Secondary radar lost

The last recorded secondary surveillance radar position was at 17:21:13 UTC. A transponder dropout at the FIR boundary delayed recognition that the aircraft had left its cleared route: civil ATC relies on Mode A/C/S, and primary radar coverage over the Gulf of Thailand is limited. Royal Malaysian Air Force primary radar, reviewed after the fact, showed a westbound track across the peninsula and up the Strait of Malacca. The investigation found no evidence of a squawk 7700 (emergency), 7600 (radio failure) or 7500 (unlawful interference) at the moment of dropout.