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How Many NM To Enable The APP Switch?  
User currently offlinewardialer From United States of America, joined Sep 2001, 1183 posts, RR: 0
Posted (4 years 7 months 1 week 3 days 11 hours ago) and read 4730 times:

Hi,

I was wondering on the rule of thumb on how many nautical miles out from the airport or the localizer do pilots enable the APP switch when doing an ILS approach?

Is it 26NM or perhaps 12NM?

I suppose 30 or 26NM is way too much...Would be like 20 to a minimum of 10NM?

Thanks.

25 replies: All unread, showing first 25:
 
User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 1, posted (4 years 7 months 1 week 3 days 11 hours ago) and read 4698 times:

Quoting wardialer (Thread starter):
Is it 26NM or perhaps 12NM?

Neither. It is not dependent on a set distance.



Generally when you have an active localizer indication inbound AND are at your assigned altitude prior to the final approach fix/glideslope intercept point.

You MUST intercept the LOC before the G/S otherwise the
User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 2, posted (4 years 7 months 1 week 3 days 6 hours ago) and read 4594 times:

So there's a gltich in my last post, but as I was saying... You MUST intercept the LOC before the G/S otherwise the A/P will not follow the G/S.

User currently offlinejetlagged From United Kingdom, joined Jan 2005, 2565 posts, RR: 25
Reply 3, posted (4 years 7 months 1 week 3 days 6 hours ago) and read 4585 times:

Quoting Fly2HMO (Reply 2):
So there's a gltich in my last post, but as I was saying... You MUST intercept the LOC before the G/S otherwise the A/P will not follow the G/S.

While it is a good idea to intercept the LOC before the G/S not doing so will not prevent the G/S capturing, at least not in every aircraft. The B747-400 for example has an autopilot option to disable G/S capture until after LOC capture. If not selected the G/S will capture before the LOC.

If you are way off the LOC the G/S gains will be not be ideal, so it may not capture and track too well.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 4, posted (4 years 7 months 1 week 3 days 5 hours ago) and read 4565 times:

Quoting jetlagged (Reply 3):
at least not in every aircraft.

True, in real life at least  

Judging from the OP's other posts, I'm assuming this is a flightsim question in which case you do need to have the LOC acquired before the G/S.


User currently offlinepilotpip From United States of America, joined Sep 2003, 3150 posts, RR: 11
Reply 5, posted (4 years 7 months 1 week 3 days 3 hours ago) and read 4525 times:

I arm it when we're cleared for the ILS. Otherwise it stays in Nav mode while we intercept the final approach course.

The 170 doesn't like being more than 15 miles on final. It will make a lot of corrections to chase the needles so I'll often keep it in VNAV and switch to approach mode as we get closer.



DMI
User currently offlineMir From United States of America, joined Jan 2004, 21678 posts, RR: 55
Reply 6, posted (4 years 7 months 1 week 3 days 1 hour ago) and read 4496 times:

I enable APP when on an intercept heading and cleared for the approach, with the proper frequency tuned and identified. Or, if it's a GPS approach, I could also just do it when cleared for the approach if I'm navigating toward something on the approach or if already established on a published portion.

Quoting Fly2HMO (Reply 4):
Judging from the OP's other posts, I'm assuming this is a flightsim question in which case you do need to have the LOC acquired before the G/S.

I don't recall this being necessary in FS - you can capture GS before LOC, and the AP will start descending.

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 7, posted (4 years 7 months 1 week 3 days 1 hour ago) and read 4482 times:

Quoting Mir (Reply 6):
you can capture GS before LOC, and the AP will start descending

Nope. Won't do it. Not in FS9 with freeware planes at least.

I did many inadvertent LOC only approaches because of that.   


User currently offlinejayeshrulz From India, joined Apr 2007, 1029 posts, RR: 2
Reply 8, posted (4 years 7 months 1 week 2 days 22 hours ago) and read 4449 times:

Quoting Fly2HMO (Reply 7):

lol..
Actually, in many paywares, the aircraft is configured to do autoland with a complete LOC and G/S capture till Landing/Flare.
Best example..PMDG 747-400.

But there are some freeware planes too.

