no-hoper:
What the microphones are recording...
Post #2985 was nearly correct,but refering to the CAA rules.
Page 16 and 17 showing clearly the difference FAA/CAA;
ACARS ARCHITECTURE - Description and Operation - System Description_°Ù¶ÈÎÄ¿â
That looks to be an old document. Current FAA regs also require hot mics for the flight crew microphones.