Clearance interpretation problem. Altitude deviation altitude excursion.

Attributes
ACN	305050
Time
Date	199505
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : rst
State Reference	MN
Altitude	msl bound lower : 9000 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zmp tower : grb
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	descent other
Route In Use	enroute : direct
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	other : unknown
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : instrument
ASRS Report	305050
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Flight from msp to rst and level at 11000 ft. Handoff from departure control to ZMP was normal. On contact with ZMP, we noted traffic at 2-3 O'clock position low. We received a vector direct rst VOR, altimeter setting, and what we thought was clearance to 9000 ft. PNF read back all including 11000 ft for 9000 ft. The clearance to 9000 ft is normal at this point en route to rst. I started a slow descent from 11000 ft. At 10500 ft center queried our altitude. I replied (PNF off frequency for PA) 10500 ft for 9000 ft. At that point the controller said we were not cleared for descent (pause) cleared to 10000 ft. Ensuing conversation revealed it was the traffic descending through 9000 ft that was referenced and not a clearance for us. Controller indicated no separation problems. Captain called the facility, after landing, to ensure no conflicts existed and to note what we perceived as the cause, which was: foremost, the phrasing (which I can't recall) that combined the clearance with the traffic's passing altitude. Expectation of a descent clearance and hearback error and readback.

Original NASA ASRS Text

Title: CLRNC INTERP PROB. ALTDEV ALT EXCURSION.

Narrative: FLT FROM MSP TO RST AND LEVEL AT 11000 FT. HDOF FROM DEP CTL TO ZMP WAS NORMAL. ON CONTACT WITH ZMP, WE NOTED TFC AT 2-3 O'CLOCK POS LOW. WE RECEIVED A VECTOR DIRECT RST VOR, ALTIMETER SETTING, AND WHAT WE THOUGHT WAS CLRNC TO 9000 FT. PNF READ BACK ALL INCLUDING 11000 FT FOR 9000 FT. THE CLRNC TO 9000 FT IS NORMAL AT THIS POINT ENRTE TO RST. I STARTED A SLOW DSCNT FROM 11000 FT. AT 10500 FT CTR QUERIED OUR ALT. I REPLIED (PNF OFF FREQ FOR PA) 10500 FT FOR 9000 FT. AT THAT POINT THE CTLR SAID WE WERE NOT CLRED FOR DSCNT (PAUSE) CLRED TO 10000 FT. ENSUING CONVERSATION REVEALED IT WAS THE TFC DSNDING THROUGH 9000 FT THAT WAS REFED AND NOT A CLRNC FOR US. CTLR INDICATED NO SEPARATION PROBS. CAPT CALLED THE FACILITY, AFTER LNDG, TO ENSURE NO CONFLICTS EXISTED AND TO NOTE WHAT WE PERCEIVED AS THE CAUSE, WHICH WAS: FOREMOST, THE PHRASING (WHICH I CAN'T RECALL) THAT COMBINED THE CLRNC WITH THE TFC'S PASSING ALT. EXPECTATION OF A DSCNT CLRNC AND HEARBACK ERROR AND READBACK.

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.