Basically, in real life, a pilot may capture it anywhere in 15Nm.But its boring to do a autoland :P



Keep flying, because the sky is no limit!
User currently offlineMir From United States of America, joined Jan 2004, 21678 posts, RR: 55
Reply 9, posted (4 years 7 months 1 week 2 days 21 hours ago) and read 4447 times:

Quoting Fly2HMO (Reply 7):
Nope. Won't do it. Not in FS9 with freeware planes at least.

Hmm. Well, I guess that's what I get for not having played FS in more than a year.

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 10, posted (4 years 7 months 1 week 2 days 17 hours ago) and read 4384 times:

Intercepting the G/S before the LOC can get you violated since most places REQUIRE you to be ESTABLISHED on the final app course before leaving the final app alt. Obviously on a visual app this does not aply. All the jets I've flown require capturing the G/S from below as well.

User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 11, posted (4 years 7 months 1 week 2 days 2 hours ago) and read 4262 times:

Quoting CosmicCruiser (Reply 10):
Intercepting the G/S before the LOC can get you violated since most places REQUIRE you to be ESTABLISHED on the final app course before leaving the final app alt.

You can be on the final approach coarse without engaging LOC. So it's possible for G/S to engage without LOC engaging and still meet the requirement.

Quoting Fly2HMO (Reply 1):
You MUST intercept the LOC before the G/S otherwise the
Quoting CosmicCruiser (Reply 10):
All the jets I've flown require capturing the G/S from below as well.

It's an option...it's a really common option, but an option none the less and you don't *have* to do it.

Tom.


User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 12, posted (4 years 7 months 1 week 2 days 2 hours ago) and read 4253 times:

Quoting CosmicCruiser (Reply 10):
Intercepting the G/S before the LOC can get you violated since most places REQUIRE you to be ESTABLISHED on the final app course before leaving the final app alt.

I can't think of any case that doesn't have that requirement to begin with.

But someone enlighten me if otherwise, this CFII is getting rusty   

Quoting tdscanuck (Reply 11):
and you don't *have* to do it.

Technically, no, but you would be wise not to. Otherwise you risk capturing a false G/S.

Still, you have to check the altitude once you intercept the glideslope as a reality check.

Just for kicks, here's a great example where significant crosschecking of charts and instruments saved lives:

http://www.youtube.com/watch?v=GelRBhJ4gmI


User currently offlinemusang From United Kingdom, joined Apr 2001, 865 posts, RR: 7
Reply 13, posted (4 years 7 months 1 week 12 hours ago) and read 4060 times:

Good Day ALL.

If we're talking about the real world, many of us use 10nm as a rule of thumb. There's a guaranteed area of reliable coverage expressed in terms of angle and range, deviations being notified. Can't remember the dimensions. I go with the ten mile "rule" due to experience.

Tried a 17 mile G/S couple at CDG a few years ago. Observed the g/s pointer go steadily from above to below the centre, i.e. the aircraft had over-flown the G/S intercept. Said to the co-pilot "Look what its doing! Lets see what it does next" (it was 50 mile visibilty, no cloud). One half dot high, the aircraft (a classic 737) realised what was going on and ever so gently lowered the nose to a 2000+ fpm descent rate which would have caused a flap overspeed (we were flap 5 180 knots at ATC request) had I not disconnected. That was the aircraft's fault.

I find the 737 classics' A/P sloppier in pitch than any other airliner I have flown (ATRs, RJ100s [plus King Airs, Caravans, Do228s]), and indeed worse than some light aircraft I have flown.

Naples runway 24 is a rollercoaster if you engage g/s too far out, presumebly due to the terrain. I once did, just to see what happened, and we experienced level flight, gentle climbs and 2000 fpm descents with the associated huge thrust changes. I now use VNAV (with DME/Altitude checks) down to about 10 miles, which nails it.

Geneva runway 23 is disappointing also. Just y'day we had an unusually long 22 mile final. No terrain between us and the runway (its over the lake the whole way) and reasonable distance/height checks, so I engaged at about 15 miles. Not as bad as NAP but I won't be doing that again!

Quoting Fly2HMO (Reply 2):
You MUST intercept the LOC before the G/S otherwise the A/P will not follow the G/S.

Not so on our 737 Classics and the RJ100s before that. Maybe a software tweak can prohibit it.

Quoting CosmicCruiser (Reply 10):
All the jets I've flown require capturing the G/S from below as well.

Ditto!

