A trainee controller reported an aircraft he thought was descending to 11000 feet continued descendi...

Attributes
ACN	1486649
Time
Date	201710
Local Time Of Day	1801-2400
Place
Locale Reference	ELP.TRACON
State Reference	TX
Aircraft 1
Make Model Name	Learjet 55
Operating Under FAR Part	Part 91
Flight Phase	Descent
Route In Use	STAR MOLLY4
Flight Plan	IFR
Aircraft 2
Make Model Name	Citation II S2/Bravo (C550)
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	IFR
Person 1
Function	Trainee Approach
Qualification	Air Traffic Control Developmental
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

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.