Gentlemen,
I need to emphasize here that the only way I can consider unvoluntary unbuckling due to minor or moderate accelerations is the way described and tested by the ATSB during the inquiry into the QF72 incident : that the buckle gets snagged under the armrest in the right position for the release to catch the armrest and open the buckle.
The ATSB tested this, and manual forces were enough to open the buckle. No need for dozens of Gs here.
Potential for inadvertent seatbelt release : Six passengers reported to the ATSB that they were seated with their seatbelt fastened at the time of the first upset, but that the seatbelt became unfastened and did not restrain them in their seats. Three of those passengers advised that they had their seatbelts tightly fastened, and three advised that they had their seatbelts loosely fastened. None of the six passengers could provide details of how their seatbelts released. As advised in the first Interim Factual Report, the investigation identified a scenario whereby seatbelts could inadvertently release. For this to occur, the seatbelt had to be loosely fastened and the buckle had to be positioned in a vertical orientation underneath the right armrest prior to an upward force being applied. The lift-latch could then catch on the armrest and the buckle release. The ATSB has conducted further examinations of this inadvertent release scenario on one of the operator’s A330 aircraft. Those examinations found that, for this scenario to occur on those aircraft, the seatbelt had to be adjusted so that there was at least 25 cm of slack in the belt (comparing the length of a firmly-fastened seatbelt with one that was loosely fastened to the minimum extent necessary to enable the inadvertent release scenario to occur). The certification requirements for aircraft seatbelts required that the possibility for inadvertent release of seatbelts is minimised. However, design and testing requirements for seats and seatbelts are based on the principle that seatbelts are ‘properly worn’.