Light Transport; Low Wing; 2 Turboprop Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 14
Communication Breakdown
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic; Flight Crew Returned To Clearance
Procedure
I issued a 'Cleared KRANN3 arrival Runway 27 Transition' to Aircraft Y and the pilot read it back correctly. I exchanged traffic information with both aircraft. Aircraft Y began his descent. I caught the descent when Aircraft Y was leaving 10;800; and instructed the pilot to maintain 11;000. I instructed Aircraft X and Aircraft Y to maintain visual separation from each other. Both pilots read it back timely and correctly. I issued a 'Brasher' warning to Aircraft Y and asked what happened? His response was that he received a 'descend Via' clearance.In reference to all of the RNAV changes in recent months/years; I feel that all of this change is confusing to flight crews and controllers. I specifically told the crew that there was crossing traffic and issued traffic; and the crew still descended!My recommendation is to not use the word 'cleared' in a clearance associated with RNAV procedures. If we are going to use this phraseology; I recommend re-iterating to the pilot the altitude to maintain. 'Cleared KRANN3 Runway 27 Transition; maintain 11;000'. We shouldn't have to; but this is an added measure of safety until we find a better way of doing this.
Reporter states difficulty with pilot in understanding the difference between cleared and descend via phraseology which led to an operational error.