Malaysian Airlines MH370 contact lost

[BillS]

Towed Pinger Locator data

The TPL specifications give a max. detection range in excess of 3km with an operating depth of up to 6000 msw. Unconfirmed reports suggest that 600m above bottom is the close range localisation path.
With a cable length of up to 18km - turn around time is not surprising.

This paper: Flight Recorder Localization Following at-Sea Plane Crashes provides technical detail.
It goes beyond my comfort zone but perhaps others might highlight the important points.