A 320 FMGS and visual approaches

[austra1998]

hi,

i just wanted to know if someone out there can tell me how accurate is the predictions of the fmgs on the vertical path during a visual approach!

i'm basically talking about the dounats that appear on the primary flight display.

i hope i explained properly....

thanks

austra

[Kirks gusset]

What! accurate to what? a visual approach is just that..visual, you see it, fly it and land it.. donuts? no idea, sure you don't mean a NPA using fPV?

[Norman Stanley Fletcher]

austra 1998 - ignore the previous post.

There is a limitation on the doughnuts (Brit spelling of donuts!) in a visual approach and they should not be considered reliable. My own preferred technique is to extend the centreline before turning onto the final approach. If it is marginal visibility or there is a real chance of a mis-identification of the runway due city features, long roads etc then I leave the autopilot in and push NAV on the FCU. (NAV thus appears armed in blue on the FMAs below TRK as I would be using TRK/FPA). That guarantees a turn onto the extended centreline - the first disaster of being locked onto somewhere other than the runway is thus avoided! In terms of vertical profile I have pre-entered the runway in use - say 'EGKK26L' or whatever in the prog page bearing/distance field. I then use 3xDist + elevation of the runway to get the correct position in space on a 3° slope. For example, at Gatwick (elev 200') at 4nm I would wish to be at 1400' (3x4+ 200'). For all practical purposes this will be good enough to get you very nicely in the right place at the right time. Hope that helps.

[Kirks gusset]

Austra, please don't get suckered into bad habits, if it's a visual approach you must be able to see the airport and landing runway, not getting the chance to "lock on" to the M6 or M42 is not an option. Now, I will admit that even when visual there can be environmental elements that can be misleading, and extending the c/l is making good use of the information available.. however,, as for the rest? yes we can all calculate a 3 degree glide based on elevation, but come on, dont cook up a home grown approach using the automatics, in fact, it's not legal. Next thing will be creating PBDs within Stars rather than sequencing and it all goes down hill from here. NSF, I am not saying your advice is technically incorrect, the a/c will happily do this, but what's the point if you want to fly a visual..

[CJ1234]

Actually Austra, though I appreciate NSF maybe far more experienced than I on the 'bus, I would be tempted to side with Kirks and largely ignore NSF's post.
If there's "marginal visibility" or "a real chance of a mis-identification of the runway due city features, long roads etc", go down the ILS, other instrument approach or divert- Kirks is right - if you can't see it, don't f about. The kind of procedure NSF has cited is reminiscent of an instrument NPA - not a visual approach.

1234

[Clandestino]

how accurate is the predictions of the fmgs on the vertical path during a visual approach?

Dunno.

Never used them for visual approach.

Always found the runway, crossed threshold at appropriate height and proper speed/config/power. And I don't think its something to brag about, its just part of the job. Gets easier with practice.

I assume that if I ever bothered to draw nice pattern on FMGS using PBD or (gasp) LAT/LONG, and put all nice CSTRs, it would guide me nicely to threshold but I never had an urge to see if it works that way. Actually my scan for visual approaches is the same for ATR, A320 and Dash8: runway-speed-power-runway-speed-power. ND or VNAV never came into play.

[guiones]

To answer your question, not personal technique.

The donut is accurate for its purpose of REFERENCE only IF you have your flight plan properly sequenced. If you do that, it will give you a very good general idea of your descent PATH to the RWY.

Now, nothing prevents you from inserting the ILS APPR for a visual approach if available for a more accurate reference.

A visual appr is just that, visual; but it is nice to have this tools available.

G

[guiones]

Norman, what limitation are you talking about, not flaming; your posts are very good, just curious.

I do agree with extending the centerline btw.

G

[Norman Stanley Fletcher]

Kirks Gusset and CJ1234 - no one is cooking up a home-brewed NPA approach. What I am suggesting is using all the information you have available to you rather than ignore critical information. You will no doubt be aware that countless visual approaches go wrong for one reason or another and numerous have ended up being peformed on the wrong runway or a road very similar to the runway. The problem is that visual approaches are often not performed in CAVOK conditions and that can lead to big snags. That being the case, it would seem prudent at all times in flight to use all the aids available to you. To completely ignore the plethora of information you have available to you in the cockpit of an Airbus is simply crass ineptitude. A visual approach is just that - a visual approach. That does not preclude the use of other sources of information to assist in the correct performing of that approach. The human brain, however, being limited in the way it is can frequently give incorrect guidance to its owner! As I said, there are so many examples of visual approaches that have gone wrong in tricky weather conditions (still legally 'visual') or when a road seems more attractive than the runway. The technique I have described is by no means the only way of doing it, but it does work every time. I do not really care what method you use to carry out a visual approach, but to open the door to unnecesary difficulty by failing to use all the resources available to you in any situation is just plain foolish.

