Air Traffic Control Fully Certified
Communication Breakdown
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance
Human Factors
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.
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.