Originally Posted by
Sailvi767
Winglets are highly susceptible to flutter. A 737-800 at cruise is already near VNE. A pushover would quickly result in putting the winglets into the flutter zone and separation from the aircraft. Flutter is primarily a TAS issue and it doesn’t take much speed increase at altitude to exceed VNE. Most aircraft with retrofit winglets have their VNE reduced for this reason. With high altitude winds the distance is very plausible.
Concur, with caveat. The winglet (in fact all winglets) reduces the flutter boundary of a wing. The B737 took additional mass addition and stiffening in order to get a stable Nyquist plot of the system to a perturbation. The chance that there was a problem with the winglet that caused the upset is remote, the flutter of a winglet is much more likely to be a resultant symptom of an overspeed of the aircraft in an upset, or an overspeed by some other unknown cause. The plane jane wing doesn't have any particular issue with flutter. The older wings of the stumpy classics are not bad at all. Any wing if pushed enough will have an aeroelastic issue at some point. One B734 splash showed that the abuse of the boundary of the design was so severe that there was evidence of roll control reversal, which was adding to the disastrous condition the drivers got themselves into. On the day of this accident, there was some isolated buildups in a line E-W around Guangzhou- Xiamen, and there was moderate jetstreams to the north by recollection. Flutter often gives an interstitial tear failure in a composite when it finally lets loose, which was what the winglet looked like having from the start.
AFAICS, there are not any obvious situations that come to mind that a winglet that would cause a flutter event within the normal envelope of the aircraft, assuming the structure did not have a history for structural damage and a poorly conducted repair...
Caveat: The effect of a winglet is well established as far as aeroelastic effects go. TBC was aware of the effect on the early wing structure as was APB, and the structure was amended to restore the boundary to a reasonable margin. The approved envelope was also amended (by memory, a while ago) and TBC had separately published a pretty good AERO article on the subject. The aircraft meets the certification requirements and the operational boundary was set as a result. There is no "safety" issue related to having winglets that are correctly designed and certified. That flutter likely occurred indicates that the envelope was either exceeded severely, or the structure had an issue that lowered the flutter boundary, which would be something like undetected damage or am inadequate repair of known damage.
Boeing AERO 17 BLENDED WINGLETS