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Descent Path and Speed Calculation


snapshot21

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1316520467_2018-9-11_18-39-39-365(2).thumb.png.6d62b261267ce037b66eedd27f947baa.png

 

 

As you can see, the aircraft does not seem to calculate the descent path too well. I think it treats "Above FLXXX" constraints as "Cross At FLXXX". This causes the aircraft to level off at some waypoints instead of calculating a path in which it descends continuously without leveling off and while respecting altitude constraints.

Furthermore, It does not seem to follow the required speed that it should be doing at some waypoints. The waypoint in the screenshot requires it to be at 280 knots, which is also reflected correctly in the MCDU, but as you can see it is aiming for 240 knots.

This example is in KSFO SERFR3 Arrival using the A321.

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I was getting nearly the same behavior, same model (A320 IAE; latest update) last night on my arrival, CAPSS3 into KDCA.  It seemed to be handing the altitude restrictions alright, but at some point began ignoring speed data and was sticking me with the same 215 kt target speed and 200-220 allowable range (which DES mode seemed to be ignoring to some degree anyway).  I just chalked it up to the SID/STAR anomalies that have already been reported, so I just switched into selected speed and handled the approach semi-manually (as I usually tend to do anyway based on ATC instructions).

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  • 3 weeks later...

Just to provide another data point, I had some more STAR wackiness last night approaching KLGB on the ROOBY3 arrival.  A320 IAE, 1.2.1.0.

 

On this particular procedure, there is a 280 kt speed restriction beyond CHURO.  As you can see in the picture, managed speed wants nothing to do with the restriction.  As a result I flew most of the procedure in selected mode (or as I like to call it "manual automatic").  As I did use V/S mode earlier in the descent (to avoid the open descent freefall "bug") could it be possible that I caused the FMS to mess up?

2018-10-2_23-17-50-791.thumb.jpg.82cbfb009d851e7d81b9d7adbf48d46a.jpg

 

Past ROOBY, things started getting really messy with the descent path calculation.  As you can see below, I'm right about where I should be based on the restrictions yet the FMS thinks I should be up around 12000 feet based on the VDEV reported on the MCDU:

2018-10-2_23-25-47-317.thumb.jpg.3f2c414317373db854df38ed6cf3a521.jpg

 

For the landing I was given the interesting RNAV 12 approach which basically entails flying a tight noise abatement 180 degree turn just before final (not unlike the Canarsie approach into the KJFK 13s).  I didn't capture any screen shots, and don't want to sound harsh but the non-precision approach guidance was useless.  Bird on, approach armed and managed speed and heading, the aircraft went into an open descent dive after BREKE, at which point I broke off the autopilot and flew the approach manually.  The "brick" stayed pegged at the top of the altitude ribbon for the entire approach, despite the bird appearing to show the correct cues.

 

Also, perhaps unrelated, past BREKE the airspeed dropped and the A/THR didn't seem to be responding to the selected speed until after I jogged the thrust levers to MAN FLEX and back to THR CLB.  After that the A/THR again responded to speed selections and I was able to pull off one of my better landings.  I can't really provide more detail given that everything up through this point got so messed up that it's entirely possible I messed something up (or several things) to cause this to happen, though I wouldn't have expected any of it to "break" the autothrust system!

 

in spite of all the craziness, I did stick one of the better landings I've ever made, so at least it was quite satisfying in that regard.

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On 9/11/2018 at 1:06 PM, snapshot21 said:

1316520467_2018-9-11_18-39-39-365(2).thumb.png.6d62b261267ce037b66eedd27f947baa.png

 

 

As you can see, the aircraft does not seem to calculate the descent path too well. I think it treats "Above FLXXX" constraints as "Cross At FLXXX". This causes the aircraft to level off at some waypoints instead of calculating a path in which it descends continuously without leveling off and while respecting altitude constraints.

Furthermore, It does not seem to follow the required speed that it should be doing at some waypoints. The waypoint in the screenshot requires it to be at 280 knots, which is also reflected correctly in the MCDU, but as you can see it is aiming for 240 knots.

This example is in KSFO SERFR3 Arrival using the A321.

I made the same Approach and the speed remained at 280 kts.. T/O 10L to PRB FL250 and then SERF3 Arrival....Alt select to approach altitude......

Can't explain why Yo have this Speed setting

KSFO.png

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