B757 roll oscillation on approach

[AC_Vital_to_Emerg]

In "Flight" magazine recently there's been coverage of a B757 roll oscillation problem. News to me, and I used to fly it. Has anyone experienced it?

[chevvron]

I've noticed a sideways oscillation of the forward fuselage after departure; this was even evident on the prototype.

[Golden Rivet]

"Analysis and flight testing has shown that with flaps set at 30, spoiler deployment from large control wheel inputs (approximately 40 degrees or more) can cause a sudden onset of airflow separation at the outboard trailing edge flaps. This loss of lift may result in an unexpected roll. Abrupt control wheel inputs by the flight crew to counteract the roll can lead to unexpected roll oscillations during the final phase of the landing approach manuever. If these pilot inputs occur at a high rate, they may become out of phase with airplane response, causing the pilot to further increase the magnitude of the wheel inputs. This phenomenon where the pilot inputs become out-of-phase with the airplane reaction is known as pilot induced oscillations (PIO).
The airflow separation over the flaps causing the potential for PIO is what we consider a "trigger" situation. That is, installation of the flap vortilons will not necessarily eliminate a PIO event from occurring, but they do help eliminate a potential "trigger" for PIO.
Extensive study and analysis has identified three areas where improvement is needed to help prevent inducing PIOs. First, vortilons (or vortex generators) were added to the flaps to help prevent airflow separation. Second, flight crew awareness was addressed by a revision to the Operations Manual. Third, a control wheel force damper has been added to the mechanism underneath the First Officer's control column.
Additionally, research has shown a correlation between control wheel breakout force (centering breakout forces about neutral) and the likelihood that a pilot will get into a PIO situation. The correlation has not been quantified, but data suggests that the higher the wheel breakout force, the more potential there is for PIO to occur. Therefore, Boeing has established that 6.5 lb of wheel breakout force (42 in-lb torque) will be the optimum setting for all model 757 airplanes. This is reflected in new AMM procedures."

[Basil]

Roll PIO was known when I flew the B757 in the early nineties. Had it occur once and took standard PIO recovery action - stopped yanking the controls around :O Worked immediately (TF!)

[AC_Vital_to_Emerg]

Basil, without in any way compromising your anonymity, was it a UK airline? If yes, was it BA or otherwise?

My UK airline got B757's in the mid-90's and I don't remember anything about this.

Golden Rivet, when did the Ops Manual get revised, please?

[Basil]

Can't remember if it was Caledonian or BA.
At the time I understood it to be a known aircraft characteristic (bit like leaving the power on into the flare) which was known and, certainly in this case, easily contained. Because of the foregoing no ASR was filed.
I flew the type for about seven years and it was not a common occurrence. Roll feel & response was one of the characteristics I liked about the 757 cf the 767.