Page 83 Final report
The first search phase aimed at detecting and locating the acoustic signals transmitted
by the Underwater Locator Beacon (ULB) fitted on each flight recorder
(8)
. As a priority,
the aeroplane’s planned flight path as well as the greatest possible area inside the
40 NM circle was swept by two Towed Pinger Locators (TPL)
(9)
.
No signal from either of the beacons was detected by the sensors deployed in the
area despite TPL passing by, on two occasions, not far from the debris field, on 22
and 23 June 2009.
Sonar imaging systems with the ability to recognise components on the sea bed were
deployed during the phases that followed.
I don't find any explications about why the signals were not detected
I don't find any in the BEA report about investigations on this problem (if the beacons were active at time .. or if they find some defects for explain the failure of detection ...)