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Flight path doesn't fit with the calculated CSTR in the FLP


Aldarean

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First of all I would like to apologize for my english. It's not my mother tongue.

So far, the departure with the Airbus A320 always worked well, but meanwhile sneak in with me again and again inconsistencies. I always use ATCPro / X for flying, as you can create good flight plans and the ATC also supports SID and STAR. However, it’s becoming more and more frequent that my flights differ from a good flight profile. For an example of my flight yesterday with the hope that I can get help here in this forum:

My flightplan from ENGM (Oslo) to EDDB (Berlin-Schönefeld) looked as follows:  

OKSAT L996 KELIN DCT SALLO UM44 KOGIM UM725 RODEP T208 GOLBO

The STAR was definitely something with GOLB ;) (can’t take a look at the moment) and a transition via LANUM. The flight plan I created was activated by ATCPro / X, so I had immediately a Company Route for the MCDU available. The flightplan looked wonderful, also the constraints (CSTR) corresponded to the charts of EDDB on runway 7L (ILS APPROACH). As CRZ ALT, FL330 was entered into the INIT page. Up to the CRZ ALT everything has also been good so far. After reaching the CRZ ALT, I have set a new FL380 cruise altitude on the PROG page of the MCDU since it was displayed as optimum (also PROG). Even before the T/D I started the Descent on the instruction of the ATC (first FL240, then FL160, FL110 etc. In each case, before reaching the instructed FL, a lower level was instructed.) The altitude instructions of the ATC should not take influence on the calculated descent path of the bus). However, I quickly noticed with a view to the ND that the V/S is not enough to bring the bus down before the point LANUM to the CSTR of 5000 feet, so I had to expedite the descent and even a holding fly (more a 360° rotation) to decrease the altitude. Nevertheless, the CSTR was marked with a purple star and marketed on the ND with a purple circle, although I could not keep the CSTR. When I finally was on 5000 feet I looked at the PROG page and noticed that I obviously flew more than 3000 feet below the calculated descent profile. How does that fit together? On the one hand, the CSTR calculated by the FMGS and the significantly different descent profile? Actually, the profile should adapt to the CSTR. I could at most imagine that the T/D with the change of the CRZ ALT from FL330 to FL380 not recalculated and therefore not further was moved forward. That would at least explain why I have flown in the DES several thousand feet under the profile.

Does anyone have an explanation? I will fly the route today with FL330 again and at the announcement of the ATC I will enter 5000 feet, instead of the hip height. Let's see if it then drops to WPT LANUM to the required 5000 feet. But theoretically, the altitude entered in the FCU does not matter and does not affect the descent profile as long as it is below the CRZ ALT or am I wrong at this point?

Greetings
Ludwig

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This, I think, is a 'quirk' of the Aerosoftbus...

 

On more than one occasion, I've begun drafting a post about this; but as it can be tricky to describe concisely, and because I've been hoping that the revised Airbus systems simulation developed for the A330 might improve it (this the primary reason I was keen to see the A318 Research Edition), I haven't addressed it before now.

 

Put simply - Managed Descent profiles tend to be pretty optimistic. This theory is perhaps borne-out by the fact the 'bus has an option for a 'sim-FO' to swing on the speedbrake all the way down. I really don't like this option, and have never enabled it - I was brought up in the school of the old ATC funny...

 

Quote

 

"Somebus 123 descend flight level two wun zeddo, be level by somewaypoint."

"Unable that rate, Somebus 123."

"Somebus 123, don't you have speedbrakes?"

"Yes, but they're for my mistakes, not yours!"

 

 

I also run Pro-ATC/X, and spent weeks examining this issue because it can be incredibly frustrating (my first flights to LFMN and LEPA in the 'bus had me arriving at IAF thousands of feet above where I needed to be). Actually, the more I flew, the more I realised that Pro-ATC/X's instructions don't really contribute to difficulties...

 

When Pro-ATC/X instructs you to descend before the FMGS' computed TOD, and you select Managed Descent (by entering the descent altitude and pushing the selector knob), the A/C will begin to descend at 1000fpm - until (if) it intersects the computed descent profile. This much is accurate Airbus behaviour. The problems, in my opinion (from the many hours of flight testing I performed), tend to begin when the 'bus is on VNAV profile; it can be 'upset' by constraints in certain circumstances - but I hasten to add, not all.

