My Little Runaway
Since I am a college professor, I say "It's more complicated than it appears" and "Wait until we have all the facts before we draw a conclusion" often. That's why I haven't said anything about AF447, except for one tweet about this post when it became clear that the aircraft had stalled. I felt that I was in no position to judge the flight crew or its actions. "It's more complicated than it appears," I thought, "Wait until we have all the facts."
I'm glad I waited. An article today in the German Der Spiegel raises some questions about the Airbus's automated flight control system. It appears that both of the Airbus's side sticks were commanding "nose down," as expected. But the nose didn't come down, and the horizontal stabilizer moved in the nose up direction. The captain evidently took a seat with a comment from one of the co-pilots like "Here, you try it!"
They did the right thing.
The airplane didn't.
I am hesitant to draw a conclusion about French aircraft and stabilizer trim, but it is tempting to see a relationship between the recent grounding of the Falcon 7X fleet and this report. A Falcon 7X had a pitch trim runaway, so the fleet was grounded. Good move.
Those of us who fly smaller airplanes might wonder why the flight controls didn't overpower the stabilizer trim. In some heavier airplanes (starting with the Lear 25), the stabilizer trim is much more powerful than the flight controls, especially in the situation where the whole stabilizer moves, rather than a tab (tabs are bad because in some situations at high airspeed they can cause control reversal). United lost a DC-8 in 1983 due to a mis-set stabilizer trim (NTSB data here). Alaska lost an MD-80 to a stabilizer trim problem. Next time you're around airliners look at the stabilizer: there are usually marks on the tail showing the maximum and minimum stabilizer position, so it can be verified during the walk-around.
I've had two pitch trim problems, neither one of which caused a crash. The first was in my Taylorcraft: I was a little too enthusiastic with the trim crank and pulled the cable off the pulley, leading to a full nose-up trim condition. The T-craft uses a single trim tab on the left elevator, and while it was annoying I had no trouble pushing the nose down and completing the pattern. After that I learned my lesson: I trimmed slowly (which is the right way), and told riders "That's the trim crank; don't touch it!" No problem.
The other was in a Seneca. The electric pitch trim wouldn't respond. I played with it for a while and, consistent with my philosophy that circuit breakers are for pulling but not for pushing, I used manual trim for the rest of the flight. Good thing: there was a short in the system, and there were sparks behind the panel.
I've also had an autopilot runaway, in a Cessna 414A. Wait, I've had two autopilot runaways, but the second was caused by a pilot with an unpleasant personality who wanted to see how I'd react. (He didn't last long with the company.) The real one was on the final leg of an air ambulance flight. It was about 0400 and the final leg was less than 50NM in night VFR conditions. But it was 0400 and I was tired. I engaged the autopilot and the nose shot up into the darkness. Huh? Whoa! Phew!. I hit the autopilot disconnect and lowered the nose.
But it was 0400, so I engaged the autopilot again. Huh? Whoa! I disconnected it and hand-flew the rest of the way home.