Quoting tdscanuck (Reply 11):
So it's possible for G/S to engage without LOC engaging and still meet the requirement.

I guess so, IF an aircraft has a means of engaging glideslope without also coupling to the Loc. On the 737, to couple to the G/s you press the APP button. If there is a Loc detected, it will lock onto that also, even if the Loc wasn't engaged previously. Which types have seperate LOC and G/S buttons?

I believe I'm correct in saying that a G/S signal would never be radiated without its associated LOC??

Regards - musang


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 14, posted (4 years 7 months 6 days 17 hours ago) and read 3942 times:

The normal LOC coverage is 25 nm 10 degrees either side of the approach centerline and 17 nm out to 35 degrees either side. In some places, mainly in the US, this has been reduced to 18/10 nm in order to facilitate more facilities in a given area without frequency congestion. There are also extended range facilities, I know they exist with 75 nm coverage within ten degrees.

GP coverage is typically 10 nm and 8 degrees either side of the centerline. Outside the coverage volume there are no guarantees on the quality of the GP signal, meaning it is not legal to descend on the GP. The GP will typically droop as you diverge to either side, giving you a lower GP angle and reducing your obstacle clearance. That's worth keeping in mind. It is not rare to see a GP with reduced coverage. I'd strongly recommend to get that LOC needle close to centered before descending.

And no, you cannot have a GP without a LOC. That's also why you have an ident signal on the LOC but not on the GP.

Cheers,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 15, posted (4 years 7 months 6 days 13 hours ago) and read 3872 times:

Quoting Fly2HMO (Reply 12):
I can't think of any case that doesn't have that requirement to begin with.

I agree and I was saying that just to emphasis the point that if you're cleared for the ILS app and you're at the final app alt you need to be established on the loc before descending with the g/s.

Quoting tdscanuck (Reply 11):
You can be on the final approach coarse without engaging LOC. So it's possible for G/S to engage without LOC engaging and still meet the requirement.

hey tdscanuk, I'm not sure I follow that. Are you saying you can be flying the loc but not tracking it? My comment was strictly referring to already being at the final app alt. Last night we were 1000' above F.A.A. and we were clearded for the visual (the ILS would be the same) though not on the loc we descended to the final app alt but no further until eswtablished.


User currently offlinemusang From United Kingdom, joined Apr 2001, 865 posts, RR: 7
Reply 16, posted (4 years 7 months 6 days 12 hours ago) and read 3857 times:

Thanks for the hard numbers FredT.

I would emphasise that we were visual and Loc established in the three glideslope experiments I mentioned earlier!!

Regards - musang


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 17, posted (4 years 7 months 5 days 10 hours ago) and read 3733 times:

Quoting CosmicCruiser (Reply 15):
Quoting tdscanuck (Reply 11):
You can be on the final approach coarse without engaging LOC. So it's possible for G/S to engage without LOC engaging and still meet the requirement.

hey tdscanuk, I'm not sure I follow that. Are you saying you can be flying the loc but not tracking it?

"LOC" (in capitals and in the context of "engagement") is an autoflight mode. You can be flying on the localizer course ("on the loc") without the LOC mode engaging on the autoflight system (either to drive the flight director or the autopilot). It's like flying an ILS on an aircraft that doesn't have an autopilot or flight director. You're flying the proscribed course (and flying the glideslope, or not) but haven't actually engaged the relevant flight control modes.

I'm not aware that this happens as a part of regular operation, or even ever, but it's physically possible with some aircraft.

Tom.


User currently offlinePGNCS From United States of America, joined Apr 2007, 2833 posts, RR: 45
Reply 18, posted (4 years 7 months 5 days 9 hours ago) and read 3727 times:

Quoting Fly2HMO (Reply 2):
You MUST intercept the LOC before the G/S otherwise the A/P will not follow the G/S.

That is not true in all aircraft, most notably Boeing aircraft. Different aircraft handle this in different ways. There are techniques and procedures used to prevent this from happening. On Boeing aircraft I always arm LOC until the course is alive to prevent the capture of the GS first, especially when being very close in and at or close to the GS Intercept Altitude. Once the LOC is active I arm the approach, otherwise you get into the scenario CosmicCruiser was discussing where you can descend on the GS without lateral course guidance, which is a very bad move.

