Enroute controller described an airspace violation when the aircraft involved apparently confused FL...

Narrative:

I was working a B737 (FL340). The SOP gives me control to descend to FL330 from the adjacent sector (X). I cleared the B737 to FL330 and initiated a hand off to sector Y (beneath me). After the hand off was accepted I switched communications to sector Y. In my next scan around I saw that the B737's data block showing FL324. I asked the sector Y controller to check that altitude for me. The pilot told her that he was descending to FL230! The pilot was then assigned FL300. I can't verify the original read back; nor do I know what the pilot said when he checked on with sector Y. No other traffic was affected by this action; however the airplane did descend below FL330 in sector X and into sector Y. It is interesting because I have had this problem in the past; with pilots reading back 230 instead of 320; and I'm a little bit paranoid about it. I started saying flight level three two zero; thirty two just for emphasis. I don't recall doing that this time; of course. Many times when I say it that way; the pilot reads it back so perfectly clear; like I didn't need to say the extra part and I feel like I'm just adding extra words. Perhaps that's really why it's clear. Since that method seems to work I will try to use it all the time.

Original NASA ASRS Text

Title: Enroute Controller described an airspace violation when the aircraft involved apparently confused FL330 with FL230; the reporter listed a phraseology modification that might prevent future altitude assignment confusion.

Narrative: I was working a B737 (FL340). The SOP gives me control to descend to FL330 from the adjacent sector (X). I cleared the B737 to FL330 and initiated a hand off to Sector Y (beneath me). After the hand off was accepted I switched communications to Sector Y. In my next scan around I saw that the B737's Data Block showing FL324. I asked the Sector Y Controller to check that altitude for me. The pilot told her that he was descending to FL230! The pilot was then assigned FL300. I can't verify the original read back; nor do I know what the pilot said when he checked on with Sector Y. No other traffic was affected by this action; however the airplane did descend below FL330 in Sector X and into Sector Y. It is interesting because I have had this problem in the past; with pilots reading back 230 instead of 320; and I'm a little bit paranoid about it. I started saying flight level three two zero; thirty two just for emphasis. I don't recall doing that this time; of course. Many times when I say it that way; the pilot reads it back so perfectly clear; like I didn't need to say the extra part and I feel like I'm just adding extra words. Perhaps that's really why it's clear. Since that method seems to work I will try to use it all the time.

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.

Attributes
ACN	1005118
Time
Date	201204
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Aircraft 1
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

Enroute controller described an airspace violation when the aircraft involved apparently confused FL330 with FL230; the reporter listed a phraseology modification that might prevent future altitude assignment confusion.