Narrative:

Flying eastbound at 11,000 MSL, VFR on top of a solid layer, inbound to des moines, I was cleared to 5000' (I think) after being handed over to dsm approach. I asked for a practice ILS to duck my autoplt, and was being vectored at 5000' when the controller advised opposite direction traffic at 5000'. We both deviated, then continued our flts. The controller said he cleared me to 6000. I haven't heard the tape yet so don't know who is right. The controller and I noted the conflict right after he cleared the other aircraft to 5000. He had evidently expected me to be at 6000' and told the other pilot so. I had been level at 5000' for some time before the incident, probably 5-10 minutes. Callback conversation with reporter revealed the following: reporter states that he did not hear playback on TRACON tape but did receive a report on it. The controller did clear small aircraft to 6000' and it was acknowledged by pilot. For some unknown reason pilot did not maintain 6000' but descended through to 5000'. He had transponder on with an IFR squawk and mode C capability. For another unknown reason the controller did not notice his altitude as 5000' as reported by mode. C it was when the controller was giving traffic advisory to both aircraft that he read the 5000' on aircraft X and gave an avoidance turn. Both aircraft were in and out of the base of the overcast at 5000'. Could not see each other. Estimated miss distance was 1 mi (6000'). Aircraft Y was a twin engine aircraft. Reporter had about 50 hours in type of equipment and may have been distraction by low time and concentration on use of autoplt. At the present time the reporter has received a letter from the FAA advising him of an investigation into the incident.

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Original NASA ASRS Text

Title: ALT BUST. PLT DEVIATION FROM CLRNC. POTENTIAL CONFLICT LESS THAN STANDARD SEPARATION.

Narrative: FLYING EBND AT 11,000 MSL, VFR ON TOP OF A SOLID LAYER, INBND TO DES MOINES, I WAS CLRED TO 5000' (I THINK) AFTER BEING HANDED OVER TO DSM APCH. I ASKED FOR A PRACTICE ILS TO DUCK MY AUTOPLT, AND WAS BEING VECTORED AT 5000' WHEN THE CTLR ADVISED OPPOSITE DIRECTION TFC AT 5000'. WE BOTH DEVIATED, THEN CONTINUED OUR FLTS. THE CTLR SAID HE CLRED ME TO 6000. I HAVEN'T HEARD THE TAPE YET SO DON'T KNOW WHO IS RIGHT. THE CTLR AND I NOTED THE CONFLICT RIGHT AFTER HE CLRED THE OTHER ACFT TO 5000. HE HAD EVIDENTLY EXPECTED ME TO BE AT 6000' AND TOLD THE OTHER PLT SO. I HAD BEEN LEVEL AT 5000' FOR SOME TIME BEFORE THE INCIDENT, PROBABLY 5-10 MINUTES. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: REPORTER STATES THAT HE DID NOT HEAR PLAYBACK ON TRACON TAPE BUT DID RECEIVE A REPORT ON IT. THE CTLR DID CLEAR SMA TO 6000' AND IT WAS ACKNOWLEDGED BY PLT. FOR SOME UNKNOWN REASON PLT DID NOT MAINTAIN 6000' BUT DESCENDED THROUGH TO 5000'. HE HAD XPONDER ON WITH AN IFR SQUAWK AND MODE C CAPABILITY. FOR ANOTHER UNKNOWN REASON THE CTLR DID NOT NOTICE HIS ALT AS 5000' AS REPORTED BY MODE. C IT WAS WHEN THE CTLR WAS GIVING TFC ADVISORY TO BOTH ACFT THAT HE READ THE 5000' ON ACFT X AND GAVE AN AVOIDANCE TURN. BOTH ACFT WERE IN AND OUT OF THE BASE OF THE OVERCAST AT 5000'. COULD NOT SEE EACH OTHER. ESTIMATED MISS DISTANCE WAS 1 MI (6000'). ACFT Y WAS A TWIN ENGINE ACFT. REPORTER HAD ABOUT 50 HOURS IN TYPE OF EQUIPMENT AND MAY HAVE BEEN DISTR BY LOW TIME AND CONCENTRATION ON USE OF AUTOPLT. AT THE PRESENT TIME THE REPORTER HAS RECEIVED A LETTER FROM THE FAA ADVISING HIM OF AN INVESTIGATION INTO THE INCIDENT.

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.