Quoting jetlagged (Reply 3):
While it is a good idea to intercept the LOC before the G/S not doing so will not prevent the G/S capturing, at least not in every aircraft.

  

Quoting Fly2HMO (Reply 4):
Judging from the OP's other posts, I'm assuming this is a flightsim question in which case you do need to have the LOC acquired before the G/S.

I am talking about the real aircraft; I have never used MS FS. If all aircraft behave this way in FS, then FS is incorrect.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 19, posted (4 years 7 months 5 days 8 hours ago) and read 3695 times:

Quoting PGNCS (Reply 18):
That is not true in all aircraft, most notably Boeing aircraft.

It's an option on Boeing's. The default is to allow G/S to engage without LOC engaged (they'll both arm at the same time if you push the APP button but they can engage independantly), but you can select the option to inhibit G/S engage until LOC engages. Under the default configuration, to implement the same function, you just hit the LOC button instead of APP...that arms only LOC, while the pitch mode remains in whatever you had before. Then you're free to engage G/S whenever you want, later, by hitting APP.

Tom.


User currently offlinePGNCS From United States of America, joined Apr 2007, 2833 posts, RR: 45
Reply 20, posted (4 years 7 months 5 days 5 hours ago) and read 3664 times:

Quoting tdscanuck (Reply 19):
Quoting PGNCS (Reply 18):
That is not true in all aircraft, most notably Boeing aircraft.

It's an option on Boeing's. The default is to allow G/S to engage without LOC engaged (they'll both arm at the same time if you push the APP button but they can engage independantly), but you can select the option to inhibit G/S engage until LOC engages. Under the default configuration, to implement the same function, you just hit the LOC button instead of APP...that arms only LOC, while the pitch mode remains in whatever you had before. Then you're free to engage G/S whenever you want, later, by hitting APP.

Tom.

Thanks Tom. I can only speak as a pilot; the (multiple) operators I have worked for apparently selected the default option. I described in my earlier reply 18 the exact technique you have explained when in Boeing aircraft. To prevent the GS from capturing first, I select LOC and once the LOC is captured, arm the approach. The operational key is to note when the GS comes alive that the approach is armed, and if not, why not? It's all about the FMA.

BTW, I don't like this feature of Boeing's. Personal preference.


User currently offlineMir From United States of America, joined Jan 2004, 21678 posts, RR: 55
Reply 21, posted (4 years 7 months 5 days 5 hours ago) and read 3654 times:

Quoting CosmicCruiser (Reply 15):
Are you saying you can be flying the loc but not tracking it?

Yeah, for instance if the approach fixes are loaded in the FMS and you're flying with FMS as your nav source. If you don't switch over to NAV, you'll fly along the localizer, but your track will be based off of where the FMS thinks the line connecting the fixes is, and not where the localizer is (though the two should be quite close).

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 22, posted (4 years 7 months 5 days 4 hours ago) and read 3651 times:

Yes I'm aware of this we do it from time to time especially if it's a hard turn to final. FMS will round out the turn much better than LOC armed will. I just didn't follow his statement since I was talking about not descending on the g/s prior to loc established where I didn't really mean LOC armed. I could be hand flying using raw data but still need to be within a dot of centerline. Just my misunderstanding.

User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 23, posted (4 years 7 months 5 days 2 hours ago) and read 3634 times:

Quoting PGNCS (Reply 18):
That is not true in all aircraft,

I know but the day I made that post I was getting annoyed at the glitches I was getting so I didn't bother saying that was only true for FS and some real world planes.


User currently offlinePGNCS From United States of America, joined Apr 2007, 2833 posts, RR: 45
Reply 24, posted (4 years 7 months 4 days 13 hours ago) and read 3560 times:

Quoting Fly2HMO (Reply 23):
Quoting PGNCS (Reply 18):
That is not true in all aircraft,

I know but the day I made that post I was getting annoyed at the glitches I was getting so I didn't bother saying that was only true for FS and some real world planes.

No worries!   Just trying to make sure nobody has any misconceptions.

What is up with all the glitches, BTW?


User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 25, posted (4 years 7 months 4 days 13 hours ago) and read 3553 times:

Quoting PGNCS (Reply 24):
What is up with all the glitches, BTW?

Heck if I know. Every complaint I make over on Site Related falls on deaf ears so I don't even bother anymore  


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