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notanastronaut
23rd Feb 2010, 05:03
Could someone kindly share the fine points of differentiation between the two.
FYI, I did go to Jepp Vol. 1 and red the legend, however some expansion would be appreciated.
Thanks,
NA

Microburst2002
23rd Feb 2010, 08:27
If I recall correctly, FAF is a fix. A two dimension fix on the approach path, where the final segment begins. The FAP is the point along that path where you capture the glide slope.
I don't know why they make a difference. Maybe because different true altitude depending on temperature, or ILS beam bending or something like that? Maybe because of the nature of the approaches, FAF in NPA, FAP in PA?. It is not mandatory to start descending at the FAF. It is mandatory not to descend before it, which is different. But reaching the FAP, at the published altitude, you have to intercept and follow the G/S.

plain-plane
23rd Feb 2010, 08:45
FAF has a specific geographical location.

FAP location can vary,,- ie, because of timing on the outbound segment / initial approach segment. local wind and the speeds you fly at will then change the ponit at which you will establish indbound... (think timed NDB approaches...)

US based info..

CaptainX
23rd Feb 2010, 08:46
FAF is the point that marks the beginning of the final approach segment of non-precision approaches (NPA). It normally consists of a beacon or DME. Since NPAs by their nature contain only azimuth guidance, such a fix is necessary before the final approach.

FAP is the ILS glide path intercept point, marking the beginning of the final approach segment of a precision approach.

Basically, on a NPA, you can't descend without a FAF!

Does this help?

FlightDetent
23rd Feb 2010, 16:28
FAF is a fix, a position. It is defined by coordinates, bearing/range or navaid. At that position, you leave intermediate apch altitude and start descending towards MDA or step down fix. Note, the possible dive&drive strategy is soon becoming illegal under EU-OPS.

Precision approaches, by definition, have electronic vertical guidance. There is no need to publish a (start of) final approach fix - pilots will intercept the electronic vertical guidance provided. Also, because of temperature dev from ISA and associated altimeter error, the geographical point of GS (for ILS approaches) intercept point moves inbound / outbound as the true altitude from which the GS is being intercepted changes. At last but not least, remember that normal 'old school' ILS installation has no DME device, only LLZ+GP and two markers which do not provide any means of identyfing the GS intercept location.

To summarize:
1) There is no need to provide a final approach FIX for precision approaches.
2) There is no way to establish a final approach FIX for precision approaches as the point of intercept does not have a FIXed position.
3) There is no instrument to identify distance on an ILS navaid approach.

Hence a question 'Why is the FAF called FAP for ILS?' has a short answer: 'cuz it ain't no effin' fix!' :8

My pet hate is calling FAP 'the centre fix'. The lingo was introduced around here about 20 years ago when first large batch of FMS aircraft (737) arrived. The GS intrecept is often overlying with ARINC coded course-to-intercept pseudo waypoint displayed as "CI24" for rwy 24. The then-students picked up to call it Centre Fix and the habit widespread to local pilot and ATCO community. It is no big deal, only shows what kind of pedantic sad soul I am.:ouch: Still, I'd be interested to learn more of the 'Centre Fix' genealogy if someone has more background info.

Sincerely,
FD (the un-real)

cosmo kramer
23rd Feb 2010, 19:02
CaptainX:
FAF is the point that marks the beginning of the final approach segment of non-precision approaches (NPA). It normally consists of a beacon or DME. Since NPAs by their nature contain only azimuth guidance, such a fix is necessary before the final approach.

...

Basically, on a NPA, you can't descend without a FAF!

Not quite correct.

You can indeed have a NPA without a FAF if there is only one facility available (hence not possible to make a fix, as two facilities are by nature required to make a fix).

Such approach start e.g. at the IAF by overflying a beacon placed at or near the airfield. Then flying a timed reversal procedure or timed racetrack, then inbound turn, and when established inbound descend to MDA/H.

Microburst2002
23rd Feb 2010, 21:13
Yes.

And it is interesting that FMS procedures cannot depict such approaches. I mean, there are not timed segments, are they?

DA-10mm
24th Feb 2010, 05:17
try checking the definition of each one in the AIM and it should become crystal clear.

plain-plane
24th Feb 2010, 08:18
FMS and timed approaches,,,

While it will not be timed, try to check the distance it is programmed to fly, against the distance you would expect to fly at standard speeds for the initial and intermediate approach segments of you category of aircraft.... (Zero Wind)
So far I have only been pleasantly surprised when checking these approaches...

Kiltie
24th Feb 2010, 08:33
I'm with FlightDetent. The phrase "centrefix" is a cancer throughout our community. Age old, out of date, inaccurate and should never have been accepted as standard phraseology.

notanastronaut
25th Feb 2010, 13:11
Thank you all for your input and infos.
NA