Aircraft descended to an altitude occupied by another aircraft.

Attributes
ACN	108237
Time
Date	198904
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : scy
State Reference	TX
Altitude	msl bound lower : 23000 msl bound upper : 24000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zfw tower : arr
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	descent other
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
Flight Phase	cruise other
Route In Use	enroute : direct
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
ASRS Report	108237
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 : required legal separation
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance none taken : detected after the fact other
Consequence	faa : investigated
Miss Distance	horizontal : 14400 vertical : 400
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

I was working approximately 7 jets landing dfw and several overflts. Air carrier X checked on frequency at FL240. Shortly after that air carrier Y (same company) checked on climbing to FL230. Air carrier Y was directly under air carrier X. Because of the close proximity of the 2 aircraft, the computer was swapping the mode C altitude information back and forth between the 2 aircraft and it was useless to me. I instructed air carrier X to maintain best forward speed. He acknowledged and requested a lower altitude. I called traffic to him as being, 'traffic directly below at FL230 I will have lower in a few mins.' I then called the low altitude sector that the air carrier Y was still in to get control to turn air carrier Y for sequence. While I was on the line, air carrier X responded to the traffic call by saying, '2-3-zero air carrier X.' I thought he was responding to the traffic at FL230. Air carrier X then descended to FL230. I turned air carrier Y to a 210 degree heading, and they passed by 2.4 mi and 400'. If air carrier X had used proper phraseology and said he was descending to FL230 I would have caught the error.

Original NASA ASRS Text

Title: ACFT DESCENDED TO AN ALT OCCUPIED BY ANOTHER ACFT.

Narrative: I WAS WORKING APPROX 7 JETS LNDG DFW AND SEVERAL OVERFLTS. ACR X CHKED ON FREQ AT FL240. SHORTLY AFTER THAT ACR Y (SAME COMPANY) CHKED ON CLBING TO FL230. ACR Y WAS DIRECTLY UNDER ACR X. BECAUSE OF THE CLOSE PROX OF THE 2 ACFT, THE COMPUTER WAS SWAPPING THE MODE C ALT INFO BACK AND FORTH BTWN THE 2 ACFT AND IT WAS USELESS TO ME. I INSTRUCTED ACR X TO MAINTAIN BEST FORWARD SPD. HE ACKNOWLEDGED AND REQUESTED A LOWER ALT. I CALLED TFC TO HIM AS BEING, 'TFC DIRECTLY BELOW AT FL230 I WILL HAVE LOWER IN A FEW MINS.' I THEN CALLED THE LOW ALT SECTOR THAT THE ACR Y WAS STILL IN TO GET CTL TO TURN ACR Y FOR SEQUENCE. WHILE I WAS ON THE LINE, ACR X RESPONDED TO THE TFC CALL BY SAYING, '2-3-ZERO ACR X.' I THOUGHT HE WAS RESPONDING TO THE TFC AT FL230. ACR X THEN DSNDED TO FL230. I TURNED ACR Y TO A 210 DEG HDG, AND THEY PASSED BY 2.4 MI AND 400'. IF ACR X HAD USED PROPER PHRASEOLOGY AND SAID HE WAS DSNDING TO FL230 I WOULD HAVE CAUGHT THE ERROR.

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.