B737-800

[ElNull]

When APP mode is selected, why would Glide slope captures prior to Localizer? It was inhibited on old versions. Why boeing would allow it?

[16down2togo]

It's a customer option to have it either way on Boeing a/c.

[ElNull]

That does not answer my question actually. My question is why would boeing allow GS capture before localizer, why they changed it? What is the point of the GS being captured and the aircraft decend without being on localizer?

[Denti]

Never had that on any of the 800s i flew. It is not a question to Boeing, it is a question to your airline why they specifically bought that mode. It is not the standard approach mode, it is, as mentioned above, a customer option.

[RAT 5]

EINull: Memory is not so accurate, and you ask why Boeing changed it. Go back a little further, B757/767 perhaps even B732 and I believe GS before LOC was possible. I can't remember B733. Therefore there had been change in philosophy not to allow it as standard on NG's. Could that have been an FAA/JAA thing? Or just 'why would you want to?' VMC you can follow GS guidance off LOC safely, without being captured. IMC not good idea.

[Denti]

@RAT_5 we had that on older 733s, but changed the pin programming to get rid of that behaviour and subsequently they could only capture the GS if the LOC was already captured.

[stilton]

Never flown any 757 or 767 that would capture
the glide slope before loc capture

[B2N2]

Originally Posted by ElNull

When APP mode is selected, why would Glide slope captures prior to Localizer? It was inhibited on old versions. Why boeing would allow it?

Is this with a specific amount of LOC defiection required?
As in 1/2 scale deflection or less?
I doubt it would capture a GS while paralleling an inbound course and full scale selection on the LOC.
There’s got to be some approach logic at work here.

[RAT 5]

Thanks Stilton. I wonder if that too was an option. I can't remember, but from B732, B757/767, B733 one of them sure did.

[MarkerInbound]

I can say some 744s will and some won't.

[ElNull]

Originally Posted by B2N2

Is this with a specific amount of LOC defiection required?
As in 1/2 scale deflection or less?
I doubt it would capture a GS while paralleling an inbound course and full scale selection on the LOC.
There’s got to be some approach logic at work here.

I just can't find it in FCOM and highly doubt if the LOC should be 1/2 scale for GS capture.

[engine out]

The 767 could that I flew. Often use this mode during day VMC operations, especially when parallel runway operations in use. Definitely don’t do in IMC

[NSEU]

All our 744s (incl. -ERs), all our 767-300s, and our later 737-800s allowed either loc or g/s to capture first.

What happens on aircraft types which inhibit G/S capture prior to Loc capture and the Loc ground station is inop? Surely being able to use one part of the ILS is better than none?
Also, on some aircraft, in flight, TOGA is armed when G/S is captured. What happens to aircraft which won't let you arm TOGA because of this "Loc first" rule?

[Vessbot]

Then you fly a different approach that has the required ground stations operative. This might even have to be at a different airport!

At least in the US, the rule is that you cannot descend below the assigned altitude until established on a published segment of the approach. And the ATC phraseology to back this up on every approach is "maintain XXXX feet until established, cleared for the..." I assume that the rest of the developed world is the same. (Am I right or wrong in this?) In your scenario, what segment are you established on?

There is no such thing as a glideslope-only approach.

Also, on some aircraft, in flight, TOGA is armed when G/S is captured. What happens to aircraft which won't let you arm TOGA because of this "Loc first" rule?

Is this really true, or a lack of understanding of the systems on the part of the crew? I find it extremely hard to believe that a plane can't set goaround thrust without a glideslope. So what, that type cannot fly a non-precision or visual approach?

[NSEU]

Is this really true, or a lack of understanding of the systems on the part of the crew? I find it extremely hard to believe that a plane can't set goaround thrust without a glideslope. So what, that type cannot fly a non-precision or visual approach?

There is also a flap extension trigger in the thrust logic (on 747-400/767-300), but I'm just exploring the "what if" extremes.

If you can't extend the flaps and weather has suddenly turned marginal (or worse) at all alternates, etc, then why not use all the tools at your disposal?

And if you are in VMC and the "loc is broke", then what is the harm? Isn't it something nice to have? Of course, you still have raw data to guide you.

Sorry, I don't know the rules of the air... purely an former engineer's questions/ramblings... Thanks for your response.

[Vessbot]

Originally Posted by NSEU

There is also a flap extension trigger in the thrust logic (on 747-400/767-300), but I'm just exploring the "what if" extremes.

If you can't extend the flaps and weather has suddenly turned marginal (or worse) at all alternates, etc, then why not use all the tools at your disposal?

If the weather is down and the loc is broke, the approach in question is not a tool at your disposal. You must use another approach.

And if you are in VMC and the "loc is broke", then what is the harm? Isn't it something nice to have? Of course, you still have raw data to guide you.

Sorry, I don't know the rules of the air... purely an former engineer's questions/ramblings... Thanks for your response.

If you're in VMC and the autopilot must be on, there's always the vertical speed wheel. The ILS capturing logic, in my opinion, should be designed to meet the requirements of IMC, a much more critical flight regime. And those requirements are to capture the localizer before the glideslope. If you get a bad vector that puts you into the glideslope first, (an ATC error that is not extremely rare) and the crew doesn't catch it, you really don't want the airplane automatically descending below the cleared altitude. This is a must-have, and compromising it for a "nice to have" is not the way to go.

[oceancrosser]

Originally Posted by stilton

Never flown any 757 or 767 that would capture
the glide slope before loc capture

A lot of the older ones do. In my fleet on the 757s, if you plug your headsets in below the tiller, there is a good chance thic a/c will capture GS before LOC. If allowed to...

[Skyjob]

The G/S capture first tolerance is a CUSTOMER OPTION, on many fleets.

Many of us have flown aircraft where G/S was able to capture first;
Many of us have flown aircraft where G/S was NOT able to capture first...

As it is a customer option and not a manufacturing issue, perhaps the OP need to address this issue with the operator and/or owner instead of asking a question here which cannot be answered unless speaking on the Operator's behalf.

[ElNull]

Sky job: There are some other Operators that have this option yet their OM says to use VORLOC mode first then APP Mode which contradicts what you say. I hight doubt this option is a customer option on the NEW 737-800. It is probably by default (yet it is not mentioned in the FCOM). Anyway that is not why I started the thread here. Even if it is a customer option then what is the purpose of it? Why it is being offered by Boeing? After many replies here I still can't find a valid reason for the GS to be captured before Localizer. Have an answer for that?

Thank you!

[Avenger]

This is an "old chestnut" issue on B737 and precisely why most SOPS direct that the LOC be captured before APP is armed. To illustrate how problematic this is the attached shows a GS capture at Fl390!
https://www.atsb.gov.au/media/.../ao2013049_final.docx

Incase it won't open VH-VYE 25th Feb 2013