Less than standard separation between air carrier Y and small transport X. System error.

Attributes
ACN	125574
Time
Date	198910
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : gdm
State Reference	MA
Altitude	msl bound lower : 6000 msl bound upper : 6000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zbw
Make Model Name	Small Transport, Low Wing, 2 Recip Eng
Flight Phase	descent other
Route In Use	enroute : direct enroute airway : zbw
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
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 non radar : 6 controller radar : 2
ASRS Report	125574
Person 2
Affiliation	government : faa
Function	controller : handoff position
Qualification	controller : radar
Experience	controller radar : 1
ASRS Report	125575
Events
Anomaly	conflict : airborne less severe non adherence : published procedure non adherence : required legal separation
Independent Detector	atc equipment other atc equipment : unspecified
Resolutory Action	controller : issued new clearance other
Consequence	faa : investigated
Miss Distance	horizontal : 19200 vertical : 0
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Traffic not very heavy, however slightly complex. 3 aircraft were in the vicinity of gdm, 1 eastbound and 2 swbnd. Swbnd aircraft X was at 8000' and air carrier Y at 6000'. Eastbound aircraft Z issued a descent clearance to 8000'. Small transport X issued a descent clearance to 7000'. Readback was correct this time. Wanting small transport X to expedite his descent, I issued 'no delay on the descent down to 6000'.' (meaning 7000'.) small transport X read back 'descending to 6000''. I never realized the discrepancy until a loss of standard separation had occurred, nor did I hear the pilot's readback of 6000' instead of 7000'. Supplemental information from acn 125575: conflict alert went off and the radar controller took evasive action by turning both aircraft and climbing small transport X to 7000'.

Original NASA ASRS Text

Title: LESS THAN STANDARD SEPARATION BETWEEN ACR Y AND SMT X. SYSTEM ERROR.

Narrative: TFC NOT VERY HEAVY, HOWEVER SLIGHTLY COMPLEX. 3 ACFT WERE IN THE VICINITY OF GDM, 1 EBND AND 2 SWBND. SWBND ACFT X WAS AT 8000' AND ACR Y AT 6000'. EBND ACFT Z ISSUED A DSCNT CLRNC TO 8000'. SMT X ISSUED A DSCNT CLRNC TO 7000'. READBACK WAS CORRECT THIS TIME. WANTING SMT X TO EXPEDITE HIS DSCNT, I ISSUED 'NO DELAY ON THE DSCNT DOWN TO 6000'.' (MEANING 7000'.) SMT X READ BACK 'DESCENDING TO 6000''. I NEVER REALIZED THE DISCREPANCY UNTIL A LOSS OF STANDARD SEPARATION HAD OCCURRED, NOR DID I HEAR THE PLT'S READBACK OF 6000' INSTEAD OF 7000'. SUPPLEMENTAL INFORMATION FROM ACN 125575: CONFLICT ALERT WENT OFF AND THE RADAR CTLR TOOK EVASIVE ACTION BY TURNING BOTH ACFT AND CLIMBING SMT X TO 7000'.

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.