Air carrier Y overtake air carrier X on final approach. Less than standard separation. System error.

Attributes
ACN	287656
Time
Date	199411
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : mdw
State Reference	IL
Altitude	agl bound lower : 800 agl bound upper : 800
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Controlling Facilities	tower : mdw
Operator	common carrier : air carrier
Make Model Name	Jetstream 31
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	DC-9 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : ground
Qualification	controller : non radar
Experience	controller non radar : 2 controller radar : 1
ASRS Report	287656
Person 2
Affiliation	government : faa
Function	controller : local
Qualification	controller : radar
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation non adherence : published procedure
Independent Detector	other other : unspecified atc
Resolutory Action	none taken : anomaly accepted
Consequence	Other
Miss Distance	horizontal : 12000 vertical : 700
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Air carrier X being followed by air carrier Y (DC9) on the ILS 31C approach at chicago midway. When Y reported on local frequency the local controller advised the pilot to reduce speed as he was faster than X. As the situation continued to develop, separation continued to decrease until standard IFR separation was lost. Local control didn't take an action to regain the lost separation nor did the supervisor on duty take any action to have air carrier Y sent around. The use of visual separation could not be used in this case being the aircraft were not breaking out of the clouds until approximately a 1 1/2 mi final. The major reason for this type of problem is chicago approach assigning high speeds to maintain to the final approach fix while keeping inbounds 3-4 mi intrail outside the final approach fix. Once aircraft reach the marker they reduce to their final approach speed a compression of the final sequence occurs inside the marker resulting in a loss of IFR separation.

Original NASA ASRS Text

Title: ACR Y OVERTAKE ACR X ON FINAL APCH. LTSS. SYS ERROR.

Narrative: ACR X BEING FOLLOWED BY ACR Y (DC9) ON THE ILS 31C APCH AT CHICAGO MIDWAY. WHEN Y RPTED ON LCL FREQ THE LCL CTLR ADVISED THE PLT TO REDUCE SPD AS HE WAS FASTER THAN X. AS THE SIT CONTINUED TO DEVELOP, SEPARATION CONTINUED TO DECREASE UNTIL STANDARD IFR SEPARATION WAS LOST. LCL CTL DIDN'T TAKE AN ACTION TO REGAIN THE LOST SEPARATION NOR DID THE SUPVR ON DUTY TAKE ANY ACTION TO HAVE ACR Y SENT AROUND. THE USE OF VISUAL SEPARATION COULD NOT BE USED IN THIS CASE BEING THE ACFT WERE NOT BREAKING OUT OF THE CLOUDS UNTIL APPROX A 1 1/2 MI FINAL. THE MAJOR REASON FOR THIS TYPE OF PROB IS CHICAGO APCH ASSIGNING HIGH SPDS TO MAINTAIN TO THE FINAL APCH FIX WHILE KEEPING INBOUNDS 3-4 MI INTRAIL OUTSIDE THE FINAL APCH FIX. ONCE ACFT REACH THE MARKER THEY REDUCE TO THEIR FINAL APCH SPD A COMPRESSION OF THE FINAL SEQUENCE OCCURS INSIDE THE MARKER RESULTING IN A LOSS OF IFR SEPARATION.

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.