In an earlier post titled Garbage in, garbage out, I wrote about how the NTSB discounted the crew’s testimony and tried to rely solely on the physical evidence, which consisted of the CVR, FDR, and the damaged aircraft. The picture included here shows the missing #7 slat.
It’s easy to see why the NTSB investigators, as well as investigators from Boeing and ALPA, zeroed in on the missing slat. A plane spirals out of control, rolling to the right. It’s a huge, obvious clue.
But there were other clues that were completely overlooked.
- The simulator tests as well as an earlier flight test showed that a single slat extended in isolation was an easily controllable flight condition, even at 39,000 feet.
- Boeing’s own engineers estimated that the slat actuator, piston, and rod could not sustain an airspeed in excess of 363 knots, which would have meant that the slat, if extended at 39,000 feet, should have been torn from the plane much earlier in the dive.
- The crew reported a failure flag in the lower rudder yaw damper. The yaw dampers and rudder power control units were replaced on the incident aircraft but never examined.
- There was evidence of hydraulic leakage near the lower rudder actuator.
- The over extension of the right landing gear was evidence of a sustained nose right sideslip, pointing to a rudder hardover as the most likely cause of the upset.
The alternative theory pointing to the rudder and away from the #7 slat didn’t surface until nearly five years after the accident when a former aerospace engineer from Grumman Aerospace re-examined the incident. His written analysis combined with what was learned from the roll-over accidents in Colorado Springs and Pittsburgh, Pennsylvania, are all new clues to this very old cold case.
You can view more images at N840TW Photographic evidence.