Jan 11, 2008

Clear Air Turbulence?

Moe's question (see previous post) led me to this National Post article which seems to suspect that an Air Canada A319's experience of sudden and severe turbulence was associated with the jet stream. This is certainly one of the first things the investigators will look at. The interaction of jet streams and the underlying Rocky Mountain ranges make this part of the continent a prime spot for encounters with clear air turbulence (CAT).

I pulled a graphic from a recent weather map and you can see that the jet stream takes a sharp bend as it crosses the mountains. It also suggests that moderate turbulence is to be expected between flight levels 250 to 370 in the same general region that this aircraft had its 'event.' (* Please note: this chart has been issued since the Air Canada incident and I have no idea what the charts pertinant to that flight showed. Air is always flowing and changing and even the latest weather maps are "history" by the time they're published.)



If aviatrix was still posting she'd probably have an excellent explanation all about mountain waves, jet streams and turbulence. I don't. But good ole Wikipedia has lots on all three topics:

http://en.wikipedia.org/wiki/Mountain_waves

http://en.wikipedia.org/wiki/Jet_stream

http://en.wikipedia.org/wiki/Clear_air_turbulence

As to why the pilot would mention the autopilot in his post event announcement: Excursions in airspeed beyond the normal envelope will cause the autopilot to disconnect producing a loud warning so the pilot knows this has happened. It's a limitation of the automated system to ensure that it isn't actually causing the problem. This illustrates once again, that automation is only designed to do certain things. Then we much-maligned "carbon-based units" are required to step in, figure out what went wrong and set things right. Sudden changes in air speed during jet stream and mountain wave encounters have often triggered such disengagements.

And one last comment - when airlines announcements recommend that you keep your seat belt fastened at all times while seated - they mean it. This is exactly why.

Video Report from Global T.V.

One point in this report I find significant is that apparently the pilot did not issue a turbulence warning to other pilots after this encounter. The possibility of an autopilot computer malfunction is mentioned. But another possibility isn't talked about. There is always a slim chance that one of the pilots accidentally "fell" into the control joystick somehow - perhaps while getting in or out of his/her chair... I've never heard of that happening, but often wondered when it would.

The ultimate cause of this incident and whether the airplane was damaged will be determined by investigators using data from the black box and cockpit voice recorders. And until their work is complete, all our speculations are just that. Speculations.

2 comments:

MathFox said...

Getting 1g accelleration "down" (which is needed to get the reported "passengers hitting the ceiling") is hard to achieve with flight controls only; it can be done by pushing the plane down abruptly. I wonder whether the A320's flight computers allow such a manouver.

All in all it is likely that the weather contributed to this incident; auto disabling of the autopilot too: In my flight sim I can control the plane fairly good (the autopilot is far better), but when I disable the autopilot there atways is a disruption. I guess that professional pilots notice that too when they take over control.

Now imagine what happens when the autopilot suddenly dumps control back into the hands of an unexpecting pilot.

Anonymous said...

This description does fit with the sudden onset of a moutain wave encounter:

"Another passenger describes "three big drops." Andrew Evans says the plane "rolled a bit after that," and that he saw dishes fly through the air. But he says it was all over in a matter of seconds."