Capturing the G/S

[BMI701EGCC]

Just a quick Q,

Is it normal to start descending on the G/S before you have captured the LOC?
This will only apply if your on right/left base and closer than the start of the G/S wouldnt it?

comments much appriciated,

Scott Waterworth
BMI701EGCC

[Intruder]

NO!!!

You are not allowed to descend from your previously cleared altitude until you are cleared for and established on a segment of the approach.

However, if you are using a full approach procedure instead of vectors to the LOC, you can descend per the procedure once cleared for it.

[catchup]

For my 2 cents there are two aspects.

1.If you are already cleared for ILS but not on LOC you may descent to MSA.

2.If you are only cleared for LOC, you may not further descent without clearence of ATC.

regards

[BOAC]

BM - I guess your question is really about 'safety' and not the ATC side of things?

Most airline SOPs preclude descent on the GP before you are 'established', some allow descent when within 5 deg of the localiser. As long as there are no ATC considerations, and as long as you are quite CERTAIN you have no terrain or airprox problems, there is nothing safety-wise stopping you. It does happen that for various reasons LOC is not 'captured' before GP intercept and provided all the above are OK, it is OK, but, no, it is not 'NORMAL'.

I'm not sure whether 'opnot' answered in 'ATC' one of your other threads (Tech Log) about ILS capture to your satisfaction, but in relation to the a/c you saw 'fly through' the localiser- as he said - if the arrival is from the south and for R06, the FULL MIRSI arrival goes through the c/l before the approach is commenced (MIRSI being north of the c/l) but ATC often 'short-circuit' the procedure to vector from the south, or occasionally having just passed through the c/l but not got to MIRSI. Maybe this will explain your query in Tech Log?

[john_tullamarine]

I would be very careful about capturing the GS prior to being very sure where you are ... following on from BOAC's comments, consider two f'instances

(a) gave an experienced crew a routine approach with crew navigation. They were running normal autoflight control. For whatever reason (we were assured after that the sim wasn't programmed to capture false LOC) the box turned in on a false LOC and commenced descent on the GS.

The crew lost SA and, had the incident being in flight rather than the box .. would have been unpleasant ...

(b) have a read about the Air NZ Apia 767 false GS capture .. was about 4-5 years ago as I recall.

There's a great deal to be said for being procedurally rigorous when the aircraft is near to the bumpy bits ...

[BOAC]

:ok: JT! We lost something when some airports took away the approach NDB which enabled a 'confidence check' on the correct localiser position. Now we have LNAV and MAP which are fine as long as they mean what they say! Too many people rely on the MAP display for SA.

Perhaps BM could amplify his question a little so we can see what he is getting at? For example, if one were flying a visual approach using ILS as a 'monitor', it would be 'NORMAL' and safe to start using the GP as a further clue to profile before being on the localiser, and indeed to be desending below published GP 'capture altitude', as one would on base for a visual circuit.

As John says, though, IFR - with EXTREME caution (best to let ATC help you!), and with JT's 'lumpy bits' around...........oooohhh! I recall Brussels had a 'false localiser capure' NOTAM for R25 for years - it may still have?

PS Not too many 'lumpy bits' there:D

[Waspy]

BRU NOTAM is still valid. Correct statement.

Have a try at VCE 04R, coming from the north. VERY FREQUENT false LOCs (and it is not a NOTAM but real stuff). If too low and north of axis, there are some man made obstacles worth a lookout. Considering the quality of ATC at VCE (telling their lives on TWR freq when you're at 2 DME from THR...), I would consider decending on GS before stable on LOC as a NO NO!

;)

[OzExpat]

Hey BOAC, are you trying to tell me that there's somebody else of MY vintage who remembers the said "confidence check" provided by a NDB/Locator beacon? Sounds like you grew up in a procedural control environment too! :}

That said, I don't know of too many places where ATC will vector me to a position from which I will intercept the GS before the LLZ. So far as I'm aware, the golden rule in ATC is to vector for a LLZ intercept at an altitude below GS capture. I know that there was a learning curve in this for ATC but, even where I've been for the last too many years, they mostly get it right these days.

Most ATCOs are very well aware that, when they're vectoring an aircraft, the pilot has buggarall chance of maintaining SA. This is why there's been, for a real long time, a lot of emphasis on telling the pilot how many miles to touchown, or to LLZ intercept.

In some places, with workload permitting, pilots can keep some idea of of SA by reference to navaids, GPS, etc. But, obviously, the caveat here is "workload permitting". Plus a fair amount of local knowledge!

I have no problem in accepting some GS guidance prior to LLZ intercept in conditions that allow visual cross-reference. However, in IMC, I insist on LLZ capture first. And then I'm also cross checking DME distance to ensure that I avoid capturing a false, or erroneous (e.g. Apia), GP.

Judging from some previous threads on this topic, I think that I'm fortunate to have done the majority of my flying in countries that provide a DME/Altitude table, plus an indication of when to expect GS intercept, on the charts. It saves a lot of mental maths - and unexpected surprises!

So I guess that my answer to the original question is that, without other indications, I would certainly not be guided by the GS indication - in IMC or at an unfamiliar location - without first being assured of LLZ capture, or being above MSA.

As some folks on here will know, I also design these procedures, so I know that there can be some really BIG rocks just outside the ILS protection area that can really ruin you day! :eek:

[ifleeplanes]

Some autopilots will not capture the GS until after they have captured the LOC.

