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When to press the Appr button?

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What (or when) is the approved procedure for pressing the "Appr" button on an ILS landing ...1) Anytime during the approach2) Only after the Nav captures the ILS but before the GS is active3) Only after the GS become activeI am never quite certain exactly when I'm under the GS ( no instrument warning ?) until the bar starts moving. But I wonder if the "Appr" will still capture the GS as you descend under it if it is armed above it.I've managed to get into trouble on short final expecting the GS to handle the descent in only to see the altimeter rising and then falling erratically ... convincing me to panic, intervene and hand it in. It seems to me the JS kind of "dives in" at the last minute and maybe one should keep his fingers off the buttons and wait for it to happen! But there must be a more relaxing way to handle the GS? Ken Park

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Ken:I'm sure more experienced pilots will weigh in here (and I'll learn something as a result). That said...I have done numerous ILS approaches and landings in this beautiful bird and have never felt that she wanted or needed to dive to the threshold at the last minute. I review the ILS approach plate prior to flight in order to better anticipate the required course and altitude to best intercept and acquire the GS. In flight, assuming I'm flying the approach as per the plate, I approach the FAF at approach speed and slightly below the GS, and I press the APR button before I even pass the FAF. I've got the ILS localizer frequency tuned on NAV1, of course. The plane quickly acquires the GS at that point and begins the appropriate descent as I control descent speed via throttle management (assuming flaps have been deployed as per proper procedures). She floats right down to the threshold...a joy to land in this manner. No diving or erratic changes in altitude.Hope this is helpful, and the PMDG tutorial is very helpful.

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2) Only after the Nav captures the LOC but before the GS is active3) Only after the GS become active
Either should work. Notice I modified slightly your (2).I have slight preference for (2) - no need to pay very close attention to GS in order not to be late. This is time to complete landing checklist so your attention could be diverted.

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Thank you very much for the helpful responses.I'm still not quite sure though if the Appr can be armed above the GS and if it will then automatically lock on as you descend into it ... In my days of flying the default FS planes I would usually wait for the GS button to light up and then hit approach ... not having that available in the JS, it looks like you're advising me to keep my powder dry until I see the whites of the runway and then push the magic Appr button.As to the erratic behaviour of the altimiter on occasion I think that I'm answering my own question when I admit I was probably too high at the IAF and the AP was still controlling the descent to the ALT selected when I swooped into the GS ... maybe then it CAP'd ... found me too low and started to climb then descend again.I never mind going in by hand though if I can see the Rwy ... she is such a jewel to hand fly.Thanks again ... I'm going to take your advice and "do circuits" !!!Ken Park

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I'm still not quite sure though if the Appr can be armed above the GS and if it will then automatically lock on as you descend into it ...
Depends on a particular AP. But regardless, you should avoid doing it - bad piloting technique.
it looks like you're advising me to keep my powder dry until I see the whites of the runway and then push the magic Appr button.
I definitely never advised that!

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If you look at the Approach Plates (free at http://edj.net/cgi-bin/echoplate.pl) find the Final Approach Fix (a Maltese Cross) and look at the altitude. You want to be AT that altitude by the time you get to it (preferable before if there are no obstacles, which makes it easier). Before the cross there is often another fix further out. If you have both of these fixes programed into your FMS then the aircraft will track and fly these fixes. Generally I found that if you intercept the localizer beam at a 30 degree angle or less, and are at the right altitude, are at the correct speed with flaps and gear down, you can press the V/L button at somewhere around 15 miles, and then the APR button and when you see the APR go to CAP then you can just use the throttle to stay on the Glideslope (but wait for the CAP). From here ... you could probably fly right to the ground with throttle only (if you're around 700' per minute) and not damage anything ... except maybe your pride. But of course you would normally disconnect the autopilot at about 300' so you could flare out properly. Anyway .... this works for me. It may not be exactly proper ... I don't know, but it works every time for me ... so far. :(

