A320 Descent profile
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Join Date: Nov 1999
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A320 Descent profile
I'm new to the A320, and after a few attempts at getting a nice descent profile into the FMGC I've given up.
The scenario is the CDA approach into AMS, it's an RNAV approach, which has altitude limits to encourage pilots to fly a 3 degree glidepath minimum from FL70.
On the 737 I flew before, the VNAV would fly this approach perfectly, only reaching the minimum profile at glideslope intercept, and this all the way with idle thrust. This is as the approach was meant to be flown, with little noise.
Now the A320 seems to dive for the first constraint, fly level until passing it, then immediately dive for the next constraint, and fly level, until it finally reaches 2000' in level flight 9 miles from touchdown.
Even worse, if I delete all the altitude constraints, it aims to be 2000' approx 13 miles from touchdown! And then fly level until the glide.
Now for all the 'bus claims about FMGC will help decrease fuel consumption and reduce noise pollution this doesn't seem to fit, and I was wondering wether others have this problem as well. For now, I work out my own profile and fly it with idle thrust and it works out fine. But I'll be up to 3000' above the FMGC profile at times, sometimes more.
Thanks for anyone who can explain!
The scenario is the CDA approach into AMS, it's an RNAV approach, which has altitude limits to encourage pilots to fly a 3 degree glidepath minimum from FL70.
On the 737 I flew before, the VNAV would fly this approach perfectly, only reaching the minimum profile at glideslope intercept, and this all the way with idle thrust. This is as the approach was meant to be flown, with little noise.
Now the A320 seems to dive for the first constraint, fly level until passing it, then immediately dive for the next constraint, and fly level, until it finally reaches 2000' in level flight 9 miles from touchdown.
Even worse, if I delete all the altitude constraints, it aims to be 2000' approx 13 miles from touchdown! And then fly level until the glide.
Now for all the 'bus claims about FMGC will help decrease fuel consumption and reduce noise pollution this doesn't seem to fit, and I was wondering wether others have this problem as well. For now, I work out my own profile and fly it with idle thrust and it works out fine. But I'll be up to 3000' above the FMGC profile at times, sometimes more.
Thanks for anyone who can explain!
Join Date: May 2000
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Now the A320 seems to dive for the first constraint, fly level until passing it, then immediately dive for the next constraint
Curious! I've never seen managed descent do anything like that, it normally hits the level restriction right at the fix with unerring accuracy. As I'm sure you know, it should start down with a V/S of 1000fpm until it meets its computed descent profile then descend with idle power until it reaches the constraint, unless you start the descent above the profile or descend with selected speed.
Even worse, if I delete all the altitude constraints, it aims to be 2000' approx 13 miles from touchdown! And then fly level until the glide.
This could be something to do with the positioning of the decelleration point in the profile. Any particular RNAV arrival into AMS, I'll have a look next time I'm there.
Curious! I've never seen managed descent do anything like that, it normally hits the level restriction right at the fix with unerring accuracy. As I'm sure you know, it should start down with a V/S of 1000fpm until it meets its computed descent profile then descend with idle power until it reaches the constraint, unless you start the descent above the profile or descend with selected speed.
Even worse, if I delete all the altitude constraints, it aims to be 2000' approx 13 miles from touchdown! And then fly level until the glide.
This could be something to do with the positioning of the decelleration point in the profile. Any particular RNAV arrival into AMS, I'll have a look next time I'm there.
Join Date: Apr 2000
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Try making any height constraints conditional, ie -7000, or +7000. Once you pass a constraint, if you then do a direct, it cancels the profile associated with the constraint and will try to regain the original profile.
Check the speeds associated with the procedure, if they are too high, it will fly a high rate of descent profile, try inputting the speed you want to be at a certain part of the approach, that usually steadies it down
Check the speeds associated with the procedure, if they are too high, it will fly a high rate of descent profile, try inputting the speed you want to be at a certain part of the approach, that usually steadies it down
Or at least, it would if I let it 
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...do yourself a favour, forget about all this managed business and fly your descent in selected speed/open descent. Works much better, is more comfortable as the aircraft is pitching around less and forces you to continually re calculate your profile thus keeping you more 'in the loop'.
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Just remembered, this happened into Cancun with our 330. If you flew about 1500 above profile it locked on perfectly. It would be worth flagging it to engineering or Airbus for them to check as it now sounds like a software/database problem.
