Joined: 06 August 2009
As I understand it:
+ The transponder will respond to secondary radar interogation signals, and will fall silent once outside radar range.
+ ACARS will send regular signals provided it is enabled and scheduled. If it was not planned to be used during the flight (to reduce communication costs) there would be no indication if it were disabled during the flight. And if it were planned but disabled in-flight, the break in transmissions should have raised a red flag immediately.
The transponder will naturally fall silent as soon as the flight goes beyond range of secondary radar. The only way to tell it has been turned off is if the aircraft stops responding to secondary radar interogations. If it wasn't seen on secondary radar, only on military (primary) radar, it wouldn't have received any such interogation requests and it would have been silent whether it was turned on or not.
And if ACARS was disabled, there is no other remote link to the aircraft systems to be able to see whether or not the transponder has been turned off.
So if the ACARS was turned off at such an early stage, why didn't MAS raise the alarm earlier because of the sudden break in transmissions? And with ACARS switched off and the plane not spotted by secondary radar, why are MAS so convinced the transponder was turned off? How would they tell?