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The topic gets more interesting as I dig down. It appears there may be more than one GS beam ie False Lobes at multiples of 3 degrees available in certain cases. It would certainly explain the erratic altimiter behaviour I experienced while descending with the ILS frequency loaded in NAV1 and the NAV button on and the APPR button "armed".I commend the forum discussion at the site shown below to those interested in the subject. It appears to be an exchange of experiences by professional pilots on the topic at hand. Also, an Airbus pilot confirms that certain aircraft can capture the GS from above ... ! If we can go to the moon you'd have to think such a feat would not be impossible ... hence my original question re arming above the GS and letting it lock on when I got down to it. Although the FAF is not necessarily the Maltese Falcon on the chart but rather where you capture the GS (you're then on final) I agree that holding off arming the APPR until you're down close to the required altitude or when the GS becomes active is a procedure less likely to get you into my sort of difficulties.http://www.pprune.org/archive/index.php/t-358117.htmlMy thanks to all contributors.Ken Park

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It would certainly explain the erratic altimiter behaviour I experienced while descending
Not sure what you mean - altimeter is purely barometric and not connected with ILS in any way. So it shows you correct altitude (if set to local airport).
... hence my original question re arming above the GS and letting it lock on when I got down to it.
But I am not sure what you want to accomplish. Certain autopilots can do it but why is it so important to you? This is bad flying technique. You should always approach ILS while at constant altitude (ALT HOLD) and fly into the GS beam - if you do it this way you will always capture GS from below and this is the way how pros do it, how 99.9% of ILS are flown. Anything else is simply a bad airman-ship.

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Although the FAF is not necessarily the Maltese Falcon
For a Non-Precision Approach the Maltese Cross would be the FAF. In reality there is no FAF for a Precision Approach ... but a Final Approach Point (FAP) which could be radar directed and at/or where you intercept the Glideslope ... usually at the Lightning Bolt on the Approach Plates (If you're looking at a CAT I) .... and just a few seconds before the Maltese Cross.

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I've seen most CAT II & III ILS approach charts missing the FAF, but every CAT I I can recall with my imperfect memory does have the cross symbol and the FAA handbook refers to it as the FAF where it is used to start the stop watch for the approach. I was taught to time every approach from the FAF inbound in case I lose GS I can then fly the LOC only mins.

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You should always approach ILS while at constant altitude (ALT HOLD) and fly into the GS beam - if you do it this way you will always capture GS from below and this is the way how pros do it, how 99.9% of ILS are flown.
We'll see this change in the next years I'm afraid. The CDA (Constant Descent Approach) system implementation tries to avoid those level flight sections (more power needed = more noise + more fuel burn).

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I believe capturing an ILS GS from above does pose a theoretical danger of encountering a false glideslope and is therefore frowned upon. As for CDA/TA's arent't these being done with RNAV approaches?

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As for CDA/TA's arent't these being done with RNAV approaches?
No, not really. They are yet another beast. And even if TAs are going to be widespread one day it will still be a while before aircraft like J41 flying to some secondary airports will be affected. Because of special equipment and other requirements big jets flying to large airports are the primary target of TAs.

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I'm still not quite sure though if the Appr can be armed above the GS and if it will then automatically lock on as you descend into it ...
According to the textbook I'm studying, antenna design (and reflections I guess) often cause energy to be radiated in multiple beams and you can get GS indication on one of the unintended beams. The antenna design ensures that the lowest beam is the correct one. Therefore, if you enter from below you can be sure of capturing the correct GS. If you enter from above, you may capture a phantom GS.Hopefully the textbook is telling the truth!Nick.

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The poster above is correct about being at the "Platform" altitude before capture. The FAP will be referenced to a DME distance on an ILS, so ensure you are outside this distance and at the correct platform altitude. This will ensure the GS is above you.To answer the original question the time to arm the APP (or APR) button is when you are within 90

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Most has been said here. Just always remember not to arm the APPR button if the ILS indicates that you are above the glide slope. Descend accordingly if so. Another point to remember. ILS sytems "prefer" to capture the localiser first (And this how one is trained to operate using the ILS). Meaning that if they capture only the GS the a/c will not descend. In order to start the descent on ILS the a/c must already be established on the localiser. At 10nm out you should intercept the GS below 3,000 AGL. Thus allowing the a/c to line up with the localiser before intercepting the GS. The doctrine: "A good landing is made from the top of the glide slope". Should always be kept in mind. Also remember (even though it is just a sim meaning you can do what you like) that at major airport such as EGGL pilots must always use the ILS and not make a visual approach. SVFR approaches are only granted by PPR, when there is little traffic and when weather conditions permit.Vololiberista

