An emergency AD requires certain Learjet 45 horizontal stabilizer actuator assemblies to be replaced with new assemblies before further flight. The directive (AD 2003-06-51) was issued March 20 after a Learjet 45 in Australia reported a severe vibration followed by a rapid nose-down pitch. Investigation revealed that the acme screw–the primary load path of the actuator–was fractured, and the backup assembly had failed.