Question regarding GS interception when approach mode armed

[WhySoTough]

Just wondering as I have never done anything but raw data in flight school.
Your FAF, where the GS starts to come alive and you descend with the glide is for example 2000 feet.
If you were at 4000 feet with your approach mode armed, I believe it will intercept the localizer, but will it also descend down to your FAF and further with the glide, or do you have to descend to 2000 feet for the GS to become active and descend with the glide?

Also, is there a certain altitude or distance with your approach mode armed for the localizer to start intercepting?

Thanks!

[Wizofoz]

First part-No, you can intercept the Glide Slope from any altitude.

Second-no, LOC will capture from any distance a valid signal is received BUT some countries have rules which limit navigation by localizer to 25NM.

[Capt Claret]

In the Douglas/Boeing 717, if the ILS is left to auto tune, it will do so at Aprox 24DME.

If manually tuned and terrain doesn't interfere with the signal propagation, then 90+ DME.

If on the LOC it will fly same from 50+DME and G/S from wherever it intercepts.

[WhySoTough]

Thanks for the response guys.
I remember watching a training video where the PF arms the approach PB at something like 6000 feet(visibility was very low) and expected the A/C to intercept LOC and come down with the glide, but it never intercepted the glide slope, and he overshot the runway at 6000 feet.. :}
Not sure why or HOW that happened..

[john_tullamarine]

Not sure why or HOW that happened

Automatics are no different to any other bit of gadgetry in that they are subject to

(a) design protocols - the end user needs to use them as the designer intended.

(b) error trapping and tolerance - does the gadgetry have a reasonable capability to recognise things which aren't quite right ? .. either internal or external to the gadget

(c) operator incompetence (in the technical sense .. ie not knowing enough about how to operate the system)

(d) database errors - the system and the operator might both do the right thing but the source database creates a GIGO result

That is to say, to trust gadgetry without suspicion is done at one's peril.

So far as the end undesirable outcome is concerned, such things occur when the pilot is not on the ball. The automatics help the operator do whatever it is he/she is seeking to do ... but they do need to be watched closely in case they do something a tad strange .. which they are wont to do at times.

If it ain't working like you want it to ... then do something different. In general, that involves reverting to a lower level of automatics involvement. That, in turn, might involve anything from pressing a button which the pilot hadn't intended pressing right through to pressing the big "OFF" button and flying it like a real aeroplane.

[galaxy flyer]

If in FAA airspace, there is a FAA Information For Operators letter (InFO 11009) on the subject. KLAX is semi-famous for the ILS approaches having very long segments with numerous stepdown fixes prior to the final segment, on the order of 25 nm. Follow the glide slope and you will bust several of them and they are, in somes cases, for terrain/obstacles. The technique is to arm APPR, upon interception of the LOC return to NAV mode so the A/P won't follow the the glide slope until APPR is rearmed. Your plane may vary but the idea is there.

Google that InFO, if interested.

GF

[grounded27]

In a simple term, Stabilized Approach! LOC CAP, do not expect your G/S to be reliable if the aircraft has to fight for it! Furthermore there are criteria (even if you have the raw data) for the G/S to capture, normally RA validation of a certain hight.

[Denti]

I had an encounter once in flight when the autoflight system captured an ILS, both LOC and GS, at FL350 and started to climb with quite a rate. Very unpleasant as we were just put on heading and told to descent. During that button push exercise my colleague inadvertently pressed on the APP button which armed the approach mode. Since then i always tune a VOR instead of an ILS during cruise to make sure the APP mode won't work.

[john_tullamarine]

Another anecdote - I had a 732 routine recurrency sim crew on descent via an arc and cleared them to intercept the ILS.

The boys (somewhat foolishly, I thought at the time) selected approach .. whereupon the box shortly thereafter captured a false localiser .. then the glideslope ... and down they went.

After a little while apparent crew discomfort set in followed shortly after by a degree of overt anxiety. They eventually figured out what was what far later than might have been prudent and, in the process, learnt a salutory lesson about the virtues of capturing the localiser (and checking its authenticity) prior to arming GS for the second capture.. not to mention a range of other checks which are available to save one's butt out there in the big bad world.

[Wizofoz]

I remember watching a training video where the PF arms the approach PB at something like 6000 feet(visibility was very low) and expected the A/C to intercept LOC and come down with the glide, but it never intercepted the glide slope, and he overshot the runway at 6000 feet..
Not sure why or HOW that happened..

If the aircraft is in a mode that maintains altitude (E,G ALT or VNAV ALT) and already above the glideslope, it never gets to the glideslope in order to intercept it, so the aircraft maintains level flight.

It happens quite often when given tight vectors to intercept the ILS.