 

I was pleased to find, for example, that flying [from cruise ~FL300] into EHAM 18R via LAMS1A with SUG3B transition, that VNAV works pretty well. I can use Managed Descent right from TOD, and arrive at IAF/FAP at the right altitude and speed. It's tight, and it helps if you're quick to select Flaps Conf 1 once you're in the 220kt speed constraint to give a bit more drag for the following descents, but it does work.

 

On the other hand, if you fly [from cruise ~FL360] to LEPA 24R via TOLS1P and POS/MUROS, Managed Descent will get you nowhere near the airport [much laughter in the tower as you declare missed approach about 20 miles out!]. Pro-ATC/X can be taken out of the equation - enter the platform height for ILS 24R (3000') at TOD, and push the knob. The 'bus will initially follow its computed descent profile, but as soon as it reaches a speed constraint, it comes 'unstuck' - the a/c pitches-up to reduce speed, and the VNAV profile drops away, far enough that you haven't any hope of recovering it without the speedbrake.

 

This, I don't like. In effect, the FMGS' computed profile requires speedbrake. The speedbrake is literally wasting airframe energy which you've already paid for with fuel - not efficient, not very Airbus.

 

Of course it's a complicated issue, made more difficult by the fact descent/deceleration performance is affected by winds, which [ATM] the FMGS can't be clued-in about.

 

In an ideal world, we'd be able to program winds, and with that information and the a/c GW, have the FMGS compute a VNAV profile which it can follow without speedbrake in most circumstances [excepting those with very tight combinations of constraints]; while that's not possible, I would've preferred the simulated FMGS to fly to 'pseudo constraints' whereby it allowed some distance for a reduced ROD which would enable it to meet speed constraints without loading-up VDEV. Unfortunately, what we've got is an [IMO a bit dirty] auto-speedbrake.

 

Hopefully, with the redesigned Airbus systems which Aerosoft have been working on, Managed Descent will be more workable. In the meantime, there are a couple of ways you can workaround it...

 

First, make more use of Open Descent (enter new altitude and pull the selector) - this isn't ideal, as it tends to turn the 'bus into a falling rock. If there are drinks trolleys loose in the aisle when you pull it at TOD, you might find one bursting through the flight deck door! Also, you tend to get lower than you need to be faster than you need to - also not ideal because making-up distance before further descents with level flight at lower levels (in thicker air) costs fuel (and in extreme cases is quite obvisouly silly - you spend a while counting fish in the Mediterranean if you use Open Descent from TOD to LEPA).

 

The other option is to use mixed modes for descent and constantly monitor the ND to check you're going to reach levels at about the right place/time. For instance, flying into Aerosoft's beautiful Genoa (I can't remember the actual STAR, but heading for the ILS end on a route from EGCC), when Pro-ATC/X issues my first descent instruction I select something like -2200/-2400fpm (Open Descent would be more like -3000-4000 fpm) and monitor the ND. I can relax the VS approaching the FL10/250kt speed limit in order to meet it, and from there it's possible to continue with Managed Descent (although again, you'll want to get the flaps out as soon as you're able - particularly if you've removed the INTERCEPT waypoint from the flight plan so that the a/c turns left to establish (which Pro-ATC/X expects) rather than looping around to the right (which will cause you to get nagged by Pro-ATC/X)).

 

It's a bit of a PITA, but I love the Aerosoftbus so much that I can put up with working around it; but again, I would love to find that the new systems simulation improves it.

 

Sorry to have written such a long post (I still feel to accurately describe difficulties and the circumstances in which they arise, I would need to write more - probably my failing!); but I hope it's been some way useful.

 

Best regards,

 

Rob.

 

P.S. I'm not certain, but I think using Cost Index 50 seems to help. For a long time I was flying with CI 15 (more realistic (but still generous) for a scheduled flight), and found descents to be that much more tricky. [This one is much more a hunch than result of strict flight testing.]

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Hello Rob,

thanks for the really very detailed answer. That sounds very interesting in any case and I can also only hope that Aerosoft will improve. For as I understood you, you are pretty broken without the standard FO and have to do everything alone. The Speedbrakes is, of course, not a permanent solution either. Then I will test it in the next flights and try different CRZ ALT. Ultimately, it is enough to monitor everything about the ND. I hope. The CI will also be testet. I will post my results in this threat to get closer and to help find a better soloution. 

