Remember the American Airlines A300 that crashed in the US after the vertical stabiliser broke off? The co-pilot over stressed the tail by pumping the rudders in an attempt to get out of wake-turbulence.
In a similar situation in 2008, an Air Canada A319 encountered wake turbulence and the pilot’s rudder & side stick inputs put the aircraft into a slip, stressing the vertical stabiliser beyond certified limits. The pilot wasn’t aware that they were encountering wake turbulence and thought it was a problem with the flight control computers (perhaps similar to the Qantas A330 problems off the coast of West Australia).
At first I thought the Airbus flight computers on the A319 were supposed to keep the aircraft within a flight envelope and prevent stressing the airframe. Then I thought maybe they’d gone into Alternate Law but there still should be some protection, right?
Well, here’s what I found when I read through the Canadian Transportation Safety Board’s report:
- The pilot’s actions in trying to correct the problems actually made things worse and also caused the flight computers to drop down into Alternate Law, removing some of the flight envelope protections they usually provide
- The A319 flight computers don’t provide envelope protection in yaw, meaning they won’t prevent a pilot from overdoing it on the rudder
- The A319’s rudder control system has a similar design to that of the A300 and can allow full rudder deflections at high speed even when only small inputs are made on the pedals (so imagine what happens when a pilot is reacting in a shock situation)
- Like many airlines, Air Canada’s “upset training” focused more on pitch issues, not yaw issues. Also, the issue of rudder reversals during wake turbulence was not covered during recurrent training and training material from the A330/A340 that covers the issue wasn’t available to A319/A320 pilots.
Interesting stuff and certainly eye opening for me when I had been thinking that the post-A300 Airbus flight computers were supposed to protect from excessive inputs. Yet more confirmation of the Swiss Cheese error model and the ability of a surprised pilot to override the systems. Fortunately the vertical stabiliser on the A319 was able to take the strain, closing one of the holes in the cheese.
Oh yeah, did I mention that incident/accident reports make for fun breakfast reading for anyone interested in aviation :)