Strange autopilot behaviour, and ideas?
Thread Starter
Joined: Nov 2006
Posts: 784
Likes: 102
From: Europe
Strange autopilot behaviour, and ideas?
A319, cruising at FL370, smooth conditions, ALT CRZ (and had been for more than 2 minutes, thus "soft altitude" hold), same problem both APs.
One other very strange item: in the following video look at the V/S versus the actual change of altitude on the tape (visible at 18 seconds and more clearly at the 1 minute mark). Related?
https://www.youtube.com/watch?v=9hk--9-fTlI
Very strange! Any ideas?
One other very strange item: in the following video look at the V/S versus the actual change of altitude on the tape (visible at 18 seconds and more clearly at the 1 minute mark). Related?
https://www.youtube.com/watch?v=9hk--9-fTlI
Very strange! Any ideas?
Joined: Jul 2007
Posts: 812
Likes: 19
From: uk
Kerosine: What you experienced was a classic standing wave encounter. Ignore any turbulence, but if you see periodic steady, off zero, VSI conditions which reverse between descent and climb, then you are "riding the wave".
Joined: Jun 2006
Posts: 240
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From: B.F.E.
Wave conditions are most prominent with an unstable layer sandwiched between 2 stable layers, one being near mountaintop level. That creates the classic "trapped" lee wave that propagates far downstream.
Slightly more conditional instability above ridge level allows an "untrapped" wave system that has more of an upward component and less downstream. This one becomes strongest at its "cap", which is the tropopause. Being that the trop is the mother of all "elevated stable layers" this is why wave action and sometimes CAT can strengthen in the high flight levels above very strong lee wave systems.
Seen it above thand Rockies and the Sierra Nevada in the USA many a time. Similar effects to what you describe can be encountered above wave systems at lower altitudes as well; if not near the ridge top or an inversion layer it is very possible to encounter almost totally smooth wave conditions.
If it is an A330 try idling both engines and wave soaring on that giant wing!
On second thought don't.
Slightly more conditional instability above ridge level allows an "untrapped" wave system that has more of an upward component and less downstream. This one becomes strongest at its "cap", which is the tropopause. Being that the trop is the mother of all "elevated stable layers" this is why wave action and sometimes CAT can strengthen in the high flight levels above very strong lee wave systems.
Seen it above thand Rockies and the Sierra Nevada in the USA many a time. Similar effects to what you describe can be encountered above wave systems at lower altitudes as well; if not near the ridge top or an inversion layer it is very possible to encounter almost totally smooth wave conditions.
If it is an A330 try idling both engines and wave soaring on that giant wing!
On second thought don't.
Joined: Feb 2001
Posts: 6,729
Likes: 104
From: The Winchester
There's an oldish techie paper on the subject here:http://www.ann-geophys.net/15/823/19...5-823-1997.pdf
FWIW the surface flow down here on the 15th/16th was a strong northerly, due to a low over Corsica, with a fair dump of snow on the mountains...not a nice day at ground level.
Joined: Sep 1998
Posts: 1,615
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From: wherever
Ive had standing waves in the middle of the south atlantic coming off the Andes so 10 minutes is not at all "too Long".
The other thing to keep an eye on is the temperature which will cause Mach to fluctuate rapidly.
In the good old days we would switch off the AT, set the thrust and watch the Mach roll up and down. Only adjusting thrust if we were getting close to max or min limits.
The other thing to keep an eye on is the temperature which will cause Mach to fluctuate rapidly.
In the good old days we would switch off the AT, set the thrust and watch the Mach roll up and down. Only adjusting thrust if we were getting close to max or min limits.
Joined: Jun 2014
Posts: 1,052
Likes: 72
From: Village of Santo Poco
I agree, I've had them starting a good 100 miles EAST of DEN and continue until well past GJT, so a good 20 min. One of them even threw us right in the barber pole, certainly enough to make you pay attention.
Joined: Dec 2015
Posts: 507
Likes: 1
From: France
A319, cruising at FL370, smooth conditions, ALT CRZ (and had been for more than 2 minutes, thus "soft altitude" hold), same problem both APs.
One other very strange item: in the following video look at the V/S versus the actual change of altitude on the tape (visible at 18 seconds and more clearly at the 1 minute mark). Related?
https://www.youtube.com/watch?v=9hk--9-fTlI
Very strange! Any ideas?
One other very strange item: in the following video look at the V/S versus the actual change of altitude on the tape (visible at 18 seconds and more clearly at the 1 minute mark). Related?
https://www.youtube.com/watch?v=9hk--9-fTlI
Very strange! Any ideas?
Joined: Jan 2008
Posts: 211
Likes: 3
From: farmm intersection, our ranch
I agree, I've had them starting a good 100 miles EAST of DEN and continue until well past GJT, so a good 20 min. One of them even threw us right in the barber pole, certainly enough to make you pay attention.
Thread Starter
Joined: Nov 2006
Posts: 784
Likes: 102
From: Europe
There's an oldish techie paper on the subject here
The other thing to keep an eye on is the temperature which will cause Mach to fluctuate rapidly.
Your best option in that sort of case is to transfer it to Airbus who will decode the DFDR/QAR/DAR data.
Just a side note: if you see something similar in the SIM think quickly about jammed stabilizer / THS runaway.
