Originally Posted by
Mac the Knife
Forgive me for enquiring of the elite, but I'd like to ask...
"By the time the rudder surface position display blanks out, there were 7 control wheel reversals and 5 rudder pedal reversals, all in about 7 seconds."
This is a large aircraft, not an aerobatic biplane - surely it could hardly have even begun to respond to one control input before another was initiated (and so why the rapid reversal?)?
Just asking, if you don't mind.
Originally Posted by
misd-agin
The A300 has the lightest control forces, and perhaps the quickest response(havn't timed them) of any large jet a/c I've flown or jumpseated in. Flown would be 5 large jets, j/s'd would add three additional types.
I've been with folks that have over controlled roll inputs. All the hydraulically powered jets with roll spoilers react very quickly. Review the NW 757 @ SEA video and watch how quickly the a/c responds once the roll spoilers extend to a signifcant angle.
Originally Posted by
misd-agin
On a scale of 1-100 abrupt rudder inputs at 160 kts is similar to taking off in gusty conditions. Call it a 10. 210 kts rudder inputs would be 20-30, and 250 kt rudder inputs are 100.
The magnitude of the lateral movement/load increases exponentially. So much so that the AA pilots union had expert testimony about the sensitivity being close to a POI response rate.
Hey
Mac the Knife … I’m not sure about the “elite” remark, but … your question is an excellent one; “why the rapid reversal?” THAT is the $64 question! I think if you watch the animation of the FDR provided by the NTSB, you will see that the aircraft
did begin to move in the direction mandated by the control inputs. It IS a large aircraft … and the momentum it would build up would be substantial … and to reverse that momentum would take A LOT of force. But,
THAT is precisely the point. Of all the control reversals noted on the FDR in that 9 seconds, I believe only one was held long enough to potentially allow the aircraft to get close to stabilizing with
THAT particular control input. In each of the other cases, the momentum was established … that is, the aircraft began movement in the commanded direction … and then, because of control reversal, that momentum was stopped and reversed … multiple times … in a
VERY short period of time. As any structures expert will confirm, while rapid control movement
will stress the structure, a significantly larger stress, by far, is generated from applying full control input opposite the movement of the structure.
I don’t know that we’ll ever know the “why” behind this particular accident. It would appear that the F/O was pre-disposed to the aggressive application of rudder, and that he was also very quick to apply what he thought was necessary control input to prevent what he
thought was happening, or was about to happen, to his airplane. Again, the FDR traces of the cockpit controls (i.e. column, wheel, and pedal) indicate that those controls were displaced – and since they are not “back-driven,” someone in the cockpit had to displace them.
I hope that the readers here don’t think I’m saying that the complete responsibility of the accident is the fault of the F/O. However, he WAS “on” the controls … so he bears at least part of the responsibility. But, as I’ve said, I think he had significant “help” in getting to the scene of the accident. The control sensitivity of the airplane helped. (I’m not rated on the A-300 so I appreciate
misd-agin’s comments about light control forces and quick responses.) The training to which he was exposed helped. The understanding of “design maneuvering” speed (or more likely, the “mis-understanding” of that term) helped. His own control application strategy helped. Not being able to recognize and escape from a “pilot induced oscillation” (PIO) also helped.
With these “helpful” pieces all fitting together, along with the comments offered by
misd-agin regarding the exponential increase in structural “load” with airspeed increase, I think that the result of the sequence of these particular events is rather straight-forward. My hope is that pilots will take away from this event several important pieces of information … an understanding of the way aircraft are certificated; an understanding of what control inputs due to the airplane; an understanding of what a PIO really is, how to recognize one, and, most importantly, how to get OUT of one if one does develop.
Let me add one additional and very important point here. I think it important to say, and to say clearly, the development of a PIO is
NOT necessarily an indication of poor piloting technique. It’s somewhat like a squad of soldiers marching across a bridge … such an event is ALWAYS done “out-of-cadence” as a precautionary measure to avoid setting up a resonant frequency vibration. A PIO is similar. A PIO is almost always initiated through some triggering event – that may, or may not, have a pilot input involved. But once the trigger has occurred, the pilot’s response is critical to either adding to or damping out that PIO. It’s hard to dampen out a PIO unless you know that you are in one, AND you know HOW to get out of one. Maybe these things will be included in future pilot training programs.