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A300 - PROFILE descend behavior - Is it really accurate?


Felix448
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Hello, 

I have a few questions regarding the IniBuilds A300, specifically when it comes to descending and how the AP and FMS are working together. 

A slight bit of introduction as I havent been around in the forum at all yet: 

I´ve been flying in simulators (P3D / DCS) for a few years now and have been using most popular addons from the common devs. Whether it be the FSL Concorde, 320 series, FTM Maddog X, Majestic Q400, PMDG aircraft and some more specific stuff. I believe that I have built quite a good knowledge over how different aircraft respond and where there are differences between modern and older systems. I´ve picked up the A300 a while ago, together with XP 11 and Active Sky, to see what XP can do and because I´ve always wanted to fly an A300. Especially since yours has been so highly regarded. 

I´ve done 6-7 flights now and still cant get around why the AP is behaving so... wacky... in PROFILE descend mode. 

A quick glance in the A300 FCOM section for Profile Mode:

image.png.127ca5eab48ffdab6c2060368ad67ed9.png

Now, what usually happens when I reach T/D is that I quickly press FL/CH and then change back to PROF upon which the FMA shows P.DES. The aircraft starts to descend, but can hardly stay on the vertical path computed by the FMS. It does recognize speed limits and altitude constraints, but in between it has a very hard time and is constantly either over or under the path. Sometimes quite severe, sometimes only slightly. It´s seems to be constantly fighting itself, often also going into 3-5° nose down descends. (Instead of just easing off the engine power?) 

I was just on a flight, descending down to FL100 where, for the next 10 miles after reaching the next waypoint, there was a restriction to stay above FL100 @ 250kts. The aircraft was already down to 250kts and going through FL120 with around -1500fpm to reach FL100, still before the waypoint with the mentioned restriction was reached. Instead of just smoothly going down as there was enough room with a few miles ahead, the FMS constantly asked for "more drag". This simply wasnt necessary and (in the end) the aircraft reached FL100 normally without the speedbrake and also still held 250kts. 

Going down further to 4000ft to intercept the ILS, it´s often still the same thing. The A300 reaches the requested altitude at CI, but HOW it does it always confuses me. It´s always a good bit above or below the vertical path and almost never exactly on it. 

Why is the AP struggling so much with the FMS paths? Even something as similarly old like the MD-80 can stay on it´s calculated vertical path as long as there´s no strong tailwind. 

 

This behavior is kinda annoying, especially when flying online, there is just very little predictability how the AP will handle a descend given by the FMS. This often causes that I have to manually manage my way down with V/S / SPD. (Which is fine, but why isnt the AP capable enough to do it´s task? If it´s actually like that in real life, fair enough. But I cant really believe that there really is such a huge problem here, so far.)

 

What I also noticed: The vertical path indicator on the right side of the ND immediately shows "V/DEV" no matter if the aircraft is spot on the diamond or above / below it. Is there any reason for this? 

 

Thanks very much for any help in advance.

Best Regards, Felix

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As Felix448 has not visited the forum earlier I find your reply not overly helpful. I have obsereved the same or similar issues as Felix and I have been trying to find a proper answer in the forum for quite some while.
IniBuilds' Airbus series is great and without doubt among -if not- the best for X-Plane 11. However, the superb tutorial videos are very complex and it is therefore very time consuming to find a certain topic if somebody has an issue which needs clarification. 
Why not simply point us to the respective video (or other relevant source) and we will find the solution ourselves?

Best regards
Uwe

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6 hours ago, Daniel Corbe said:

It's not our job to do your research for you.  Perhaps you might be more comfortable with the toy aircraft that come with X-Plane. 

Yeah, thats just the community attitude we all need. Amazing toxicity. 👍 

 

So does anyone have a *real* answer or do we just continue to get replies that basically say nothing? If the reason for this is so obvious and it´s well accepted that the A300 just cant do it. I´m sure someone could explain in a few sentences why the issue is so apparent? By the way, the manual states that the aircraft will vary with the speed on descend, but nowhere that it´s unable to follow the FMS´s vertical path. 

 

Regards

Edited by Felix448
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6 hours ago, Daniel Corbe said:

It's not our job to do your research for you.  Perhaps you might be more comfortable with the toy aircraft that come with X-Plane. 

Plesae note this quote from the IniBuilds invitation to join the forum:

Forum Support

Get help faster from knowledgeable moderators, community members and experienced product users at the official iniBuilds Forum.

 

So, there is no need to become abusive.

For me this particular discussion is closed.

Uwe

 

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Hi, 

Thanks for your post. 

I will try and give a summary of the systems behavior and link with some evidence.  

The A300 was indeed an early implementation of VNAV and was not all that successful. The current implementation in the sim is actually a little bit too good compared to the real aircraft. Most crews that flew the A300 / A310 said they used the TOD marker as a starting point when it comes to descent planning but never really used the coupled P.DES mode. Or if they did use it they often had to step in and help the aircraft in someway. 

Please see the linked video with a A300 captain talking about the issues they had with the A300s P.DES mode. As you can see this is no where near the same level as the A320 and even the A320 can have issues! 

The level of accuracy for the VNAV is not to the level that it can be managed from TOD to ILS intercept. Give ideal conditions it can do this, but as you mentioned the system often can't quite work that out. And that is realistic. 

When you said that why is the A300 pitched 3-4 degrees down. This is expected as it should be able idle. It's hard to see when the A/THR is at idle in the A300 as you don't get the flashing IDLE memo on the E/WD like on the A320. But idle in the A300 can be 60%N1 at higher FLs so you might indeed see high N1 on the engines but they are at idle. You can check by putting the aircraft into LVL change and seeing if the thrust changes. Or if RETARD is not shown on the speed column. 

Hope this helps,

Thanks.    

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@Cammac Thanks for taking the time to give a thorough answer on the topic! I was already kinda expecting that the A300 isnt perfect with the AP on profile mode (generally), but this clears up the question. 

Also thanks for linking the specific video, I was clicking through some of the training videos but didnt find the right one yet. 

I dont know how much time everyone behind the scenes at IniBuilds has. Maybe developing a small PDF that comes with the aircraft which answers some of the more tricky questions would be great. Kinda like a quick reference before heading to forums etc.

 

Best Regards,

Felix

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