Airbus: Cat III Single or Cat III Dual... what's the difference?

[FlyKingfisher]

I believe that pilots flying Airbuses use Cat III Single (Autopilot1 engaged) while flying in good weather and visibility, but Cat III Dual (Autopilot1 & 2 engaged) is deployed only in poor weather and visibility conditions. I read somewhere that deploying both autopilots on an Airbus engages 'Autoland'. What exactly is Autoland and how does a Cat III Single approach differ from a Cat III Dual approach? Any Airbus pilots here?

[Talkdownman]

Any Airbus pilots here?
Probably not in Spectators Balcony (Spotters Corner) Forum! This is much too difficult for spectators and spotters! Why not try Flight Deck Forums? You might find some Airbus pilots there...

[FlyKingfisher]

Probably not in Spectators Balcony (Spotters Corner) Forum! This is much too difficult for spectators and spotters! Why not try Flight Deck Forums? You might find some Airbus pilots there...

RightO. Thanks.

[avionic type]

I cannot speak about the Airbus ,but a simple answer to Cat 2 and Cat 3 is Cat 2 the a/pilot will controle in all axis as a normal a/p and will follow the Flight Management System ,compass demands,plus speed via auto throttle and radio demands ie glide slope and ILS but cannot be used to land the plane automaticly in auto pilot A OR B but for full Autoland BOTH autopilot channels must be engaged to land the a/c "Hands Off" in bad visability company policy dictates minimum visibility . they all work should 1 channel disagree with the others and on modern a/c there are 4 the odd one out will automaticly be disconnected and the system will be down graded. Must admit it has been quite a while since I worked on the systems but imagine it is the same now with more bits added:hmm::hmm:

[Big_Mach]

There are 2 fully functioning independent autopilots on the A320. If the captain is flying, he engages AP1 when he wants the aircraft to fly itself; the first officer would engage AP2 when he is flying. For most phases of flight, both cannot be engaged at the same time - pressing the other button would just switch systems. When flying the approach phase it is possible to engage both autopilots and then the aircraft is capable of an autoland.

The category of ILS (Instrument Landing System) approach - CAT I, II or III - is dependent on visibilty and cloud base and have different heights at which you need to see the runway before continuing to land. A high percentage of approaches are flown down to CATI minima before deciding whether to continue and land manually or "go-around".
If the weather drops below a certain value CAT II and CAT III approaches will be flown. Some airports and some aircraft are only capable of CATII approaches; the airbus can fly a CATIII approach. Basically this means the aircraft can get closer to the runway before you need to see it or the lights to continue and land.

In our company, if we are using the autopilot, we do engage both of them even for a CATI approach, even though only one is needed. This is to do with redundancy - if one AP suddenly fails the other one is there to take over instantaneously. When the flying pilot is happy and ready he will disengage them both and continue flying manually.

We can also do practice autolands - to keep ourselves current for when we need to do one for real. In this case, even on a bright blue day, both autopilots will be engaged on the approach and, even though it is a CATI approach, the aircraft will fly and land itself.

It is a bit confusing but basically CAT I means one autopilot and a manual landing, CAT II or III means two autopilots and an autoland.

[FlyKingfisher]

@Big Mach: Thank you very much for your reply.

What exactly is Autoland?

I'm a flight simmer and my aircraft of choice is the A320. During and ILS approach, when only one A/P is engaged, the PFD shows "Cat III Single" when both A/Ps are engaged it shows "Cat III Dual".

During Autoland does the a/c select flaps, speed, etc. by itself? Even when single A/P is engaged the a/c still lands itself by intercepting the LLZ and G/S (by engaging LOC and APR buttons). So what's the difference when both are engaged? The only difference, I reckon is, as you said, to be safe in case one A/P suddenly fails. So, I'm still confused about Autoland...

[Big_Mach]

An autoland is, as it suggests, when the aircraft automatically lands itself. By that I mean it will fly down the glideslope and along the localiser and once it senses it is over the threshold via the radalt, it will perform the flare and roll-out itself. Some aircraft are even more sophisticated (the A320 is one) and will reduce thrust and steer down the centre-line once on the runway - all automatically with no input from either pilot.

