LatinVFR forums

Hey there!
I’ve been loving the A321 so far. I got it some time ago and I’ve never had problems with it. Untile today.
I did my longest flight ever, from Helsinki to Reykjavik with a Finnair A321 and at a certain point I noticed that the plane was pretty far away from being on the line of route. It was very much on the right. So I tried so tricks that I found out to work with A320 when something like has happened. But this time it didn’t. I tried to switch AP on and off many times. I tried to move the plane in the right position again and the activating AP but as soon as I pressed on AP the plane started going back to the right…
How’s it possible? Is it a bug or?
Again, it’s the first time ever I had troubles with the A321.
I don’t know if it can help, but at a certain point the plane started going a bit crazy for like 1/2 seconds from right to left, but then went back normal.

Francesco wrote: ↑

Mon Feb 20, 2023 2:11 am

Hey there!
I’ve been loving the A321 so far. I got it some time ago and I’ve never had problems with it. Untile today.
I did my longest flight ever, from Helsinki to Reykjavik with a Finnair A321 and at a certain point I noticed that the plane was pretty far away from being on the line of route. It was very much on the right. So I tried so tricks that I found out to work with A320 when something like has happened. But this time it didn’t. I tried to switch AP on and off many times. I tried to move the plane in the right position again and the activating AP but as soon as I pressed on AP the plane started going back to the right…
How’s it possible? Is it a bug or?
Again, it’s the first time ever I had troubles with the A321.
I don’t know if it can help, but at a certain point the plane started going a bit crazy for like 1/2 seconds from right to left, but then went back normal.

Not capturing the LNAV after takeoff can be caused by several things. The first waypoint might be a discontinuity or VECTORS. This means the aircraft will continue to fly on its current heading. If it gets too far away from the flight path, you have to manually fly the aircraft back to the flight path.

The easiest way to do this is to copy the second waypoint into the scratchpad by pressing the key next to it. Then copy the scratchpad into the first waypoint. If managed mode isn’t automatically enabled, then enable it. A new route has been generated to get you back on track.

VNAV (using managed mode) is confusing because sometimes it doesn’t do what we think it should be doing. There are three key parts to understanding VNAV: altitude, speed, and flight path. All three are dynamically computed.

If you are flying at 300 knots @ 18,000 ft and you want to descend to 250 kts @ 10,000 ft. But the VNAV doesn’t seem to be responding to what you entered. If your flight plan shows the next two waypoints at 18,000 ft, the VNAV will keep flying at 18,000 ft and then start descending to 10,000 ft. It looks like it is doing nothing while it is waiting. It is possible to override the VNAV to start descent now and your vertical path takes you below those two waypoints. But if those waypoint restrictions are to fly aircraft over mountains, overriding the VNAV might guide you into mountains!

Let’s use the same example without the waypoints. If 250/10000 is shown in your MCDU, the VNAV should start descending, right? The other part of the VNAV calculation is the speed. You are having the VNAV slow the aircraft from 300 kts to 250 kts. When is the aircraft going to slow down? Before descent, after descent, during descent? If the aircraft slows down before descent, then the V/S will be lower. If you watch the speed, you’ll see the VNAV slow the aircraft before starting descent. Again it looks like the VNAV isn’t doing anything. The A321 is a very aerodynamic aircraft and is difficult at times to slow down. This is the time to extend flaps or use the spoilers to slow the aircraft quickly.

This happened to me during my last flight from Manila to Taipei, and I should've extended flaps to increase drag during my descent. Instead, I didn't, so I used the spoilers.

If you look at the waypoints in the MCDU, some of the waypoints have normal size numbers such as 250/10000. These are waypoint restrictions from navigation charts.

Unfortunately ATC messes up VNAV. They often give altitude and speed restrictions that don’t fit the VNAV path. IRL pilots will turn off VNAV and use V/S. In MSFS we have the ability to ignore or turn off ATC and fly VNAV without losing our pilots license.

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