Air carrier X had less than standard separation from air carrier Y. Pilot deviation? System error.

Attributes
ACN	155488
Time
Date	199008
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : hou
State Reference	TX
Altitude	msl bound lower : 29000 msl bound upper : 35000
Environment
Flight Conditions	Mixed
Light	Night
Aircraft 1
Controlling Facilities	artcc : zhu
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	climbout : intermediate altitude
Route In Use	enroute : direct
Flight Plan	IFR
Aircraft 2
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	enroute : direct
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Experience	controller radar : 4
ASRS Report	155488
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance non adherence : published procedure non adherence : required legal separation
Independent Detector	other controllera
Resolutory Action	none taken : insufficient time
Consequence	faa : investigated
Miss Distance	horizontal : 24000 vertical : 1400
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Operational Error Pilot Deviation

Narrative:

Air carrier X was requesting FL350 en route from houston, tx to tulsa, ok. I cleared air carrier X to FL290 for crossing air carrier Y at FL310. I never issued traffic to either aircraft as I estimated that they could stay at least 3 or 4 mi apart and never see each other. Air carrier X had been deviating for WX and when he cleared it, I issued the clearance: 'air carrier X, cleared direct okm, direct tul, initial heading 3-5-0. Air carrier X read back: 'roger, direct okm, tul, 3-5-0, air carrier X.' I thought he was reading back the clearance as I'd intended, a route clearance only with no altitude instructions. I diverted my attention to another aircraft, issued him a routing clearance, and entered the amended route into the computer. When I looked back at air carrier X, his altitude showed FL299, and I asked him his altitude. He said, 'out of FL3-5 climbing to FL3-5-0.' had I used better phraseology in the initial clearance, 'air carrier X, fly heading 3-5-0 until receiving okm, then cleared direct okm, direct tul,' the aircraft might not have climbed and the deviation not occurred.

Original NASA ASRS Text

Title: ACR X HAD LESS THAN STANDARD SEPARATION FROM ACR Y. PLT DEVIATION? SYSTEM ERROR.

Narrative: ACR X WAS REQUESTING FL350 ENRTE FROM HOUSTON, TX TO TULSA, OK. I CLRED ACR X TO FL290 FOR XING ACR Y AT FL310. I NEVER ISSUED TFC TO EITHER ACFT AS I ESTIMATED THAT THEY COULD STAY AT LEAST 3 OR 4 MI APART AND NEVER SEE EACH OTHER. ACR X HAD BEEN DEVIATING FOR WX AND WHEN HE CLRED IT, I ISSUED THE CLRNC: 'ACR X, CLRED DIRECT OKM, DIRECT TUL, INITIAL HDG 3-5-0. ACR X READ BACK: 'ROGER, DIRECT OKM, TUL, 3-5-0, ACR X.' I THOUGHT HE WAS READING BACK THE CLRNC AS I'D INTENDED, A RTE CLRNC ONLY WITH NO ALT INSTRUCTIONS. I DIVERTED MY ATTN TO ANOTHER ACFT, ISSUED HIM A RTING CLRNC, AND ENTERED THE AMENDED RTE INTO THE COMPUTER. WHEN I LOOKED BACK AT ACR X, HIS ALT SHOWED FL299, AND I ASKED HIM HIS ALT. HE SAID, 'OUT OF FL3-5 CLBING TO FL3-5-0.' HAD I USED BETTER PHRASEOLOGY IN THE INITIAL CLRNC, 'ACR X, FLY HDG 3-5-0 UNTIL RECEIVING OKM, THEN CLRED DIRECT OKM, DIRECT TUL,' THE ACFT MIGHT NOT HAVE CLBED AND THE DEVIATION NOT OCCURRED.

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.