Airbus : Keeping discontinuity in SEC FPLAN for circling approach

[alexious85]

Hello
Airbus in its FCTM (PR-NP-SOP-190-GUI ) , with regard to preparing a Circling approach states that after you insert the intended landing rwy in the SEC F-PLN , you keep the discontinuity .
Can you think for what reason Airbus suggests to keep the discontinuity?
Thank you for your inputs .

[Douglas Bahada]

Fplan sequencing?

[alexious85]

I don't see how that helps. When you activate the SEC FPLAN , all the waypoints from the active ( at the time of creation ) will exist . You create the secondary by copying the active and making lateral revision at destination to change the rwy .

[Roj approved]

. You create the secondary by copying the active and making lateral revision at destination to change the rwy .

You can make that Lateral Revision anywhere along the route.

Example 1: IMMEDIATE RETURN TO LAND

Copy ACTIVE FLPN

LAT REV at last WPT on the SID, or first WPT on the route (your choice)

New Dest

New RWY and Approach

There will now be a LAT DISC between last WPT and IAF on the new Approach.

When you activate, you can delete last route WPT which will give you PPOS then a LAT DISC then the Approach, or DCT to IAF, or DCT IAF Radial IN. This will give you reasonable accurate track miles without having to finger bang the box.

[compressor stall]

ROJ - for the circle, the procedure is to make LAT REV on the DEST and insert LDG RWY only.

Alexious - It may be done partly to reduce the likelihood of wanting to fly the Visual circuit in NAV but it does get you the CF on the landing runway.

Remember, the primary navigation instrument in a circling approach is the window.

[Roj approved]

oops, should have read the OP's post. sorry

So when you activate the SEC FLPN you will be in TRK with a LAT DISC. So no NAV, but you will get offset distance from the inbound track from the CF so you can judge your spacing.

[8314]

Wouldn't your flight plan for the instrument approach and your landing rwy both end on the extended centerline. So you would have to define the circling direction with a self tailored waypoint.

[Nick 1]

The reason you activate the second flp with the other runway , in a circling , is because you will have the
Mini GS calculation for the landing runway , so there is no need for a sequenced route .

[alexious85]

Nick I suspect the reason Airbus is emphatic about keeping the discontinuity is different than simply a matter of not needing to because you end up getting the GS mini function . In the FCTM is states to keep the discontinuity so you don’t really see the emphasis much . In a 2011 article in the Airbus FAST magazine about circling approaches it states this again and is emphatic about it ( it has it in bold letters ) . When Airbus is emphatic about something is more of a tactical / system related reason. It is more actually from this article that I started asking

[alexious85]

Ok I found the answer in THALES FMGS manual . Its not what we imagined .
Here it is .
I could not imagine that the logic would work this way . It is a bit confusing but I can spot a contradiction of this logic based on the sequencing rules of DCS-22 20-30-10-5 P3/12 .
https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/1080x922/sec_fplan_disc_bb00f1000a5a12b26a7462704fda6703d1dd8ad5.jpg

[grrowler]

From memory, if you clear the discon it will/may sequence onto the MAP as you pass abeam threshold on down wind, which is not ideal.

[compressor stall]

Good find Alexious. You never stop learning.

[CaptainMongo]

Here is a link to the safety first article alexious85 mentioned:

https://safetyfirst.airbus.com/app/themes/mh_newsdesk/documents/archives/the-circling-approach.pdf

Things we lose by keeping the discon and not extending the runway (DIR TO) are an exact numerical offset and V/DEV doughnut. Things that definitely enhance SA during the visual maneuver.

I’m not arguing against the logic that the box may “eat” the runway when passing abeam, only wondering if there is a workaround. I guess the PM could extend the runway when passing abeam the landing threshold and at least have use of the doughnut (to late to correct any offset by that point)

[alexious85]

Interesting fact :
the piece of information ( attached above ) from the THALES FMGS manual is for version 2 revision 1. For version 2 revision 2 , the exact same page does not mention the fact that you s loose the runway under these conditions so all these might not be absolute .

[Qwark]

The FCOM describes the waypoint sequencing logic within the AUTO FLIGHT system chapter. To quote my company manual;
"In HDG/TRK mode, a flight plan waypoint is sequenced if the following conditions are met;
- The waypoint is behind the aircraft and the aircraft is less than 7NM from it
- The angle between the current aircraft track and the track of the leg is less than 90 deg"

If the flightplan discontinuity is removed then depending on the final course of the instrument approach, the runway waypoint might sequence. This however would be quite a rare situation. Airbus SOP usually is designed to consider all "worst case" scenarios.

On a slightly different topic, if you have conducted circuits in the simulator, the same situation can happen if deleting the FPLAN DISCONTINUITY before take-off. On the upwind or crosswind leg the runway will sometimes sequence as you satisfy the sequencing logic. Once you turn downwind the aircraft track no longer meets the sequencing logic.

Once the runway is sequenced the ND no longer shows X/TRK indications and the "YOYO" is no longer useful.

[CaptainMongo]

The chief of our instructors checked this issue out in one of our simulators and this is what he said to me:

Everything worked just fine per the Manual
We did have a PPOS and discontinuity in the secondary flight plan
When we activate the SEC F-PLN all was fine, but shortly after the PPOS ate itself and the DISCON. We weren’t anywhere close to a base turn
The CF remained.
Abeam the runway we went (DIR TO the runway on the inbound course) and had good FD vertical and lateral guidance

[Alpine Flyer]

LH Group is starting to roll out circling FMS overlays now. Don't know how they work exactly on the airbus but I assume that you'd want to keep the circling in secondary, sequence together and activate secondary once you are sure you'll fly the circling. In case of a botched circling, the coding includes a balked landing as well. There are talks on prohibiting circling altogether for non-coded circlings. Most probably a boost to safety but somehow another skill no longer trained. (OTOH, circling approaches at published visibility minima are a strange proposition for jets manoeuvring at 150kts, especially given the few occasions to keep current.)