Originally Posted by
mm_flynn
Ian,
Are you certain there is actually a logoff ACARS message (as compared to 'logging off' in the airplane as a planned way of causing the system to stop sending messages). I can't find any indication that such a message type exists. Equally, I can't see a 'logon' ACARS message (as compared to the satcom logon). The closest message type to logon seems to be Link Test.
Also, the 1:07 message, based on the Inmarsat log, looks to be 'large' (30 signalling units). What appears to be the ACK is only three signalling units. I would expect a logoff message to be similar in size to the ACK. That coupled with the fact that no official documents has said 'the last ACARS message at 1:07 was a log off message' leads me to believe the early comments of ACARS being 'turned off' were imprecise comments that should technically have been 'ACARS stopped transmitting expected messages or responding to interrogations some time after the last routine message at 1:07'.
One can easily understand how a briefer in the heat of the moment would use version one vs version two.
ACARS was only licensed and activated on VHF by the Malaysians it was not linked to INMARSAT SATCOM or to Boeing or Rolls Royce health monitoring. So the ACARS logon/logoff would be an ARINC/SITA protocol over VDL which I believe is ARINC 622 'ATS Data Link Applications over ACARS, or 618 Air/Ground Character-Oriented Protocol Specification.