Undoubtedly the BEA will be examining closely the reasons why the ULBs were not located.
In my mind, there are three reasons:-
The first is that there was only one functioning ULB as the ULB attached to the SSFDR was most likely struck and destroyed by collapsing frames/stringers during the initial impact sequence. Hence the reason it was not attached to the recorder.
Secondly, the SSCVR ULB finished up getting itself half buried and the effective output was attenuated by at least 3dB along with some localized acoustic absorption caused by silt. Overall, the radiated output was down by around 6dB as the polar ends are the least effective radiators.
Thirdly, the grid line that was being towed was north/south from 3°30'00"N to 3°00'00"N and there was the potential for the TPL to drift off line caused by the turn to head back north on the next grid line. If the towing vessel turned too early and gave insufficient time for the TPL to realign, then there was a potential for a nadir (shadow) to appear in the area covered by the tow. The fact that the tow (now deep at around 3,000m) may not have lined up until some time after passing through 3°00'00"N provided the opportunity for missed detection, coupled with the lower than optimal output by the remaining ULB.
The answer lies in the detail.
