Aircraft descended through altitude of another aircraft.

Attributes
ACN	124605
Time
Date	198910
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : hou
State Reference	TX
Altitude	msl bound lower : 10000 msl bound upper : 10700
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	cruise other
Route In Use	arrival other enroute airway : zhu
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	descent other
Route In Use	arrival other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
ASRS Report	124605
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	conflict : airborne less severe non adherence : published procedure non adherence : required legal separation
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance
Consequence	Other
Miss Distance	horizontal : 21000 vertical : 300
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Air carrier X was cleared at FL230 with clearance to cross gland at 10,000'. Air carrier Y was cleared at 17,000'. Small transport Z was assigned heading 100 degree to intercept the efd 254 radial and to cross 45 mi southwest of efd at 7000'. Air carrier Y was being descending to 9000' reference small transport Z, and when small transport Z left 8000', air carrier Y was assigned heading 360 degree to intercept the iah 234 radial at 9000'. I observed small transport Z descend below 7000', and when I questioned the pilot he reported at 5000'. Small transport Z was about to enter another controller's airspace so I told him to climb to 7000'. Meanwhile air carrier Y was converging with air carrier X, and I realized that 5 mi lateral separation could not be maintained. Air carrier X was at 10,000' and air carrier Y at 10,700'. I cleared air carrier Y to maintain present altitude and started a right turn. I contacted hou approach and asked them to start air carrier X descending out of 10,000', then I assigned air carrier Y heading 070 degree, minimum lateral separation was 3.5 mi. Air carrier X had decreased his airspeed to 250 KTS as required on the gland 6 arrival, and while I was distracted with small transport Z, I did not observe that my planned separation was not working.

Original NASA ASRS Text

Title: ACFT DESCENDED THROUGH ALT OF ANOTHER ACFT.

Narrative: ACR X WAS CLRED AT FL230 WITH CLRNC TO CROSS GLAND AT 10,000'. ACR Y WAS CLRED AT 17,000'. SMT Z WAS ASSIGNED HDG 100 DEG TO INTERCEPT THE EFD 254 RADIAL AND TO CROSS 45 MI SW OF EFD AT 7000'. ACR Y WAS BEING DESCENDING TO 9000' REFERENCE SMT Z, AND WHEN SMT Z LEFT 8000', ACR Y WAS ASSIGNED HDG 360 DEG TO INTERCEPT THE IAH 234 RADIAL AT 9000'. I OBSERVED SMT Z DSND BELOW 7000', AND WHEN I QUESTIONED THE PLT HE REPORTED AT 5000'. SMT Z WAS ABOUT TO ENTER ANOTHER CTLR'S AIRSPACE SO I TOLD HIM TO CLIMB TO 7000'. MEANWHILE ACR Y WAS CONVERGING WITH ACR X, AND I REALIZED THAT 5 MI LATERAL SEPARATION COULD NOT BE MAINTAINED. ACR X WAS AT 10,000' AND ACR Y AT 10,700'. I CLRED ACR Y TO MAINTAIN PRESENT ALT AND STARTED A RIGHT TURN. I CONTACTED HOU APCH AND ASKED THEM TO START ACR X DESCENDING OUT OF 10,000', THEN I ASSIGNED ACR Y HDG 070 DEG, MINIMUM LATERAL SEPARATION WAS 3.5 MI. ACR X HAD DECREASED HIS AIRSPEED TO 250 KTS AS REQUIRED ON THE GLAND 6 ARRIVAL, AND WHILE I WAS DISTRACTED WITH SMT Z, I DID NOT OBSERVE THAT MY PLANNED SEPARATION WAS NOT WORKING.

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.