Narrative:

While being vectored for the ILS runway 30R approach to stl, I inadvertently descended through my assigned altitude of 3500 ft. I caught the mistake and initiated a climb at 2700 ft. I called approach to verify my assigned altitude, to which he replied, '3500 ft.' I continued the climb to 3500 ft as assigned, and completed the remainder of the approach uneventfully. Stl approach assigns unusual altitudes while vectoring aircraft for instrument approachs. Assigned altitudes of 5500 ft, 4500 ft, 3500 ft, 2500 ft, etc, are not uncommon altitudes to be assigned by stl approach on downwind as well as base and final. I was told to 'maintain 180 KTS,' to 'expect a close in base turn,' and 'caution wake turbulence following B747' to same runway. I could hear aircraft ahead of me being assigned 2500 ft and was expecting the same myself. With this in mind, as well as wake turbulence concerns, preparations for a close in base turn and pre-landing checks, I inadvertently flew through my assigned altitude. When I began correction, I noticed the autoplt was disconnected. I do not recall disconnecting it myself or hearing it disconnect. This autoplt has disconnected without audible warning in the past. I do not recall hearing a warning from the altitude alerter either. This item has been intermittently inoperative in the past. Perhaps a copilot on board, better maintenance or more vigilance on my part would have prevented this altitude deviation.

Related reports:

Google
 

Original NASA ASRS Text

Title: A CESSNA 425 PLT RPTS THAT HE FLEW THROUGH HIS ASSIGNED ALT WHILE MANEUVERING FOR AN APCH AT STL. ERROR ADMITTED.

Narrative: WHILE BEING VECTORED FOR THE ILS RWY 30R APCH TO STL, I INADVERTENTLY DSNDED THROUGH MY ASSIGNED ALT OF 3500 FT. I CAUGHT THE MISTAKE AND INITIATED A CLB AT 2700 FT. I CALLED APCH TO VERIFY MY ASSIGNED ALT, TO WHICH HE REPLIED, '3500 FT.' I CONTINUED THE CLB TO 3500 FT AS ASSIGNED, AND COMPLETED THE REMAINDER OF THE APCH UNEVENTFULLY. STL APCH ASSIGNS UNUSUAL ALTS WHILE VECTORING ACFT FOR INST APCHS. ASSIGNED ALTS OF 5500 FT, 4500 FT, 3500 FT, 2500 FT, ETC, ARE NOT UNCOMMON ALTS TO BE ASSIGNED BY STL APCH ON DOWNWIND AS WELL AS BASE AND FINAL. I WAS TOLD TO 'MAINTAIN 180 KTS,' TO 'EXPECT A CLOSE IN BASE TURN,' AND 'CAUTION WAKE TURB FOLLOWING B747' TO SAME RWY. I COULD HEAR ACFT AHEAD OF ME BEING ASSIGNED 2500 FT AND WAS EXPECTING THE SAME MYSELF. WITH THIS IN MIND, AS WELL AS WAKE TURB CONCERNS, PREPARATIONS FOR A CLOSE IN BASE TURN AND PRE-LNDG CHKS, I INADVERTENTLY FLEW THROUGH MY ASSIGNED ALT. WHEN I BEGAN CORRECTION, I NOTICED THE AUTOPLT WAS DISCONNECTED. I DO NOT RECALL DISCONNECTING IT MYSELF OR HEARING IT DISCONNECT. THIS AUTOPLT HAS DISCONNECTED WITHOUT AUDIBLE WARNING IN THE PAST. I DO NOT RECALL HEARING A WARNING FROM THE ALT ALERTER EITHER. THIS ITEM HAS BEEN INTERMITTENTLY INOP IN THE PAST. PERHAPS A COPLT ON BOARD, BETTER MAINT OR MORE VIGILANCE ON MY PART WOULD HAVE PREVENTED THIS ALTDEV.

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.