Originally Posted by
tcasblue
The Boeing symposium I attended was quite interesting with focus on aircraft flown by the company. Here is what they said on one particular subject.....
"The 787 aircraft fuselage is flexible and vibrations can develop(called Body Structural Modes). These vibrations can be transmitted by the pilot to the control column(known as Airplane Pilot Coupling).
There is a new section in the 787 FCTM about Body Structural Modes.
The 787 has a Vertical Mode Suppression function as an integral component of the flight control system. It is active during manual flight in Normal Mode and Landing Flaps manual flight for the 787-9 and in Normal Mode and any flap setting when below 315 CAS on the 787-10.
Note: Enhancements to the 787-9 VMS with FCM CBP5 SB787-27A0044, released December 18, 2018.
It is suggested to keep a relaxed grip on the controls and avoid sharp or oscillatory control inputs."
The unforeseen interactions of parts of a complex system is an interesting topic. When I was flying the Extra 300, in high speed descents the tiniest bump of turbulence would excite a yaw oscillation that would continue indefinitely, fed by my body swaying left and right in the seat and pressing alternating rudder pedals in phase. So I would have to stiffen up my legs and it would go away.
Or, the CRJ 700/900 equipped with IRSes, at or near idle N1's, the fan excites a harmonic of the IRS mounting, which sometimes causes about a 4Hz pulse of the stick with autopilot engaged. It's in our manuals, as long as it's not "excessive" or causes pitching of the airplane, disregard and treat as normal.