Yaw Damper
Independent of AP and FD. Engaged following AFCS power up. Disengaged when YD button is pushed, the FBW system turns to direct mode, FBW system indicates the rudder has failed,
EICAS messages
YD OFF - yaw damper switched off
FLT CTRL FAULT - FBW in direct mode
RUDDER FAIL - FBW detects rudder failure