The pilots are still responsible for configuration changes - gear and flaps, deploying the thrust reversers on landing, and braking if the autobrakes don't work. We can also control the speed at which we fly because the aircraft doesn't know, for example, that it is getting too close to the aeroplane in front and therefore needs to slow down. However if we don't need to fly a requested speed from ATC we can let the aeroplane decide when it wants to slow down (flying over the 'D in a circle' to activate the approach) as long as we are in "managed speed" (i.e. the speed knob has been pushed, not pulled so there is no speed showing in the MCP window).
It will then slow down until it gets to "Green Dot", "S" speed or "F" speed which are minimum speeds it can fly at without having to extend slats or flaps (these speeds are on the left of the PFD on the speedtape). It won't go below these speeds because otherwise the aircraft might stall, so it waits until we extend the flaps before reducing speed further. If there is a speed constraint in the FMGC - either part of the procedure or because we put it there - it will adhere to that as well.

I have to admit I did give you a bit of duff information earlier in that you can do a CAT III single approach on 1 autopilot, which is why you are getting that on your flightsim. But if the aircraft and airport are adequate we will always fly the highest category approach available for an autoland (CAT III dual). This means that when it is really foggy, we don't have to be able to see anything and the aircraft can continue to land.
Also, if you are cleared for the approach, you don't have to select LOC and APPR; APPR arms the localiser as well as the glideslope. You only need to select LOC if you just want the aircraft to follow the localiser and not to capture the glide - maybe you haven't been cleared to descend by ATC or you are just doing a localiser only approach!

[FlyKingfisher]

Ok, I now understand what Autoland is.

An autoland is, as it suggests, when the aircraft automatically lands itself. By that I mean it will fly down the glideslope and along the localiser and once it senses it is over the threshold via the radalt, it will perform the flare and roll-out itself. Some aircraft are even more sophisticated (the A320 is one) and will reduce thrust and steer down the centre-line once on the runway - all automatically with no input from either pilot.

Since I now understand Autoland (both A/Ps engaged) I now know I've flown these several times. But I don't think thrust is reduced in flightsim automatically even during Autoland because I can hear the aural "retard retard" when the aircraft is 20 ft above the runway. I must check this out though.

We can also control the speed at which we fly because the aircraft doesn't know, for example, that it is getting too close to the aeroplane in front and therefore needs to slow down. However if we don't need to fly a requested speed from ATC we can let the aeroplane decide when it wants to slow down (flying over the 'D in a circle' to activate the approach) as long as we are in "managed speed" (i.e. the speed knob has been pushed, not pulled so there is no speed showing in the MCP window).
It will then slow down until it gets to "Green Dot", "S" speed or "F" speed which are minimum speeds it can fly at without having to extend slats or flaps (these speeds are on the left of the PFD on the speedtape). It won't go below these speeds because otherwise the aircraft might stall, so it waits until we extend the flaps before reducing speed further. If there is a speed constraint in the FMGC - either part of the procedure or because we put it there - it will adhere to that as well.

I haven't tried flying an approach with the speeds on auto. I always "manage" my speed to slow down well in advance so that I'm not too fast when I'm cleared for the approach. I must try this "D in a circle" as well. Let's see what happens.

Also, if you are cleared for the approach, you don't have to select LOC and APPR; APPR arms the localiser as well as the glideslope. You only need to select LOC if you just want the aircraft to follow the localiser and not to capture the glide - maybe you haven't been cleared to descend by ATC or you are just doing a localiser only approach!

The LLZ aligns the aircraft with the runway centreline while G/S makes the aircraft descend... am I right? Does this mean that on a 'LLZ only' approach the PF has to manually initiate descent? This means that the rate of descent also has to controlled by the pilot (similar to a visual approach)... is it not?

[Right Way Up]

Flykingfisher,

On an autoland the thrust is automatically reduced to idle. The retard call is a reminder to move the thrust levers to idle as on most A320s you will not get ground spoilers and thence autobrakes to operate.

[FlyKingfisher]

@Right Way Up: I get your point.

[Big_Mach]

Regarding LOC and APPR: On some aircraft (eg B737) you have to arm both LOC and G/S but on the airbus it assumes that you want to follow the localiser if you want to descend on the glideslope, therefore by pushing APPR you are arming both.
The way you were doing it isn't wrong, just more work! In fact generally I will press LOC only until I have definitely been cleared to descend on the glideslope as well. So if I am cleared for the approach I press APPR; if I am only cleared to intercept the localiser I press LOC (and then when ATC later clear me on the glide I press APPR).

Regarding localiser only approaches, yes, you have to initiate the descent. Like I said on your other post, we do that at 0.3DME before the descent point because of inertia. Again this is where the TRK/FPA button is a beauty. At 0.3DME before top of drop, we dial up -3.0 (3 degress down, or whatever the glidepath angle is) in the vertical speed window, pull the V/S knob and the aircraft will descend at 3 degrees. You can do it in V/S but this is much easier.
If you have approach plates to hand, there should be a box somewhere which shows height vs distance to be on the nominal glide. So compare your DME reading vs your height and make corrections to get yourself back on the glide. E.g. at 5DME you should be at 1500ft AGL, 4DME 1200ft etc

[FlyKingfisher]

I read somewhere that Airbus pilots arm the LOC switch first and then arm the APR switch once the LLZ is intercepted. My information may be incorrect. But this is what I do.

