At IGARI pilots were required to log in with Ho Chi Minh (“HCM”) Air Traffic Service by entering the code VVTS through their CPLDC. The Boeing 9M-MRO was equipped with ADS-C, not ADS-B aqnd if you understand the difference you may then realise why it is not as simple as just switching off.
When they logged in with HCM this it logged them off with Lumpur Control and would have the same appearance as switching off their Transponder, but I do not accept they switched it off.
On FR24 there is a track playback which shows ADS-C feed after 17:21 UTC which implies they continued to give a feed through SITA to Vietnam until they reached BITOD.
What is interesting to me about the feed to SITA after 17:21 UTC is that it was devoid of altitude data.
Is that purely because once logged out with Lumpur the data was not being received or was that indicative of a partial electrical failure before all the data feed stopped?