This morning, I read a CBC story about a control problem on an Air Canada Jazz Dash-8 flying from Kingston to Toronto on 2 September 2004. From the story, it was pretty hard to figure out what had actually happened, especially given statements like these:
- “It took the strength of two men to steady the control column of an Air Canada Jazz flight veering dangerously out of control” sounds like directional control problems — jammed aileron? asymmetric flaps?
- “pilots struggling to gain control of the plane from the moment after takeoff” directional control?
- “The control column, a stick used to manoeuvre and control altitude, was forcing the plane’s altitude higher as the craft continued to pick up speed” sounds like excessive up trim, but then why would the plane be picking up speed if the nose were up?
- “loose nuts caused bolts in the plane’s elevator spring tab – a part of the aircraft that helps maintain control – to move out of place and throw the plane’s balance and control out of whack” OK, then it’s elevator trim
I didn’t feel much better informed after reading the article, so I hunted down the actual TSB report. It makes terrifying reading, not because of the trim-control problem (as troublesome as that was), but because of the risk of catastrophic structural failure.
As far as I can understand from a quick reading, the Dash-8 has elevators that can move separately on the left and right sides, and each has its own trim tab — the left and right trim tabs have to be balanced with weights so that one doesn’t pull more than the other. When the incident plane was in for painting a while before the accident, the tabs were rebalanced. The TSB’s best guess is that the AME left the bolts loose on one side in case the weight needed to be removed for more balancing, and forgot to tighten them afterwards.
Eventually, the weight fell off, leaving the tabs badly out of balance. As the Dash-8 accelerated for takeoff from Kingston, it sought to trim nose-high. The pilot flying (first officer) noticed that very little pressure was required to lift off, and soon both pilots were pushing forward hard to maintain airspeed, even with full nose-down trim. After running through some checklists, the captain disconnected the copilot-side trim (I think — this part is a bit hazy), and then was able to trim the plane for cruise using controls on his side. Althought they had declared an emergency 30 seconds after takeoff, they decided to continue to Toronto (about a half hour away) rather than landing at CFB Trenton between Kingston and Toronto.
I can understand their decision. After all, trim is mainly just a convenience for the pilots, to save us having to push or pull the yoke constantly during flight. On such a short flight, and with one of the independent trim systems (apparently) working, why not just continue the last distance into Toronto, rather than dumping up to 50 passengers on the tarmac of a military base in the middle of nowhere?
Unfortunately, things could have turned out very badly. Because of the weight imbalance, the two elevators were exerting different forces — one was trying to push the nose down, and one was trying to pull it up. According to the TSB report, this caused a twisting force on the vertical stabilizer close to its structural limit. That reminds me of the force that snapped the tail off American Airlines flight 587, though the cause was rudder oscillations, in that case. In hindsight, we know that that was the real danger of the flight and that the trim problem was only a sympton.
If the flight crew had known the real risks, I don’t doubt that they would have set down in Trenton without a second’s thought. In my own flying, I’ll try to keep in mind that any anomaly I can actually detect may just be the tip of a very large iceberg.
An interesting incident that I had not heard of before.
I remember a flight I did years ago in a Piper Arrow with an instructor. Early in the flight, I noticed that the elevator trim wheel, which was always a bit stiff, seemed harded to move than usual. It seemed to get worse during the flight. On an ILS approach at the end of the flight, I found it impossible to trim the plane. The instructor wanted to fiddle with the trim wheel to see if he could make it better. I had a strong instinct that we should leave it alone and refused to let him mess with it. The plane was flyable, it was just annoying to have to keep forward pressure on the yoke. After landing, we told the mechanic and he investigated. He later showed me that the trim cable had slipped over time, allowing a turnbuckle to bind against a pulley. Had we continued to force the trim back and forth, the plastic pulley could have split, letting the cable go slack and allowing the trim tab to move however it wanted. I’m not sure what would have happened, but I’m glad we didn’t try to use trial and error to diagnose the problem in the air.
Trim authority can be critical to aircraft control, especially in larger aircraft. I understand the pressure on commercial pilots to continue to their destination, but I would have landed immediately if I was the PIC on that flight. It’s much safer to diagnose problems on the ground, regardless of how upset the passengers might get or how late the freight might be.