Originally Posted by
mm_flynn
I have little technical knowledge of the ACARS system, but it appears to not intrinsically require a log on/log off process. It seems to just be a VHF/HF/Satcom store and forward system where individual data packages are sent to the central service for storage and then periodically polled or pushed until an ACK is received or a retry limit is hit.
It is also not clear to me that the Satcom system itself has any 'logoff' function and therefore if there would be anything to distinguish between the device being turned off/ put into standby/ loosing power/ being destroyed
The logon sets up the store and forward system and checks that you are a subscriber to ARINC or SITA messaging systems. Similarly, the SATCOM sets up a link at the lower protocol layers that also allow check of subscription against the aircraft ID, or you would not be able to start the SATCOM phone or other uses of the SATCOM system. These actions all happen on power on and are 'transparent' to the pilot. This is what was meant by the Earlier statements about 'pilots not knowing how to switch the comms systems off'. From the pilot perspective the comms can be switched off - but they are actually still connected at the network level in case the pilot wants to switch the higher level application on again.