Narrative:

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.

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Original NASA ASRS Text

Title: ZDC Controller reported that an aircraft was issued one altitude; but descended below that altitude in a holding pattern.

Narrative: 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.

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.