'm sensing that there's a gap between what Jonh Tullamarine says are procedures designers and actual pilots. How about we close that gap?

[john_tullamarine]

there's a gap between ... procedures designers and actual pilots

I would expect that most, if not all, procedures designers have significant flying backgrounds. Perhaps one should be looking at Doc 8168 for reasons as to why one sees this and that in particular plates ?

[TeeS]

15% gradient sounds like an Airbus non-approved procedure........

I'm sensing that there's a gap between what Jonh Tullamarine says are procedures designers and actual pilots. How about we close that gap?

Hi again Boeing
I think I may have to eat my words, the maximum 15% descent gradient applies to stepdown fixes and after the FAF but I can't find any requirement for that limitation to apply at the IAF or IF, sorry for the duff information and if I can find anything else, I'll let you know.
Apologies to you too Vilas
Cheers
TeeS

[Boeingdriver999]

No problemo!

When I read Doc 8168 I find that it outlines the rules and regulations but doesn't provide a "why?" And I think the "why?" is apparent if you have a background in that area but not so apparent to a lowly line pilot.

BD

[TeeS]

No problemo!

When I read Doc 8168 I find that it outlines the rules and regulations but doesn't provide a "why?" And I think the "why?" is apparent if you have a background in that area but not so apparent to a lowly line pilot.

BD
Hi Boeing
The background to the 15% is that the designer can ignore obstacles, after the FAF and step down fixes, that lie under a 15% plane starting at the earliest fix tolerance at the level of the OCA in the previous segment, less the MOC in the previous segment. If you descend at more than 15% then you will close on that plane as you descend.
Cheers
TeeS