B737-900
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 4
Situational Awareness; Distraction; Communication Breakdown
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Procedure; Human Factors
Sector 19 went into a hold for LGA airport. Before I issued holding instructions to Aircraft X; I issued an amended altitude clearance: 'Aircraft X; amend altitude; descend via the KORRY3 arrival; except cross BRAND at and maintain one-one; eleven thousand.' Aircraft X read back; 'Roger; descending via the KORRY except cross BRAND at eleven.' I continued issuing a few other clearances then gave Aircraft X holding instructions at RBV. He read them back correctly. After that exchange I said in a separate transmission 'Aircraft X; I just want to reiterate; cross BRAND at and maintain one-one; eleven thousand; I'll have traffic holding directly beneath you in the pattern.' Aircraft X read back; 'yes; BRAND at eleven.' A few minutes later Aircraft X checked in the hold at RBV: 'Aircraft X; established in the hold at ten thousand.' I rogered the airplane; but it took a minute to realize what he said; and my D-side also told me that I now had two airplanes in the same holding pattern at the same altitude. I asked Aircraft X to verify his altitude; 'Aircraft X; verify level at one-one thousand?' and Aircraft X said 'negative; we are level at ten thousand; you told us to descend via the KORRY except cross BRAND or HOLEY at eleven thousand.' I climbed Aircraft X immediately to eleven thousand. No separation was lost; nor was there a TCAS RA.I think it was a misunderstanding or a misinterpretation on the pilots part of what I had expected the aircraft to do.
ZDC Controller reported that an aircraft was issued one altitude; but descended below that altitude in a holding pattern.