B767 crew was cleared to descend on top of another aircraft.

Attributes
ACN	446297
Time
Date	199908
Day	Sat
Local Time Of Day	1801 To 2400
Place
Locale Reference	intersection : usaba
State Reference	FO
Altitude	msl bound lower : 29000 msl bound upper : 31000
Environment
Light	Night
Aircraft 1
Controlling Facilities	artcc : sbgl.artcc
Operator	common carrier : air carrier
Make Model Name	B767 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : descent cruise : enroute altitude change
Route In Use	enroute airway : ns.airway
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : sbgl.artcc
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
ASRS Report	446297
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	conflict : airborne critical non adherence : far non adherence : published procedure non adherence : required legal separation
Independent Detector	aircraft equipment : tcas other flight crewa other flight crewb
Resolutory Action	flight crew : took precautionary avoidance action
Miss Distance	horizontal : 0 vertical : 500
Supplementary
Problem Areas	ATC Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error
Situations
ATC Facility	procedure or policy : sbgl.artcc

Narrative:

TCASII traffic alert. Level at FL310, we were given a clear to descend to FL180. Traffic was exactly below us. At FL305 monitor vertical speed was given. Per company procedure, the autoplt was disconnected. Rather than slow descent, I climbed back up to FL310. First officer switched from 40 below to 20 below during this incident. Now we could see the traffic on the scope. We tried telling ATC but we got the same clearance 'descend to FL180.' very difficult to try to get across that there was a jet right below us at FL290 going the same direction on the airway. One thing was very clear to first officer and myself. Without the aid of TCASII it would have been up to the other aircraft below us to avert a major problem.

Original NASA ASRS Text

Title: B767 CREW WAS CLRED TO DSND ON TOP OF ANOTHER ACFT.

Narrative: TCASII TFC ALERT. LEVEL AT FL310, WE WERE GIVEN A CLR TO DSND TO FL180. TFC WAS EXACTLY BELOW US. AT FL305 MONITOR VERT SPD WAS GIVEN. PER COMPANY PROC, THE AUTOPLT WAS DISCONNECTED. RATHER THAN SLOW DSCNT, I CLBED BACK UP TO FL310. FO SWITCHED FROM 40 BELOW TO 20 BELOW DURING THIS INCIDENT. NOW WE COULD SEE THE TFC ON THE SCOPE. WE TRIED TELLING ATC BUT WE GOT THE SAME CLRNC 'DSND TO FL180.' VERY DIFFICULT TO TRY TO GET ACROSS THAT THERE WAS A JET RIGHT BELOW US AT FL290 GOING THE SAME DIRECTION ON THE AIRWAY. ONE THING WAS VERY CLR TO FO AND MYSELF. WITHOUT THE AID OF TCASII IT WOULD HAVE BEEN UP TO THE OTHER ACFT BELOW US TO AVERT A MAJOR PROB.

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.