Possible altitude deviation on descent to approach.

Attributes
ACN	310230
Time
Date	199507
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : mdw
State Reference	IL
Altitude	msl bound lower : 2000 msl bound upper : 5300
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : ord
Operator	general aviation : corporate
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 91
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Flight Phase	descent : approach
Person 1
Affiliation	Other
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : instrument
Experience	flight time last 90 days : 90 flight time total : 12000 flight time type : 2000
ASRS Report	310230
Person 2
Affiliation	Other
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	conflict : airborne less severe
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	Other
Supplementary
Primary Problem	Ambiguous
Air Traffic Incident	Pilot Deviation

Narrative:

We were inbound to chicago midway level at 10000 ft. The controller told us to fly a heading of 340 degrees to intercept the runway 31C localizer, descend and maintain 2000 ft. The copilot read this back and we started the descent. At approximately 5500 ft we noticed converging traffic below (a 172). About that time, the controller called and asked what altitude we were descending to, we replied 2000 ft. He said he had not given us that altitude to level off and maintain 4000 ft, which we did. The other aircraft passed about 1000 ft below us before we leveled out at 4000 ft. He had been at 4600 ft when we crossed above him at around 5300-5500 ft. No evasive action was required by either aircraft. As for the descent, I think the previous controller had given us the descent and possibly it wasn't passed on to the next guy. The only other possibility would be we just misunderstood the instructions to fly 340 degrees, intercept the localizer and mistook maintain 10000 ft for maintain 2000 ft.

Original NASA ASRS Text

Title: POSSIBLE ALTDEV ON DSCNT TO APCH.

Narrative: WE WERE INBOUND TO CHICAGO MIDWAY LEVEL AT 10000 FT. THE CTLR TOLD US TO FLY A HDG OF 340 DEGS TO INTERCEPT THE RWY 31C LOC, DSND AND MAINTAIN 2000 FT. THE COPLT READ THIS BACK AND WE STARTED THE DSCNT. AT APPROX 5500 FT WE NOTICED CONVERGING TFC BELOW (A 172). ABOUT THAT TIME, THE CTLR CALLED AND ASKED WHAT ALT WE WERE DSNDING TO, WE REPLIED 2000 FT. HE SAID HE HAD NOT GIVEN US THAT ALT TO LEVEL OFF AND MAINTAIN 4000 FT, WHICH WE DID. THE OTHER ACFT PASSED ABOUT 1000 FT BELOW US BEFORE WE LEVELED OUT AT 4000 FT. HE HAD BEEN AT 4600 FT WHEN WE CROSSED ABOVE HIM AT AROUND 5300-5500 FT. NO EVASIVE ACTION WAS REQUIRED BY EITHER ACFT. AS FOR THE DSCNT, I THINK THE PREVIOUS CTLR HAD GIVEN US THE DSCNT AND POSSIBLY IT WASN'T PASSED ON TO THE NEXT GUY. THE ONLY OTHER POSSIBILITY WOULD BE WE JUST MISUNDERSTOOD THE INSTRUCTIONS TO FLY 340 DEGS, INTERCEPT THE LOC AND MISTOOK MAINTAIN 10000 FT FOR MAINTAIN 2000 FT.

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.