Originally Posted by
Mac the Knife
AirRabbit, thank you for your courteous reply. It helps a bit, but I still don't quite see what he was trying to achieve by this.
I don't fly (in my next life!), but I do sail small boats, so for me it's as if he'd put the tiller hard over and then, just as the bows started to swing, slammed the tiller hard over to the other extreme. And then repeated this 4 more times.
This won't get you anywhere in a small boat and I can't imagine a situation when one might do this, even in a blow and a running sea. I know aircraft are not the same as small boats but it still sounds rather odd.
I'm sorry if it's a silly analogy but I'm trying to understand.
PS: On reflection, you might well break the rudder off in a dinghy if you tried this, so what happened is not that surprising. But why do it?
Well, I don’t think it’s a silly analogy, and, as I said, the “why” is the big question here. The “what” of this accident is not terribly difficult to understand – even though some here would want to argue with the findings of the NTSB. It appears that, in addition to improperly using the rudder to prevent a roll from occurring, this pilot actually developed an oscillation similar to what is called a “Dutch roll” – where the yaw-roll coupling is excited. He then managed to exacerbate this oscillation and continued to apply, and then reverse apply, maximum (or nearly maximum) roll and yaw control inputs until the tail structure failed.
You are not alone in trying, but failing, to know “why” this pilot did what he did. Given that this pilot was certainly
not a novice pilot, I would presume that the “why” included the fact that he thought he was doing the right thing to prevent what he thought was happening, or was about to happen, to his aircraft. He had just encountered wing-tip vortices from the preceding aircraft (the JAL B-747), and I presume he was responding to a second encounter. Further, it appears that he thought the proper control application strategy included the aggressive use of rudder application. Normally, a pilot is trained to use the rudder to keep the airplane in a “coordinated” flight condition. The instrument used to verify the need for such control, as well as indicating the direction and the magnitude of that control input, is the inclinometer, commonly called the “slip/skid” ball. The idea is to “keep the ball in the center,” and by doing so, keep the airplane in “coordinated” flight; that is, not skidding and not slipping. I’m not familiar with the A-300 so I don’t know if there is an electronic version of the inclinometer that was recorded as part of the FDR, or if there is only a simple, mechanical inclinometer available for pilot reference. I only know that the animation included vertical and lateral acceleration indications, which are not necessarily the same.
There was some testimony/evidence that was presented during the investigation of this accident that indicated this pilot may have been trained to use the rudder during other situations – like the recovery from “other than normal” attitudes. However, the evidence and testimony
also shows that this training always included references to “minimal use” or “smooth, coordinated use” of any rudder input. Contrary evidence indicated that at least once, earlier in this pilot’s career, he may have used essentially the same control inputs (i.e., full or almost full rudder application) to, as he was reported to have said, “level the wings due to wake turbulence.” Of course, rudder deflection
will cause a rolling moment, due to the sideslip that will develop (in fact, in some airplanes, particularly swept-wing airplanes, a competent aerobatic trained pilot may be able to “roll” the aircraft completely through 360 degrees, using only the rudder), but the rudder is
not a roll-control device; and no one seems to know where this pilot may have picked up this particular control application impression.
I know very little about boats of any size – other than the fact that they are supposed to float – but your analogy and your conclusions seem to be pretty well on target. As you say, “you might well break the rudder off in a dinghy if you tried this.” And, it would seem that this situation verifies your concerns.