A VFR small aircraft descended in the pdx traffic pattern when it had been told to maintain 2000 ft.

Attributes
ACN	338220
Time
Date	199606
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : pdx
State Reference	OR
Altitude	msl bound lower : 1200 msl bound upper : 2000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : pdx tracon : ggg
Operator	general aviation : personal
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	Part 91
Flight Phase	other
Route In Use	approach : visual
Flight Plan	VFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 135
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : private pilot : instrument
Experience	flight time last 90 days : 20 flight time total : 550 flight time type : 200
ASRS Report	338220
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	other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

After canceling my IFR clearance with portland approach, I was cleared for a right downwind approach to runway 28R at pdx. I was also told to maintain 2000 ft by the controller in the tower. I maintained 2000 ft until entering the downwind pattern at which time I began descending to the tpa (1100 ft). The controller noted this and asked for my altitude which at that time was 1200 ft. She advised me that there was a commuter aircraft on final for runway 21 at pdx and that is why she wanted me at 2000 ft. I had the other aircraft in sight and was able to avoid any 'close calls.' however, I obviously should have remained at the last assigned altitude and not assumed that once in the traffic pattern I could begin my descent. I apologized for my error and certainly learned a good lesson -- do not assume anything!! Always check before changing altitudes or headings when under positive control.

Original NASA ASRS Text

Title: A VFR SMA DSNDED IN THE PDX TFC PATTERN WHEN IT HAD BEEN TOLD TO MAINTAIN 2000 FT.

Narrative: AFTER CANCELING MY IFR CLRNC WITH PORTLAND APCH, I WAS CLRED FOR A R DOWNWIND APCH TO RWY 28R AT PDX. I WAS ALSO TOLD TO MAINTAIN 2000 FT BY THE CTLR IN THE TWR. I MAINTAINED 2000 FT UNTIL ENTERING THE DOWNWIND PATTERN AT WHICH TIME I BEGAN DSNDING TO THE TPA (1100 FT). THE CTLR NOTED THIS AND ASKED FOR MY ALT WHICH AT THAT TIME WAS 1200 FT. SHE ADVISED ME THAT THERE WAS A COMMUTER ACFT ON FINAL FOR RWY 21 AT PDX AND THAT IS WHY SHE WANTED ME AT 2000 FT. I HAD THE OTHER ACFT IN SIGHT AND WAS ABLE TO AVOID ANY 'CLOSE CALLS.' HOWEVER, I OBVIOUSLY SHOULD HAVE REMAINED AT THE LAST ASSIGNED ALT AND NOT ASSUMED THAT ONCE IN THE TFC PATTERN I COULD BEGIN MY DSCNT. I APOLOGIZED FOR MY ERROR AND CERTAINLY LEARNED A GOOD LESSON -- DO NOT ASSUME ANYTHING!! ALWAYS CHK BEFORE CHANGING ALTS OR HDGS WHEN UNDER POSITIVE CTL.

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.