Hetfield :
The pilots goofed it! Simple, but true.....
Perhaps, but an accident never has a single cause. Had Jakarta ATC the correct a/c type and flight plan , the aircraft would have never been cleared to descend to 6000 ft in that area, and be left alone after that. . A decend on a return leg to land , yes but that would have been on a different course/heading.
These errors, or rather asumptions that led to errors, explains the ATC involvment discussed ( and speculated) at length at the begining of this thread.