[BMI701EGCC]

thanks for all your comments, its much appriciated,

BOAC the answer you gave to my ATC Q was great, thanks



Scott Waterworth
BMI701EGCC

[john_tullamarine]

.. the underlying message is, I suggest, don't trust your life to the whizz bang magenta line gadgets. They can be good (or bad) but he (she) who trusts them unthinkingly .. sooner or later is going to get a fright .. or worse.

Reflect upon how many code errors exist in the software on your home PC .. sure, the TSO'd things are better than that ... or, are they ?

.. and it doesn't really matter what you use as a check for reasonableness before pitching over (subject to the normal priority preferences) .. even a pre-thought out VOR/DME or NDB/DME fix is better than just sitting back and hoping that the box is telling you the truth.

.. or are some of us still in the early generation systems where "what's it doing now ?" was the catchcry ?

[Intruder]

Is it normal to start descending on the G/S before you have captured the LOC?
I'm getting confused with all the different answers here. Maybe I didn't understand the question...

It is NORMAL to fly an ILS under an IFR clearance. Given that assumption, you may NOT descend on the G/S until the LOC is captured, because you are not on an approach segment until you are on the LOC. In the US, the rule is codified in FAR 91.175(i):

"When operating on an unpublished route or while being radar vectored, the pilot, when an approach clearance is received, shall, in addition to complying with Sec. 91.177, maintain the last altitude assigned to that pilot until the aircraft is established on a segment of a published route or instrument approach procedure unless a different altitude is assigned by ATC."

All the other circumstances noted here appear to be under other than "normal" conditions.

[NigelOnDraft]

We lost something when some airports took away the approach NDB which enabled a 'confidence check' on the correct localiser position. Now we have LNAV and MAP which are fine as long as they mean what they say! Too many people rely on the MAP display for SA. Not quite sure how true that is ?? I always try and have an aid, either NDB or VOR, to confirm the LLZ, and most still have one or the other? Indeed, when "cleared to intercept the Loc" I ensure that the Loc is ident'd, the Loc indicator to the "expected side" and that the independant aid agrees with we are near the Loc. With a NOTAM re false captures, I don't believe the Loc should be armed until the Loc indicator is off the stops AND the "aid" confirms the correct radial etc.

Of course, I am seemingly fighting a one man battle and 99% of my colleagues just press the Loc button with 1 ms of being cleared to intercept with not a thought as to ident, indication, false captures, backup aid, terrain or monitoring the intercept as it occurs etc.. :(

[BOAC]

either NDB or VOR - which is what I am saying??
EG - CPH - how many people retain the KAS VOR on one side during loc capture? Where is the NDB?

[DFC]

As far as I am aware one must be within 1/2 scale deflection of the LOC to descend on the GS.

The only time I would consider descending early (subject to ATC) is when descending to MSA or descending to the published altitude for that leg of the approach.

Look at the numbers. If one is on the GS at the minimum altitude before 1/2 scale deflection of the LOC then I believe that there is a high probability of a rushed approach and/or an unstable approach.

Regards,

DFC

[Ttailed]

Hi there,

It's simple fly what Mr Jepp or Aerad tell you as published. It is a no no to descend before being established. Most airline SOP'S also have the non flying pilot on the VOR/NDB frequency to monitor the radial before joining the pilot flying on the ILS makes sense to me.

Tt

[ManaAdaSystem]

I was under the impression most Airbuses cannot be tuned to different navaids (1 and 2) during approach?
NGs will start to cough out caution messages about rwy/tune, and in some cases draw a yellow line through the course numbers on the PFD. They will also happily capture LOC before the needle has moved at all.
NDBs are set to be removed by year??? VORs may or may not be usable for x-checking.

New equipment, new procedures. RNP/ANP is a good start.

If there is a steamdriven NDB on the chart, I will tune it, but I'm still just as happy to fly the approach without it.

[bookworm]

Is it normal to start descending on the G/S before you have captured the LOC?

Not sure anyone has really asked the question: "descend from (or through) what level?"

On a procedural ILS approach there is a minimum altitude at which the final approach segment begins. On a radar vectored ILS, there is a level to which ATC has instructed you to descend for the intercept. Obviously, one must not descend below those levels prior to LOC intercept.

But is there an issue with using the GS for guidance on the way down to that level, facilitating a more or less continuous descent?

[Intruder]

But is there an issue with using the GS for guidance on the way down to that level, facilitating a more or less continuous descent?
There is, in many cases. The GS may not be certified beyond the published intercept point, increasing the chances of false captures outside that point.

That said, it is often possible to use GS as a backup descent aid outside the normal guidance limits. For example, it is possible to capture the GS when landing to the south at SCL at over 30 miles out. If it remains stable, it will keep you above all the minimum altitudes on the published arrival. However, it is not technically legal to use the GS for that purpose, and is not safe to use it as sole guidance. Use it as a crosscheck, but use other means for primary navigation.

[BOAC]

Trouble is, without the requested 'background' to the question from BM, we are all 'guessing' at what the question was all about!! Eg 'Capturing' was never mentioned, and I suspect we have all got too technical with it?

[john_tullamarine]

... which is one strength of the forum ... a question generates a bunch of related questions .. nice if the original question gets answered also .. but we end up thinking about a few other things along the way.