Narrative:

Was crossing over atl on approach into pdx from csg. No other radio traffic noted except for air carrier aircraft on approach into atl. I had just taken the aircraft off autoplt (altitude hold) when I saw the air carrier on short final to runway 27L at atl. I was assigned 5000 ft, but I descended to 4500 ft as my attention was on the air carrier landing at atl. ATC cleared me to 3100 ft, at which time I realized I had descended to 4500 ft. Although the WX was cavv, my attention should have been inside the aircraft rather than watching dal. Contributing factors include the time of night (XA00 local) and the lack of 'radio chatter' that would've kept my attention on my plane.

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Original NASA ASRS Text

Title: C172 PLT HAD ALTDEV IN ATL CLASS B.

Narrative: WAS XING OVER ATL ON APCH INTO PDX FROM CSG. NO OTHER RADIO TFC NOTED EXCEPT FOR ACR ACFT ON APCH INTO ATL. I HAD JUST TAKEN THE ACFT OFF AUTOPLT (ALT HOLD) WHEN I SAW THE ACR ON SHORT FINAL TO RWY 27L AT ATL. I WAS ASSIGNED 5000 FT, BUT I DSNDED TO 4500 FT AS MY ATTN WAS ON THE ACR LNDG AT ATL. ATC CLRED ME TO 3100 FT, AT WHICH TIME I REALIZED I HAD DSNDED TO 4500 FT. ALTHOUGH THE WX WAS CAVV, MY ATTN SHOULD HAVE BEEN INSIDE THE ACFT RATHER THAN WATCHING DAL. CONTRIBUTING FACTORS INCLUDE THE TIME OF NIGHT (XA00 LCL) AND THE LACK OF 'RADIO CHATTER' THAT WOULD'VE KEPT MY ATTN ON MY PLANE.

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.