Aircraft descended below assigned altitude resulting in less than standard separation.

Attributes
ACN	108126
Time
Date	198904
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : fzt
State Reference	TX
Altitude	msl bound lower : 25000 msl bound upper : 29000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zfw
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 3 Turbojet Eng
Navigation In Use	Other
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
ASRS Report	108126
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 225 flight time total : 22000 flight time type : 300
ASRS Report	108238
Events
Anomaly	altitude deviation : overshoot conflict : airborne less severe non adherence : required legal separation non adherence : clearance
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	none taken : detected after the fact other
Consequence	faa : investigated
Miss Distance	horizontal : 13800 vertical : 100
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

I was the radar controller on sector 89, ZFW ARTCC. I was working a busy arrival sector, handling mostly jets landing at dfw. I instructed air carrier X to turn left heading 200 degrees for sequence. I then instructed him to descend to FL290. He acknowledged. I then instructed him to maintain 250 KTS. He replied, '2-5-zero now.' I took that as acknowledgement of the speed. I then instructed air carrier Y to turn left heading 200 degrees and descend and maintain FL250. He acknowledged. I then instructed air carrier Y to maintain 250 KTS. I was quite busy and when I scanned the data block of air carrier X I observed him passing FL260 in a descent, and less than 5 mi from air carrier Y. I instructed air carrier Y to turn left heading 270 degrees and the aircraft started to diverge. The closest they got was 2.3 mi and 100'. I attribute this error to air carrier X descending through his assigned altitude of FL290. Air carrier X did reduce his speed to 250 KTS, but also descended to FL250; I don't know why. If his phraseology had been better, I would have caught the error. Supplemental information from acn 108238: we were given a series of headings, altitude changes and airspds to maintain. All clrncs were read back and acknowledged by ZFW. The frequency was very congested at the time and the controller was speaking extremely fast and in some cases blocking out xmissions. To the best of my knowledge, we complied with all ATC clearance. At no time were we advised of any altitude deviations.

Original NASA ASRS Text

Title: ACFT DESCENDED BELOW ASSIGNED ALT RESULTING IN LESS THAN STANDARD SEPARATION.

Narrative: I WAS THE RADAR CTLR ON SECTOR 89, ZFW ARTCC. I WAS WORKING A BUSY ARR SECTOR, HANDLING MOSTLY JETS LNDG AT DFW. I INSTRUCTED ACR X TO TURN LEFT HDG 200 DEGS FOR SEQUENCE. I THEN INSTRUCTED HIM TO DSND TO FL290. HE ACKNOWLEDGED. I THEN INSTRUCTED HIM TO MAINTAIN 250 KTS. HE REPLIED, '2-5-ZERO NOW.' I TOOK THAT AS ACKNOWLEDGEMENT OF THE SPD. I THEN INSTRUCTED ACR Y TO TURN LEFT HDG 200 DEGS AND DSND AND MAINTAIN FL250. HE ACKNOWLEDGED. I THEN INSTRUCTED ACR Y TO MAINTAIN 250 KTS. I WAS QUITE BUSY AND WHEN I SCANNED THE DATA BLOCK OF ACR X I OBSERVED HIM PASSING FL260 IN A DSCNT, AND LESS THAN 5 MI FROM ACR Y. I INSTRUCTED ACR Y TO TURN LEFT HDG 270 DEGS AND THE ACFT STARTED TO DIVERGE. THE CLOSEST THEY GOT WAS 2.3 MI AND 100'. I ATTRIBUTE THIS ERROR TO ACR X DSNDING THROUGH HIS ASSIGNED ALT OF FL290. ACR X DID REDUCE HIS SPD TO 250 KTS, BUT ALSO DSNDED TO FL250; I DON'T KNOW WHY. IF HIS PHRASEOLOGY HAD BEEN BETTER, I WOULD HAVE CAUGHT THE ERROR. SUPPLEMENTAL INFO FROM ACN 108238: WE WERE GIVEN A SERIES OF HDGS, ALT CHANGES AND AIRSPDS TO MAINTAIN. ALL CLRNCS WERE READ BACK AND ACKNOWLEDGED BY ZFW. THE FREQ WAS VERY CONGESTED AT THE TIME AND THE CTLR WAS SPEAKING EXTREMELY FAST AND IN SOME CASES BLOCKING OUT XMISSIONS. TO THE BEST OF MY KNOWLEDGE, WE COMPLIED WITH ALL ATC CLRNC. AT NO TIME WERE WE ADVISED OF ANY ALT DEVIATIONS.

Data retrieved from NASA's ASRS site as of August 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.