PA28 pilot and twin cessna had less than legal separation in C90 class C airspace.

Attributes
ACN	560396
Time
Date	200209
Day	Sun
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : mdw.airport
State Reference	IL
Altitude	msl single value : 2000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : c90.tracon
Operator	general aviation : personal
Make Model Name	PA-28 Cherokee/Archer II/Dakota/Pillan/Warrior
Operating Under FAR Part	Part 91
Flight Phase	cruise : level
Flight Plan	None
Aircraft 2
Operator	general aviation : personal
Make Model Name	Cessna Twin Piston Undifferentiated or Other Model
Operating Under FAR Part	Part 91
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : instrument pilot : cfi pilot : commercial
Experience	flight time last 90 days : 70.2 flight time total : 773.3 flight time type : 1404.4
ASRS Report	560396
Person 2
Affiliation	other
Function	flight crew : single pilot
Events
Anomaly	conflict : airborne critical non adherence : clearance non adherence : far non adherence : published procedure other anomaly other
Independent Detector	atc equipment other atc equipment : radar other flight crewa
Resolutory Action	controller : separated traffic controller : issued new clearance controller : issued alert flight crew : took evasive action none taken : detected after the fact
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 500 vertical : 0
Supplementary
Problem Areas	Airspace Structure Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

I was transitioning mdw's airspace to the east at 2000 ft. The controller alerted me of a twin cessna traffic, which I quickly spotted and replied to the controller. As soon as I released the push-to-talk switch, I heard the controller say 'roger, cherokee aircraft, maintain VFR at or below 2500 ft.' I responded with 'roger, aircraft' and initiated a climb. Just before I reached 2500 ft, I saw the other aircraft (I am unsure of the type), and took immediate evasive action just before the controller caught it on his screen. I was given a phone number, and subsequently was informed that I had misheard the controller. In my opinion, it was a simple misunderstanding which could have happened to anyone. I will definitely be careful to read every transmission back from now on.

Original NASA ASRS Text

Title: PA28 PLT AND TWIN CESSNA HAD LESS THAN LEGAL SEPARATION IN C90 CLASS C AIRSPACE.

Narrative: I WAS TRANSITIONING MDW'S AIRSPACE TO THE E AT 2000 FT. THE CTLR ALERTED ME OF A TWIN CESSNA TFC, WHICH I QUICKLY SPOTTED AND REPLIED TO THE CTLR. AS SOON AS I RELEASED THE PUSH-TO-TALK SWITCH, I HEARD THE CTLR SAY 'ROGER, CHEROKEE ACFT, MAINTAIN VFR AT OR BELOW 2500 FT.' I RESPONDED WITH 'ROGER, ACFT' AND INITIATED A CLB. JUST BEFORE I REACHED 2500 FT, I SAW THE OTHER ACFT (I AM UNSURE OF THE TYPE), AND TOOK IMMEDIATE EVASIVE ACTION JUST BEFORE THE CTLR CAUGHT IT ON HIS SCREEN. I WAS GIVEN A PHONE NUMBER, AND SUBSEQUENTLY WAS INFORMED THAT I HAD MISHEARD THE CTLR. IN MY OPINION, IT WAS A SIMPLE MISUNDERSTANDING WHICH COULD HAVE HAPPENED TO ANYONE. I WILL DEFINITELY BE CAREFUL TO READ EVERY XMISSION BACK FROM NOW ON.

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.