If they ARE tracked in real time, why are we not reading that they knew EXACTLY where it was when it disappeared as opposed to reading things like "lose contact somewhere off Vietnam" and "may have turned back"?!
The 'realtime' ADSB info tx'd by aircraft is for air-air avoidance over watermass or land where there are no SITA ground stations. The data FR24 uses is from people like myself, At home capturing that data and sending it to a central point for display. ATC have different methods of this. Like us, they are somewhat blind over large water masses and rely on:
Traditional Radar (rotating statings pinging off the transponders)
MLAT - multi ground stations calculating position from 4+ points and delay in the replies
FAA ASDI data, which is from HF datapackets or sat uplink with bundled data bursts with previous active flight information.
VHF ACARS downlink - short message data
Voice Comms/Handover on HF
The difficulty with this case, is it was over short water. Between 2 countries now not wanting to take responsibility for being the last to hear from it it may seem. It was very close to the ATC boundy for handover to the destination. But they had not made contact themselves (pilots) as part of the operational process, and at the same time the departure ATC are all 'we assumed they were fine' since they had not heard from them leaving their boundries.
Then comes the starting to search over land cause 'thats where it would be by now' as apposed to looking at the guys saying 'err.. it may not be an official source but we lost contact with it while seeing a plane 20mins ahead just fine at this spot'
Major cluster. Chinese airports 'no, its OK they landed here..' or not. "it may be crashed over the coast cause its 2hrs since it left" or not.. Then changing their stories with the actual last time contacting via VHF/HF