Strange autopilot behaviour, and ideas?

[Kerosine]

A319, cruising at FL370, smooth conditions, ALT CRZ (and had been for more than 2 minutes, thus "soft altitude" hold), same problem both APs.


The aircraft showing instability in altitude hold, and after a few minutes seemed to max out a continuous oscillation +/- 150ft.


Mach varied between .73 and .79 (selected at .78).


The autothrust was trying to compensate with thrust from 70% to CLB.

Very noticeable and uncomfortable for everyone on board. We used an manual thrust setting which seemed to improve the situation.

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?

[back to Boeing]

Don't fly Airbus myself. But perhaps a mountain wave?

[EGPFlyer]

If it was yesterday we experienced mountain waves and similar fluctuations over northern England.

[Meikleour]

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".

[Kerosine]

Well, it's definitely a possibility since we were approaching the Pyrenees, though I've never seen MW so pronounced at high level!

[macdo]

Wave probably, but you often get CAT like that crossing the Atlantic. At least there was plenty of margin!

[Linktrained]

Steve Fossett got to 50,699 ft near the Andes to hold the GLIDER altitude record in a standing wave over 10 years ago.
LT

[hikoushi]

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.

[Lantirn]

It was a constant phenomenon or it was for some minutes?

[Kerosine]

Thanks all for the information, always interesting to read!

It continued for around 10 minutes before we took out the ATHR and set the power manually.

[wiggy]

Well, it's definitely a possibility since we were approaching the Pyrenees, though I've never seen MW so pronounced at high level!

The Pyrenees isn't the highest mountain range in the world (tops out at 3404metres, mount Aneto, about 100 km from chez nous) but it's worthy of attention because it's a range than can and does can generate significant wave, turbulence and spectacular lenticular cloud on a regular basis. I've certainly been in wave at FL 300 plus going over the top of the range in the vicinity of Andorra ( near the Giron waypoint).

There's an oldish techie paper on the subject here:http://www.ann-geophys.net/15/823/1997/angeo-15-823-1997.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.

[Lantirn]

10 minutes its too long for mountain wave i think.

Something should be wrong with the air data. The tubing, the module, the probes i dont know. But its too long.

[FE Hoppy]

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.

[Amadis of Gaul]

Ive had standing waves in the middle of the south atlantic coming off the Andes so 10 minutes is not at all "too Long".



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.

[KayPam]

A319, cruising at FL370, smooth conditions, ALT CRZ (and had been for more than 2 minutes, thus "soft altitude" hold), same problem both APs.


The aircraft showing instability in altitude hold, and after a few minutes seemed to max out a continuous oscillation +/- 150ft.


Mach varied between .73 and .79 (selected at .78).


The autothrust was trying to compensate with thrust from 70% to CLB.

Very noticeable and uncomfortable for everyone on board. We used an manual thrust setting which seemed to improve the situation.

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?
Your best option in that sort of case is to transfer it to Airbus who will decode the DFDR/QAR/DAR data.

[Lantirn]

Ah ok! Never noticed for so long, I had once but severe form above Alps!

[flyingchanges]

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.

I have been 80 north of DEN in a block altitude +/- 1,500 FPM between MCT and idle full boards.

[FlightDetent]

Just a side note: if you see something similar in the SIM think quickly about jammed stabilizer / THS runaway. :ouch:

[Kerosine]

There's an oldish techie paper on the subject here
Thanks for the link, I'll be reading that tomorrow stuck in my hotel room with nothing else to do!

The other thing to keep an eye on is the temperature which will cause Mach to fluctuate rapidly.
Ah! This didn't occur to me at the time, so I can't confirm what the temp was doing!

Your best option in that sort of case is to transfer it to Airbus who will decode the DFDR/QAR/DAR data.

Might have been binned by now. I'll have a word with the guys at the base and see if it was an anomaly or a trend!

Just a side note: if you see something similar in the SIM think quickly about jammed stabilizer / THS runaway.
Good to know, I'll put that on my "to look up" list for this week.

[WindSheer]

Well it snowed in Benidorm this week so slightly out of the normal for the region...