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Based on official statements, and statements made by "unnamed U.S. officials," I believe MH370's Aircraft Communications Addressing and Reporting System (ACARS) was disabled at 1:07 am MYT (17:07 UTC).
March 13: ABC News [1]
"Two U.S. officials tell ABC News" The data reporting system, they believe, was shut down at 1:07 a.m. The transponder -- which transmits location and altitude -- shut down at 1:21 a.m.
March 15: Malaysian PM Najib Razak [2]
Based on new satellite information, we can say with a high degree of certainty that the Aircraft Communications Addressing and Reporting System (ACARS) was disabled just before the aircraft reached the East coast of peninsular Malaysia. Shortly afterwards, near the border between Malaysian and Vietnamese air traffic control, the aircraft’s transponder was switched off.
Preliminary Report [3]
17:01:16 UTC MH370 reported maintaining FL 350
17:07:29 UTC last ACARS message (page 4) / Inmarsat plot satellite data
17:07:49 UTC last ACARS communication (page 3)
17:07:55 UTC MH370 reported maintaining FL 350
Note: The screen shot was captured at FlightRadar24.
1. http://abcnews.go.com/International/malaysia-airliner-pinging-indication-crashed-indian-ocean/story?id=22894802
2. http://www.theguardian.com/world/2014/mar/15/malaysian-pms-full-statement
3. https://www.facebook.com/media/set/?set=a.10152221402889355.1073741995.72613804354
