An ATR42 flight crew descended below assigned altitude.

Attributes
ACN	437030
Time
Date	199905
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : sbi.vor
State Reference	TX
Altitude	msl bound lower : 8500 msl bound upper : 9000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : bpt.tracon
Operator	common carrier : air carrier
Make Model Name	ATR 42
Navigation In Use	other vortac
Flight Phase	cruise : enroute altitude change
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	Part 91
Flight Phase	cruise : level
Flight Plan	None
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 190 flight time total : 3400 flight time type : 1200
ASRS Report	437030
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : instrument pilot : commercial
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance
Independent Detector	other controllera
Resolutory Action	controller : issued alert
Miss Distance	horizontal : 20000 vertical : 500
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were given a clearance to 'cross iah arrival smith intersection at 8000 ft.' during our descent we were given 'pilot's discretion to 9000 ft' and what we understood as 'cross smith at 8000 ft.' we visually noticed traffic and slowed our descent rate and when clear of traffic we continued down to 8000 ft. Passing 8500 ft ATC queried about our altitude and informed us that we were to maintain 9000 ft and expect smith at 8000 ft because of traffic below us. We reported the traffic in sight and ATC cleared us from our position of 8500 ft to 8000 ft. We received no TCASII alert nor did we receive a GPWS alert. Contributing factors to misunderstanding of altitude assignment could be high volume of ATC traffic commands with other aircraft resulting in an inadvertent clearance or a miscom on behalf of the flight crew with ATC due to accomplishing required checklists and duties required for arrival phase. Safety of flight was not compromised as the WX conditions were VMC, and the conflicting aircraft was idented both on TCASII and visually.

Original NASA ASRS Text

Title: AN ATR42 FLC DSNDED BELOW ASSIGNED ALT.

Narrative: WE WERE GIVEN A CLRNC TO 'CROSS IAH ARR SMITH INTXN AT 8000 FT.' DURING OUR DSCNT WE WERE GIVEN 'PLT'S DISCRETION TO 9000 FT' AND WHAT WE UNDERSTOOD AS 'CROSS SMITH AT 8000 FT.' WE VISUALLY NOTICED TFC AND SLOWED OUR DSCNT RATE AND WHEN CLR OF TFC WE CONTINUED DOWN TO 8000 FT. PASSING 8500 FT ATC QUERIED ABOUT OUR ALT AND INFORMED US THAT WE WERE TO MAINTAIN 9000 FT AND EXPECT SMITH AT 8000 FT BECAUSE OF TFC BELOW US. WE RPTED THE TFC IN SIGHT AND ATC CLRED US FROM OUR POS OF 8500 FT TO 8000 FT. WE RECEIVED NO TCASII ALERT NOR DID WE RECEIVE A GPWS ALERT. CONTRIBUTING FACTORS TO MISUNDERSTANDING OF ALT ASSIGNMENT COULD BE HIGH VOLUME OF ATC TFC COMMANDS WITH OTHER ACFT RESULTING IN AN INADVERTENT CLRNC OR A MISCOM ON BEHALF OF THE FLC WITH ATC DUE TO ACCOMPLISHING REQUIRED CHKLISTS AND DUTIES REQUIRED FOR ARR PHASE. SAFETY OF FLT WAS NOT COMPROMISED AS THE WX CONDITIONS WERE VMC, AND THE CONFLICTING ACFT WAS IDENTED BOTH ON TCASII AND VISUALLY.

Data retrieved from NASA's ASRS site as of July 2007 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.