[austra1998]

thanks norman

thank you....

your post was very usefull...

i have used the 3x method during the descent but i have never used it for the approach...yesterday i cross check it during the ils...and it was quite accurate!

only one thing made me wonder....i had 3 landings when the elevation of the runway was 80 feet...and it come down to be almost spot on ( maybe there was an error of 10 feet max) but when i tested it on an airport where the elevation was sea level, the error was much bigger....(80 feet) curiously enough!

any ideas why?

thanks
austra

[Elephant and Castle]

If the conditions are not ideal you should not attempt a visual approach period. I am also against heads down manipulation of the MCDU while maneuvering low level. It is certainly nice to avoid landing on a motorway but hitting another aircraft in the circuit is no fun either. In answer to your specific question though if the flight plan is sequenced and the landing runway is inserted in the MCDU the donut is accurate enough to bring you onto the PAPI quite nicely. Once you are on finals follow the PAPI and you will have no problems.

By all means use all resources available, tune the ILS, extend the centerline, use the miles to go, the distance to the runway,etc but above all on a visual approach look outside. Autopilot on, pushing bottons heads down absolutely not the the way to go at 1500 AGL.

[Jonty]

The doughnut works from the FMGS and the MCDU. What it tells you is purely based on what has been entered in the MCDU. On a GPS equipped aircraft, where the full approach procedure and the appropriate glide path has been entered in the MCDU, and the way points have been correctly sequenced, the the doughnut will be fairly accurate.

Any sort of map shift or an incorrect value entered into the MCDU will mean the doughnut is inaccurate. How inaccurate will depend on how big the error is!

During a visual approach it can be a useful source of reference, providing you keep in mind its limitations. But the visual picture and the PAPIs should always be the primary source.

Just for completeness, during a NPA, full scale deflection of the VDEV (brick) is 250ft, full scale deflection of the doughnut is 400ft.

[BOAC]

My ghast is completely flabbered

[Microburst2002]

_________________

[aca-98]

BOAC, love it!

[jet.man123]

Well, well, I guess Airbus have got it all wrong in their training material and BA also need to re-write their FTM on this one, for years the pilots must have been "foolish" and demonstrating " crass ineptitude" I'm shocked.. on the other hand maybe, just maybe, they did as Airbus say and " Keep it simple". Heads down during a visual is complete nonsense. It is irresponsible to promote procedures and practices outside of the recognised methods, keeping situationally aware and having aids as back-up is one thing, but actively supporting their use in an unrecognised way is another. Simple question..and answer.. If you don't practice it this way in the sim.. you don't do it in real life.. but we are all entitled to an opinion and this is not a bun fight, just a sharing of knowlegde and experience, N'est pas?

[jb5000]

Why is everyone saying/implying that using anything on the FMGC means that you are 'heads down' during a visual approach?

If the flight plan is properly arranged even before top of descent then it can be a really useful aid during the approach. The 'doughnut' is in clear view on the PFD!

[PENKO]

I'm with NSF on this one. It would be poor airmanship to ignore the wealth of information that Airbus is giving you. There is two of you in that cockpit. It takes three pushes of a button to get the info (extended centreline and dead accurate distance to go) right in front of you.

Of course it is completely wrong to fly a visual purely based on the ND, just as it is so easy to descend on the donut. As long as you don't fall into that trap, as long as your primary navigation is based on your situational awarensess of what is happening outside of the aircraft, then get the PNF to push those buttons. Again, as per NSF, why not make maximum use of all the information available.

[CJ1234]

Just to pick up on the last post:
"Poor airmanship"??
What a load of crap. Don't exaggerate.
It says nothing in my SOPs about doing the things NSF refers to. I'm not dismissing him, by reading his posts I see he has far more experience than I have. It might make things easier for some people - but I don't. And I have by now completed lots of successful, and by-the-book, visual approaches.

Careful what you say Penky!

1234

[Right Way Up]

"Poor airmanship"??

then

It says nothing in my SOPs about doing the things NSF refers to

CJ1234 I would suggest airmanship is not about what is written in the manuals but the common sense you employ to fly safer. Most ops manuals mention when on a visual approach to utilise any guidance available to backup what you are seeing. Theres been enough cockups flying visual approaches that it is always worth backing up your situational awareness.