Narrative:

I was vectoring for increased in-trail at a reduced speed and the sector the aircraft was previously in had held the aircraft for some time and then vectored out of the hold to meet our requirements. Speeds were not working so I had started to fan aircraft on the arrival out to the north. C90 called and informed us we were back in the hold so I started to head aircraft back east and amended altitudes as necessary. My lead aircraft needed to divert to fwa and wanted to climb to a higher altitude so I used headings to allow a climb sooner. A CRJ7's interim altitude of 120 was related to this circumstance and as the aircraft approached an eastbound aircraft at 110 he descended through 120. The pilot's explanation of why it happened did not make sense to me. I am not a pilot and have no first-hand knowledge of what a flight deck/FMS looks like. Traffic volume and complexity were relativity low; the weather wasn't causing any deviations in my airspace; and pilots were reading back all of my clearances and not clogging up the frequency with questions. However; I was vectoring for increased in-trail at a reduced speed and the sector the aircraft was previously in had held the aircraft for some time and then vectored out of the hold to meet our requirements. It was only 2 or 3 minutes prior to the deviation that C90 put us back into the hold while we were vectoring and 2 aircraft requested to divert as soon as they had the opportunity. The whole scenario seemed like the perfect opportunity for a pilot deviation and I couldn't dismiss the possibility that I had cleared him lower until I heard it. Maybe I'm fooling myself about the complexity level on my end; but I could certainly see this situation being complex on the pilot's end. I have no recommendations beyond constant vigilance during weather shifts on the controller's and pilot's end. If a pilot has doubts as to their clearance they should never hesitate to ask and controllers should be grateful for their professionalism.

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Original NASA ASRS Text

Title: Enroute Controller experienced a loss of separation event apparently due to a pilot's unauthorized descent.

Narrative: I was vectoring for increased in-trail at a reduced speed and the sector the aircraft was previously in had held the aircraft for some time and then vectored out of the hold to meet our requirements. Speeds were not working so I had started to fan aircraft on the arrival out to the north. C90 called and informed us we were back in the hold so I started to head aircraft back east and amended altitudes as necessary. My lead aircraft needed to divert to FWA and wanted to climb to a higher altitude so I used headings to allow a climb sooner. A CRJ7's interim altitude of 120 was related to this circumstance and as the aircraft approached an eastbound aircraft at 110 he descended through 120. The pilot's explanation of why it happened did not make sense to me. I am not a pilot and have no first-hand knowledge of what a flight deck/FMS looks like. Traffic volume and complexity were relativity low; the weather wasn't causing any deviations in my airspace; and pilots were reading back all of my clearances and not clogging up the frequency with questions. However; I was vectoring for increased in-trail at a reduced speed and the sector the aircraft was previously in had held the aircraft for some time and then vectored out of the hold to meet our requirements. It was only 2 or 3 minutes prior to the deviation that C90 put us back into the hold while we were vectoring and 2 aircraft requested to divert as soon as they had the opportunity. The whole scenario seemed like the perfect opportunity for a pilot deviation and I couldn't dismiss the possibility that I had cleared him lower until I heard it. Maybe I'm fooling myself about the complexity level on my end; but I could certainly see this situation being complex on the pilot's end. I have no recommendations beyond constant vigilance during weather shifts on the controller's and pilot's end. If a pilot has doubts as to their clearance they should never hesitate to ask and controllers should be grateful for their professionalism.

Data retrieved from NASA's ASRS site as of July 2013 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.