B777-200 lateral oscillation in turbulence
Although I trained (during the late ‘50s) as an a/c engineer, my experience of commercial aircraft has been as a passenger since 1965.
I raise this matter following a return trip to Auckland in an AirNZ 772. I have done this trip about 20 times since 1983, via both LA and HK in 747s and latterly 777s. On this trip, for the first time, I sat in the rearmost two rows of the aft cabin of a 777-200.
On the outbound two legs there was little turbulence, but what there was promoted a rapid ( 1+cycle/sec) lateral oscillation of between 2 and 6 cm. On the return trip, particularly on the AKL – HK leg the high altitude turbulance was more severe. This caused, at the rear of the aft compartment, a continuing oscillation as described above which effectively prevented any chance of rest. It was so severe at times that one’s rib-cage was hitting each arm rest every second! The aircraft was quite literally wagging it’s tail!
Being interested in this beyond self comfort I went forward ahead of row 53 – the front of the rear cabin, and into the mid cabin. The lateral movement was almost un-noticeable. From there, observing the passengers in the rear cabin was interesting; heads were rocking on relaxed necks from side to side, and the lower trunks of the heavier passengers were visibly wobbling back and forth. This phenomenon arose in anything but the mildest turbulence.
I made enquiries of the cabin crew and was told that ALL their 777s were the same, demonstrating the same oscillation in anything but still air. They acknowledged that the rearmost 14 rows were extremely uncomfortable in any turbulence for this reason.
The movement appears to be a rapid yaw and reverse about a point somewhere near the wing centreline, or perhaps the c of g.
While I am sure that Boeing’s stressmen are more than competent, I was given to wonder at the moments being created at the base of the vertical stabiliser; I estimate that this lateral oscillation was present for more that seven ours of the twelve hour leg.
Apart from resolving never again to occupy a seat in the rear cabin of a 777 I would be interested to hear any engineering comment about the phenomenon, and a confirmation that this problem is not unique to AirNZ’s fleet.