Originally Posted by OK465
We're talking about a heavy, inertia laden aircraft at 265 knots, which is decelerating, with an initial max available roll rate in roll direct of less than 20-25 degrees per second (max rate is available at about 320+ KIAS clean, less on down). Rate-wise even at full SS deflection the thing is a slug.
Yes it is a very stately slug. The problem comes from the difference between expectations of the PF versus what the aircraft actually did.
At altitude, roll damping is lower. Did PF understand he was in roll direct? Did he have appropriate training in flying in a roll direct law? Even sluggish aircraft can be overcontrolled or oscillate, they just do it slower, but they still oscillate.
Whether fighter or transport, 900 knots or 300, these things are aircraft and aircraft oscillate if not properly controlled. For an aircraft the size, mass, and control power of AF447, the roll oscillation it encountered was surprisingly quick, but the roll did not reach significant angles until after the stall. This may have deterred PNF from intervening.
The difference between divergent and convergent PIO is the amount of damping available. If the oscillation is building energy, bad things are going to happen as they did in the Sageburner accident. At some oscillation frequencies, pilots cannot effectively intervene.
OC, we are in full agreement that PF came on too strong when he took control. The problem appears to be that his initial mistake was not easily reversed.
The one surprising thing I learned reading the Sageburner file was that the aircraft was headed for an 8 g peak when the pilot released the stick in an attempt to break the oscillation. The aircraft had been trimmed in the alignment turn, and the additional nose up stabilator motion caused the aircraft to breakup somewhere around 15 g.
FWIW, On my roll PIO experience in the visual simulator, the only pause was the minor delay in visually locking on to the attitude indicator.