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captain420

When should you activate loc/appr button to intercept ILS localizer and gs?

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Is there a specific method on when to hit the loc/app button on a boeing 737/777/747 and airbus a318-a330 to capture the localizer and glideslope on an ILS runway?

 

Do you do it at the IAF/IAP or FAP/FAF? and do you intercept it at an angle or can you still capture it if you're goig straight towards it?

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Here's a screencap of the FSX Map with an approach into LFRZ from the East. The track initially at 90 degrees to the runway, followed by a 60 degree turn. Now 30 degrees off the glideslope we can activate the approach. A thirty degree turn onto final can be made by the AP nicely, but 45 degrees the extra latency in the AP can begin to introduce slalom.

computedapproaches.jpg

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Hi Aaron,

 

This is a bit type-specific and different aeroplanes will have different logic in the way the automatics will handle things, so the general health warning as always is to refer to the FCOM for the type you are flying to understand how the automatics in that particular aeroplane will work.

 

Generically, however: APP mode will result (eventually, provided the various conditions specific to that aeroplane are satisfied) in tracking the localizer and the glideslope; LOC will track the localizer only.

 

You should arm APP mode once you are on a closing heading to the localizer and cleared for the approach by ATC (ie you have permission to intercept and track the localizer and descend on the glideslope).

 

Bear in mind:

 

- Limitations of the localizer and glideslope (range, potential for false glideslopes etc)

- Limitations of the autoflight system on the particular aircraft you are flying

 

For instance, on the B747 once APP is engaged that is it in so far as that the only way to get out of LOC | GS is to either press TOGA or recycle the flight directors. If you were planning to do a circling approach to the other end of the runway, for instance, that would be a bad thing and so a more sensible thing to do would be to engage LOC only and follow the glideslope in VS until you reach your circling minima, at which point you can then go in to HDG SEL to commence the circling procedure.

 

If your aircraft is GPS equipped then you MAY be permitted to intercept a localiser direct from NAV mode (aircraft and airline policy permitting). However, there is a health warning associated with this in that it is possible, especially if you are approaching the localizer at a very shallow intercept angle, that you will end up paralleling the localizer in LNAV and never actually capture. This, obviously, is dangerous (especially if your aeroplane's automatics will capture the glide slope without having first captured the localizer. My personal recommendation is that it is safest to always capture from HDG SEL, ideally at around a 30-degree intercept angle, but that does not mean it cannot be done from NAV; just be aware of the traps and check the FMA carefully.

 

Some types will capture the glide slope prior to having first captured the localizer, some will not. Either way, remember the GS is only reliable within about 8 degrees of the localizer inbound course and if you are not on the localizer it is not guaranteed that the GS will be terrain/obstacle safe.

 

Generally you need to be below the glideslope to capture correctly; this is best practice and I would recommend you aim to do this at all times, particularly whilst you are still getting to grips with instrument flying, energy management and the automatics on your aircraft. It is possible to intercept the glide slope from above and sometimes this will be necessary due to ATC requirements, but it brings heightened risks. Check the type-specific procedure in your FCOM, and be aware of the dangers: see false glide slopes, high-energy approaches and unstable approaches...

 

Some airlines will have you engage all available autopilots when capturing the ILS, whether you are planning an autoland or not. Airbus SOP once cleared for the approach and on a closing heading to the localizer is to press APPR and engage the second autopilot (so you have, hopefully, CAT 3 DUAL and AP1 + 2 annunciated).

 

Hope that helps.

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its simple for the boeings. you will need to be under the glideslope and fly into it per se in order to capture it. As for the LOC, when you are at a maximum of 30 degrees intercept course, just press APP mode. the auropilot takes care of the rest.

 

If you want to know why i chose degrees as your max intercept angle, let me know or just PM me.

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Is there a specific method on when to hit the loc/app button on a boeing 737/777/747 and airbus a318-a330 to capture the localizer and glideslope on an ILS runway?

 

I just made a video about this, which you might find helpful. From a technical point of view you need to know that the localiser is about 10 degrees wide and 'visible' up to about 25 miles out. The GS is about 1.4 degrees high, usually slopes at 3 degrees and needs to be intercepted from below. But the procedures are different in each case, which is why you will find a unique chart for each approach at each airfield.

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