ZID controller experienced potential separation loss during aircraft communication problem.

Attributes
ACN	605876
Time
Date	200401
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : rod.vortac
State Reference	OH
Altitude	msl bound lower : 23000 msl bound upper : 28000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zid.artcc
Operator	general aviation : corporate
Make Model Name	Learjet 35
Operating Under FAR Part	Part 91
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zid.artcc
Operator	general aviation : corporate
Make Model Name	Super King Air 350
Operating Under FAR Part	Part 91
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Experience	controller radar : 18 controller time certified in position1 : 21.5
ASRS Report	605876
Person 2
Affiliation	government : faa
Function	controller : radar
Events
Anomaly	non adherence : clearance other anomaly other
Independent Detector	atc equipment : conflict alert other controllera
Resolutory Action	controller : issued new clearance flight crew : took evasive action
Miss Distance	horizontal : 12000 vertical : 1000
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft ATC Human Performance
Primary Problem	ATC Human Performance

Narrative:

This situation was scary but in a way controled at all times. Aircraft #1 was climbing wbound from columbus, oh. Aircraft #1 was assigned FL230. Aircraft #1 responded to clearance. Aircraft #1 was handed off and switched to my frequency. Aircraft #1 microphone quit functioning. Response to frequency change not heard on previous frequency. Previous controller tried to switch aircraft #1 several more times. Previous controller called me and asked was I talking to aircraft. Thinking aircraft might be sitting on my frequency but not checked on, I climb aircraft #1 to FL280. I get no response. I assume aircraft #1 still cleared to FL230. I notice aircraft #1 leaving FL245 so I start calling his call sign -- no response. Not knowing how high aircraft #1 is going, I turn aircraft #2 from about 190 degree heading to 070 degree heading. Aircraft #2 much slower and having trouble getting out of aircraft #1's way. Commotion on frequency causes aircraft #1 to realize that I don't have control of events. Using a different microphone, we re-establish communications. He assures me he is only going to FL280. Positive separation? Pilot also makes reference to having problems with microphone in past. I should not check for aircraft by issuing clearance that requires readback. If pilot knows equipment may need maintenance, pilot shouldn't use it.

Original NASA ASRS Text

Title: ZID CTLR EXPERIENCED POTENTIAL SEPARATION LOSS DURING ACFT COM PROB.

Narrative: THIS SIT WAS SCARY BUT IN A WAY CTLED AT ALL TIMES. ACFT #1 WAS CLBING WBOUND FROM COLUMBUS, OH. ACFT #1 WAS ASSIGNED FL230. ACFT #1 RESPONDED TO CLRNC. ACFT #1 WAS HANDED OFF AND SWITCHED TO MY FREQ. ACFT #1 MIKE QUIT FUNCTIONING. RESPONSE TO FREQ CHANGE NOT HEARD ON PREVIOUS FREQ. PREVIOUS CTLR TRIED TO SWITCH ACFT #1 SEVERAL MORE TIMES. PREVIOUS CTLR CALLED ME AND ASKED WAS I TALKING TO ACFT. THINKING ACFT MIGHT BE SITTING ON MY FREQ BUT NOT CHKED ON, I CLB ACFT #1 TO FL280. I GET NO RESPONSE. I ASSUME ACFT #1 STILL CLRED TO FL230. I NOTICE ACFT #1 LEAVING FL245 SO I START CALLING HIS CALL SIGN -- NO RESPONSE. NOT KNOWING HOW HIGH ACFT #1 IS GOING, I TURN ACFT #2 FROM ABOUT 190 DEG HDG TO 070 DEG HDG. ACFT #2 MUCH SLOWER AND HAVING TROUBLE GETTING OUT OF ACFT #1'S WAY. COMMOTION ON FREQ CAUSES ACFT #1 TO REALIZE THAT I DON'T HAVE CTL OF EVENTS. USING A DIFFERENT MIKE, WE RE-ESTABLISH COMS. HE ASSURES ME HE IS ONLY GOING TO FL280. POSITIVE SEPARATION? PLT ALSO MAKES REF TO HAVING PROBS WITH MIKE IN PAST. I SHOULD NOT CHK FOR ACFT BY ISSUING CLRNC THAT REQUIRES READBACK. IF PLT KNOWS EQUIP MAY NEED MAINT, PLT SHOULDN'T USE IT.

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.