Malaysian Airlines MH370 contact lost

[vapilot2004]

points of reference regarding aircraft data comm systems

Like all modern aircraft with EICAS/ECAM display systems, most of the 777 cockpit panels are merely control interfaces, or in Boeing vernacular, CDU's. The actual units are generally racked in the EE bay and not at all user friendly as to disablement.

The transponders are an easy disable - switch them to standby. Done. However, to prevent the ACARS system from transmitting data requires a bit more work. On the 777, ACARS is an integrated part of the AIMS and as such, has no off switch. One can disable the reporting module of the AIMS by either pulling specific CB's located in the EE bay (ACARS modem), de-racking the primary units (2 - ACARS modem LRU's) located in the EE bay AIMS cabinet, or (simplest method) by accessing the ACARS page on an FMS CDU and switching the data link to an unused frequency, deselecting ACARS downlink, or putting all 3 VHF radios in voice mode.

Similarly the SATCOM system is also fully integrated and while there is a control page available via the FMS CDU, there is no off switch. The cockpit CDU can however change birds and channels, but according to a senior maintenance engineer, this action would trigger a logoff signal to be sent heavenward. That signal is not part of the current set of known facts.

According to information we have so far, it appears the ACARS was disabled, while the SATCOM system remained active, allowing the aircraft to be known to be flying 4-5 hours after last comms were received.

As a side note, under current software revision, there is an FMS CDU page which will show the status of an active SATCOM link - and again going by facts known at this time, the interlopers were likely unaware of this CDU function or failed to understand the system fully. The information so far suggests the interlopers were knowledgeable at least to the level of an experienced 777 pilot, with some additional knowledge of the rarely used FMS pages to disable the ACARS.