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Can you see that??? After changing FL330 to FL380 the bus tries to descent between just two points from FL300 to 5000 feet. After calculating the FPL at the gate everything was great. 

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2 hours ago, Aldarean said:

Can you see that??? After changing FL330 to FL380 the bus tries to descent between just two points from FL300 to 5000 feet. After calculating the FPL at the gate everything was great. 

Please login to display this image.

Why are you telling it to be 315kts at 5000ft? I mean, you're currently doing 265kts right? So what descent speed is set? Is your speed managed or selected? Either way, you need to think more about the speed you will be doing at different stages of the arrival, whether that's the cause of it or not.

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@Aldarean That looks particularly optimistic, even by Aerosoftbus computed profile standards! As @HAL-9000 suggests, the speeds look as peculiar as the altitudes; but it's pretty difficult to follow exactly what's going on from the one screenshot. You say this incredible descent profile is only computed after a change in cruise level to FL380 - have you noticed what happens to the computed ToD when you begin the climb to the new cruise level? It appears that somehow you're still at cruise level in the STAR, and there's no to where ToD is supposed to be.

 

I have adjusted my cruise level in flight before now (trying to find smooth air), but I don't recall any such weird recalculation - I've only ever had to help it through tricky constraints as described previously. I'll have another look at it here (perhaps on the route you're flying as well as on one from my roster); but maybe you could show us the ND and MCDU F-PLAN page (at the top (showing last/next) as well as on the STAR) before you make the additional climb, and after?

 

All the best,

 

Rob.

 

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Following a private message from @Aldarean questioning why I'm talking about speed, I want to point out that the reason I mention speed is that, if the aircraft expects you to be doing 315kts during the descent and you are only doing 265kts, you're obviously going to end up very high due to the lack profile drag, and this might be the cause of your woes.

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Yes, the speed is very important, I agree with you and think we are talking at cross purposes. Let me try to explain ^_^. The recordings were made quite early. The picture from the previous post still during the CLB. I've seen no point in flipping the entire circuit again to check if the FLP changes when I adjust the CRZ ALT. I have a parrallel post in the german forum and give the pictures from there also gladly here. It is good to see that at WPT RADEL the ALT is provided with FL155. It can therefore be assumed that up to then a reasonable DES has taken place. The picture from the last post shows my plane during the CLB below FL100 (265ktn) and after entering the new CRZ ALT of FL380.
The big speed value (0ktn in Picture 1) shows only the current Speed (I'm Standing at the gate) not the Speed at this WPT so we don't know how high the Speed value at this wpt is. But it is also clear to see on the FPL that I am not flying any of the displayed waypoints on the FPL, since none are displayed white. I scrolled to this Point in the FLP.

 

Picture  1: FLP at the gate with calculated FL330:

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Picture 2: changing FL330 to FL 380 during the CLB (265Ktn) a few minutes earlier.

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I did nothing more than to change FL330 to FL380

 

I can't take any other Pictures before tomorrow evening.

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Vor 1 Stunde, HAL-9000 sagte:

Okay, if you're happy I'm happy.

Should I understand why you think I am happy? Obviously I have not been able to present the problem clearly enough. Probably due to the language barrier. I did not want to attack anyone personally

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1 hour ago, Aldarean said:

Should I understand why you think I am happy? Obviously I have not been able to present the problem clearly enough. Probably due to the language barrier. I did not want to attack anyone personally

You're happy that speed isn't causing you any descent path issues. Therefore I'm happy. 

 

Im sure you realise that I don't work for aerosoft, that I'm nothing more than a stranger on the Internet trying to help out another stranger on the Internet. But I don't have enough info to help you and you are confident that my initial questions about speed are irrelevant, so there's nothing more I can do. Therefore I am happy. 

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I see that I was wrong about the speed anderen what I weiter in post Nbr. 8. I am sorry about that. The speed values HAL-9000 means are compleat wrong. My fault. But then Problem ist solved so far. I didn't realized that the CSTR values are constrain the minimum and Not the maximum ALT values. After some tests from a german member he detected that everything is fine with the flight plan. But I'm still confused why the new FLP looks such different during the descent. Thanks for every answer and the help. Have a save flight!

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