A319
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 15.5
Confusion; Situational Awareness
ATC Issue All Types; Airspace Violation All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Separated Traffic; Flight Crew Returned To Clearance
Procedure; Human Factors
Aircraft X checked on at FL390. I issued a descent to FL370 to get the aircraft out of Sector 40 airspace since the aircraft was landing at DSM. The pilot read back descending to FL370. I had traffic at FL360 and FL350 that was diverging. About a minute goes by and I notice my En Route Information Display System (ERIDS) light up with conflictions. I look back to the scope and observe Aircraft X descending below the assigned FL370. It takes about 3 seconds to fully process what I am seeing. Aircraft X is descending head on threw the traffic at FL360 and is busting through my base altitude of FL350. I ask the pilot what altitude I assigned believing I misstated the original altitude of FL370 or missed the read back. While this is happening I forced a data block up on Sector 39 scope. The Sector 39 controller shouted out it was ok; he only had traffic at FL300. The pilot responds that he was assigned FL270. Still not believing what I was seeing I asked if he was at FL370 to which he responded the altitude he was descending out of. I amended Aircraft X altitude to FL330 and hand him off to sector 39.This happened my last stint before I went home. I returned that night for the midnight shift and listened to the tape. I did issue FL370. Aircraft X responded descending to FL370. I question the pilot about the altitude I assigned he responded FL270. It was a pilot deviation on his part which I didn't react to quickly enough and I had an airspace violation with Sector 39. Had Aircraft X not descended as quickly as he did there could have been 3 aircraft with TCAS alerts to respond to. Luckily no separation was lost however I was giving quite a scare.I shouldn't have questioned myself. I should have forced that data block to Sector 39 and stopped the aircraft immediately and asked question later. I've been in the agency almost years and this skill is hard to teach and obviously execute when it matters. Pilot deviations happen. It all goes back to hear back/read back on both sides of the microphone.
ZMP Controller reported of a pilot deviation. Pilot read back correct altitude to descend to but went through the cleared altitude. No Separation was lost due to the descent rate of the deviating aircraft.