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Gents-Proving once again that there are no "right" answers in aviation- let me give you a different spin.Our line's operating procedures for thet LOC/APR modes were as follows:LOC: Press when you get "cleared to intercept" an ILS. This instruction implies that you are cleared to use the approach path, but not cleared to descend... As such, using LOC will satisfy this criteria.APP: Press when you get "cleared for the ILS approach" from ATC. It is important to note that (in the real world) no ATC worth his salt will clear you for the ILS approach whilst you are still above the G/S. In my career I've never had it happen... for what that matters.... Well- there was that one day on VATSIM... but that was back in '96. :-p

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I'm in my 15th JS41 flight and never used AP for ILS approach :( ! From what you are saying I'll keep following GS and LOC by hand :(

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I'm in my 15th JS41 flight and never used AP for ILS approach :( ! From what you are saying I'll keep following GS and LOC by hand :(
Ok if you wish as it's only sim! But, if you are flying in UK airspace and wish to land at an airfield equiped with ILS you must and I repeat "must" not fly a visual approach unless you have PPR (Prior permisson required). What this means is that you must fly the ILS using the autpilot.Vololiberista

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Basically never hit the APP button till you're on the final vector for LOC intercept and have been CLEARED for the approach.

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dbw1Not technically correct, Cleared for the approach is different to being cleared to intercept the LLZ, in both cases the APP can be armed. (But you mustn't descend on the glide without clearance)VololiberistaJust because you don't arm the APP (which is basically the Flight Director) it doesn't mean you are conducting a visual approach. You can quite easily fly a raw data ILS without the aid of the Flight Director & Autopilot.

It is important to note that (in the real world) no ATC worth his salt will clear you for the ILS approach whilst you are still above the G/S. In my career I've never had it happen... for what that matters.... Well- there was that one day on VATSIM... but that was back in '96. :-p
RobTry flying into Amsterdam Schiphol, it's a frequent occurrence ;)

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Actually. based on 30+ years of doing it for a living (and i acknowledge different sop's exist for different companies) were one cleared to intercept the LOC or LLZ I would hit the LOC button. Were I cleared to intercept the LOC for an ILS (dnot yet have approach clx) I would hit the LOC button. Only after I received Approach Clx would I hit the APP button as only then would I be cleared for the Approach and own the airspace to the ground. Also, I acknowledge these procedures may vary from aircraft to aircraft.

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Actually. based on 30+ years of doing it for a living (and i acknowledge different sop's exist for different companies) were one cleared to intercept the LOC or LLZ I would hit the LOC button. Were I cleared to intercept the LOC for an ILS (dnot yet have approach clx) I would hit the LOC button. Only after I received Approach Clx would I hit the APP button as only then would I be cleared for the Approach and own the airspace to the ground. Also, I acknowledge these procedures may vary from aircraft to aircraft.
I don't know where you fly but in the US it is pretty much standard that your clearance for the LOC and ILS approach comes at the same time and it sounds something like this:XXXX, turn right heading XYZ, maintain xxxx (altitude) until established on the localizer. Cleared for the ILS runway XX ...I have only done GA IFR flying and I never heard a separate clearance issued for localizer and the approach. Perhaps if you fly big iron things could be different.

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dbw1Granted, but you said it yourself, quote - never hit the APP button till you're on the final vector for LOC intercept and have been CLEARED for the approach.What I was saying is that technically this is not quite right, because you can still hit the APP button when cleared to intercept the LLZ only, nothing is stopping this from happening. It is only your company's SOP that dictates you can't.It used to be my company's SOP to only hit LOC (or NAV in our case) when cleared to intercept only, but now it has been replaced with hitting APP when within 90

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In the United Kingdom you cannot be "cleared approach" in conjunction with ILS or Microwave, unless it's a self-positioned approach (See CAP413, page 206, also see 9.4.2 in CAP493). The instruction to descend on the ILS is given separately from clearance to join the localiser, although the two maybe combined into one transmission if the glidepath clearance is conditional and there's no conflict with procedure.I wouldn't press the APP button on the AP until you had received clearance to descend on the glidepath, because you might end up descending without authorisation. You'd have to use another LNAV mode in the interim. Even when flying overseas I wouldn't press the APP button until I'm sure I'm not going to descend in to anyone or anything.廖傑英 (Chris)

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