ZSE controller described a below authorized altitude event when an aircraft apparently misunderstood...

Attributes
ACN	980050
Time
Date	201111
Local Time Of Day	1801-2400
Place
Locale Reference	YKM.Airport
State Reference	WA
Environment
Light	Night
Aircraft 1
Make Model Name	Turbo Commander (JetProp) Undifferentiated
Operating Under FAR Part	Part 135
Flight Phase	Descent
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Fully Certified
Person 2
Function	Pilot Flying Captain
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 40 Flight Crew Total 2900 Flight Crew Type 210
Events
Anomaly	ATC Issue All Types Deviation - Altitude Overshoot Deviation - Procedural Clearance Inflight Event / Encounter CFTT / CFIT

Narrative:

I was working only 2 aircraft both inbound to ykm. An AC90 was descending to 9;000 ft and had read back the clearance. I issued 6;000 ft to the other aircraft and that aircraft read it back. I noticed that the AC90 had descended below 9;000 ft and I asked him to say altitude. He read back what the mode C was showing 8;300 ft. I immediately issued a climb back to 9;000 ft and told the aircraft my minimum IFR altitude. The AC90 said that he read back a clearance to 6;000 ft. I never heard him read back that clearance and was not busy; so I should have caught it if he did.I am always very focused on read backs and take extra care to listen. We can always listen more closely and try to avoid read back errors.

Original NASA ASRS Text

Title: ZSE Controller described a below authorized altitude event when an aircraft apparently misunderstood the altitude assignment.

Narrative: I was working only 2 aircraft both inbound to YKM. An AC90 was descending to 9;000 FT and had read back the clearance. I issued 6;000 FT to the other aircraft and that aircraft read it back. I noticed that the AC90 had descended below 9;000 FT and I asked him to say altitude. He read back what the Mode C was showing 8;300 FT. I immediately issued a climb back to 9;000 FT and told the aircraft my minimum IFR altitude. The AC90 said that he read back a clearance to 6;000 FT. I never heard him read back that clearance and was not busy; so I should have caught it if he did.I am always very focused on read backs and take extra care to listen. We can always listen more closely and try to avoid read back errors.

Data retrieved from NASA's ASRS site as of April 2012 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.

ZSE controller described a below authorized altitude event when an aircraft apparently misunderstood the altitude assignment.