Nigel Osborne
a Transponder that someone just can't turn off with a flick of a switch. Needs to be fitted so it cannot be tampered with from the cockpit, if that is indeed the actual reason of its lost signal.
From personal experience signal from a transponder may not necessarily be turned off deliberately. I am well aware that the vintage transponder in the C152 which I was flying was likely to be far less refined than that on MH370, however, in my particular case was asked by D&D to squawk 7700 and promptly complied - or so I thought. The transponder was certainly set correctly and should have been squawking but in fact was not. The request to squawk 7700 was the last transmission from D&D which I was able to hear for quite some time. D&D could hear me, but I couldn't hear D&D (or at least all transmissions from D&D were almost inaudible and muffled with no identifiable words). Communication was eventually restored via an Emirates Airbus and following a request to turn up the volume on the radio (it was already at maximum) I was asked to squawk 7700. My reply was that I was already doing that but would recycle, did so, and the problem was resolved. It is entirely possible that the crew of MH370 did squawk 7700 but was completely unaware that the transponder was not functioning.