On flightsim, you are cleared for the approach, that's it. You are then asked to contact the tower. So, LLZ and G/S interception take place between the time approach clears you and once you are fully established on the LLZ and G/S, then tower says: "cleared to land". You are not cleared separately for the LLZ or G/S on flightsim.

I have not come across ATC saying: "cleared to decend on the G/S". I used to listen to a/c chatter a lot, one of the things I've heard go something like this: "Kingfisher 316, you are 23 miles north-west, cleared for ILS approach RWY27, turn right heading 250 to intercept localiser, maintain 3,700 until established on the localiser, contact tower on 118.1, report established."

[Big_Mach]

Mate, it doesn't really matter if you push LOC first or not, but you don't have to. If you push APPR you get LOC and G/S in blue underneath whatever are your active modes (probably ALT and HEADING in your case), meaning both are armed. The airbus manual says "The pilot arms the (ILS) APPR mode (LOC and G/S in blue on the FMA) by pushing the APPR pushbutton on the FCU" [FCOM 1.22.30.51]

Flightsim is not the real world, and is based on American r/t. In the UK it used to be standard to be cleared onto the localiser first, report established, then get cleared on the glideslope. This now thankfully has changed so you can descend once established without having to report it.
The reasons being are historical, but I believe someone once descended on the glideslope without being established on the localiser and probably flew into high ground.
Occasionally, mostly at airports with parallel runways, you can still get cleared for the localiser first. This is because if you started to descend you might trigger a TCAS event.
So for these reasons, for me personally, if I am only cleared on the localiser I just push LOC. If I am cleared for the approach I push APPR. Other people, when cleared for just the localiser but are expecting the glideslope later, will push APPR straight away.
Both ways are correct and ultimately it's not worth arguing over whether you push one or two buttons!

[FlyKingfisher]

@Big Mach: Right. All doubts cleared. :ok: Thank you very much for your time and effort. I appreciate it.

PS. Just FYI: I have installed EditVoicePack which enables me to change the default (American) phraseology to ICAO standards - decimal instead of point :)

[condorbaaz]

Cleared for Approach , normally means cleared to descent on ILS, if G/S isavbl.
Else you will be specifically told " Cleared for Localizer Only Approach"

[TaiYiCor]

In reply to you question, what is the difference btn CAT3SINGLE and CAT3DUAL. THE a320 has two auto flight channels, each with different components affecting it. These components can be the Flight Director(s), ATHR (auto trust), AP (autopilot(s)), RA (radioaltimeter), FMGC etc, as well as flight controls, hydraulic systems, electrical systems, DMC (DIgital Management Computer(s)), FMGC etc.

These components play a role when it comes to each channel's operation. When one system-component becomes inoperative the entire channel drives off line for a full integrity auto land. This is when the integrity of the entire system is affected. This is what we call capability degradation because we can still autoland but with only one channel without the other complementing the operation and backing up. When one channel is off line because its components are affected, then we are operating an approach in CAT 3 SINGLE status. When no component is affecting any channel, then we are operating under CAT 3 DUAL because both channels are online.

When we have both channels on line (CAT 3 DUAL) then the autopilot as a whole can afford the failure of a component that affects a channel without compromising the integrity of the system below 100' RA (Alert Height). When we have only one channel online (CAT 3 SINGLE), then the autopilot as a whole system cannot afford the failure of a component that affect a channel without compromising the integrity of the system at any point during the approach and downgrades the airplane capability to CAT 2 or even CAT 1 (depending on the importance of the component affected or the number of components affected). Back to CAT 3 DUAL, when a component that affects a AP channel fails, the system becomes CAT 3 SINGLE unless, UNLESS we have flown down past the 100' RA mark (Alert Height), below which the AP system can afford losing any channel-affecting component without downgrading the airplane capability. That is, in CAT 3 DUAL, you will NOT see or hear (triple click) a capability degradation should ANY failure occur, including an engine failure or an autopilot failure.

In short, what CAT 3 DUAL does for you that CAT 3 SINGLE does not do, is it guarantees you an auto land if you have a failure in the most critical region of a LVO ILS approach (below 100' RA)

[Driver 170]

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http://youtu.be/-Zq3HOOkp8w