Improper descent clearance causes less than standard separation.

Attributes
ACN	343723
Time
Date	199608
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : tul
State Reference	OK
Altitude	msl bound lower : 24700 msl bound upper : 25000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zme tracon : lax
Operator	common carrier : air carrier
Make Model Name	B727 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise other descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : second officer
Qualification	pilot : instrument pilot : commercial pilot : flight engineer
Experience	flight time last 90 days : 200 flight time total : 12000 flight time type : 3000
ASRS Report	343723
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 : required legal separation other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	Other
Miss Distance	horizontal : 18000 vertical : 700
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

While cruising level FL250, encountered light chop. Requested FL210 from ATC. Controller responded descend and maintain FL240, expect FL210 shortly. Proceeded with descent and reported out of FL250 for FL240. At approximately FL247, ATC said 'confirm you're level FL250.' replied 'no, we're descending to FL240 as cleared.' ATC said 'turn right heading 180 degrees.' at this time we observed traffic approximately 200 ft below and 3 mi away. When separation was restored, ATC reclred us to FL240.

Original NASA ASRS Text

Title: IMPROPER DSCNT CLRNC CAUSES LTSS.

Narrative: WHILE CRUISING LEVEL FL250, ENCOUNTERED LIGHT CHOP. REQUESTED FL210 FROM ATC. CTLR RESPONDED DSND AND MAINTAIN FL240, EXPECT FL210 SHORTLY. PROCEEDED WITH DSCNT AND RPTED OUT OF FL250 FOR FL240. AT APPROX FL247, ATC SAID 'CONFIRM YOU'RE LEVEL FL250.' REPLIED 'NO, WE'RE DSNDING TO FL240 AS CLRED.' ATC SAID 'TURN R HDG 180 DEGS.' AT THIS TIME WE OBSERVED TFC APPROX 200 FT BELOW AND 3 MI AWAY. WHEN SEPARATION WAS RESTORED, ATC RECLRED US TO FL240.

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.