B737 with ZID was issued descent to occupied altitude resulting in operational error.

Attributes
ACN	634653
Time
Date	200410
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : zid.artcc
State Reference	IN
Altitude	msl bound lower : 30700 msl bound upper : 33000
Environment
Flight Conditions	IMC
Weather Elements	Ice Turbulence
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zid.artcc
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zid.artcc
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 210 flight time total : 20000 flight time type : 12000
ASRS Report	634653
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 180 flight time total : 5800 flight time type : 2800
ASRS Report	634654
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation
Independent Detector	aircraft equipment : tcas other flight crewa other flight crewb
Resolutory Action	flight crew : took precautionary avoidance action
Supplementary
Problem Areas	ATC Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

We were cleared to descend from FL370 to FL350. Shortly after leveling off ATC cleared us to descend to FL310. While expediting the descent for turbulence passing FL330 I noticed traffic below us and I slowed our descent so as not to get an RA. Passing FL307 and the traffic changing to white on the TCASII, ATC said that we had descended below our clearance altitude of FL330. We immediately climbed back to FL330. ATC then advised us of a deviation and to call their office. We both recalled setting FL310 in the window and both verbally and visually confirming it. I was confident in the fact that we set it correctly. After landing, we called ZID at the number given and was told that after reviewing the tapes, we were indeed cleared to FL310 and it was a controller error. Thanks again to the TCASII for the alert.

Original NASA ASRS Text

Title: B737 WITH ZID WAS ISSUED DSCNT TO OCCUPIED ALT RESULTING IN OPERROR.

Narrative: WE WERE CLRED TO DSND FROM FL370 TO FL350. SHORTLY AFTER LEVELING OFF ATC CLRED US TO DSND TO FL310. WHILE EXPEDITING THE DSCNT FOR TURB PASSING FL330 I NOTICED TFC BELOW US AND I SLOWED OUR DSCNT SO AS NOT TO GET AN RA. PASSING FL307 AND THE TFC CHANGING TO WHITE ON THE TCASII, ATC SAID THAT WE HAD DSNDED BELOW OUR CLRNC ALT OF FL330. WE IMMEDIATELY CLBED BACK TO FL330. ATC THEN ADVISED US OF A DEV AND TO CALL THEIR OFFICE. WE BOTH RECALLED SETTING FL310 IN THE WINDOW AND BOTH VERBALLY AND VISUALLY CONFIRMING IT. I WAS CONFIDENT IN THE FACT THAT WE SET IT CORRECTLY. AFTER LNDG, WE CALLED ZID AT THE NUMBER GIVEN AND WAS TOLD THAT AFTER REVIEWING THE TAPES, WE WERE INDEED CLRED TO FL310 AND IT WAS A CTLR ERROR. THANKS AGAIN TO THE TCASII FOR THE ALERT.

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.