Learjet 55
Air Traffic Control Developmental
Training / Qualification; Situational Awareness; Confusion
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic
Human Factors; Procedure
A Citation was level at 10000 feet westbound via MOLLY4. The Learjet checked in descending out of 13000 feet for 11000 feet via MOLLY4 RNAV Arrival. At MOLLY Intersection the Learjet was level at 11000 feet; and then suddenly he started to descend to 9000 feet head on with the Citation. My Instructor noticed the Learjet was descending and in conflict with the Citation. Immediately; he took over the position and instructed the Learjet an expedited descent instruction to 9000 feet to avoid the Citation.[I should] confirm with the ARTCC Controller if the aircraft has been instructed to maintain altitude above my traffic and/or restate to maintain the altitude I need the aircraft to maintain for traffic. Do not rely on pilots when they report descending to an altitude assigned by other controller. I will ensure and instruct the pilot to maintain the altitude I need the aircraft to maintain.
A Trainee Controller reported an aircraft he thought was descending to 11000 feet continued descending through 11000 feet into opposite direction traffic at 10000 feet.