Narrative:

Takeoff delayed due to a line of thunderstorms west of phl. Takeoff was uneventful. I leveled at 8000 ft MSL. I engaged autoplt and heading select to circumvent thunderstorms. We entered clouds and I commented to my first officer and jump seat rider that rising cloud formations made it feel that we had a high rate of descent. I checked the instruments and we were straight and level. The jump seat rider said he was experiencing the same sensation when he looked outside. While looking outside, I used heading select to turn and avoid a buildup. I experienced spatial disorientation, and felt we were descending. I disengaged the autoplt and started a climb, which resulted in an inadvertent altitude excursion to 9300 ft MSL. I immediately called ATC and advised them. There were no other aircraft in the vicinity as indicated by TCASII. I returned the aircraft to 8000 ft MSL and proceeded to destination. The inadvertent altitude excursion was caused by spatial disorientation, due to the rising cloud formation, the turning aircraft, and by my looking outside during the turn. When I returned to my instrument xchk, the situation cleared up immediately. Supplemental information from acn 310670: the fast rising cloud formations gave a sense of the aircraft being in a steep descent. Captain became spatially disoriented, and accidentally disconnected the autoplt. He immediately put aircraft into steep climb and 30 degree left bank. I advised him of 30 degree bank angle and climb. I immediately called departure control and advised them that we were climbing to 9000 ft to avoid WX.

Related reports:

Google
 

Original NASA ASRS Text

Title: ALTDEV CAUSED BY VERTIGO SPATIAL DISORIENTATION.

Narrative: TKOF DELAYED DUE TO A LINE OF TSTMS W OF PHL. TKOF WAS UNEVENTFUL. I LEVELED AT 8000 FT MSL. I ENGAGED AUTOPLT AND HDG SELECT TO CIRCUMVENT TSTMS. WE ENTERED CLOUDS AND I COMMENTED TO MY FO AND JUMP SEAT RIDER THAT RISING CLOUD FORMATIONS MADE IT FEEL THAT WE HAD A HIGH RATE OF DSCNT. I CHKED THE INSTS AND WE WERE STRAIGHT AND LEVEL. THE JUMP SEAT RIDER SAID HE WAS EXPERIENCING THE SAME SENSATION WHEN HE LOOKED OUTSIDE. WHILE LOOKING OUTSIDE, I USED HDG SELECT TO TURN AND AVOID A BUILDUP. I EXPERIENCED SPATIAL DISORIENTATION, AND FELT WE WERE DSNDING. I DISENGAGED THE AUTOPLT AND STARTED A CLB, WHICH RESULTED IN AN INADVERTENT ALT EXCURSION TO 9300 FT MSL. I IMMEDIATELY CALLED ATC AND ADVISED THEM. THERE WERE NO OTHER ACFT IN THE VICINITY AS INDICATED BY TCASII. I RETURNED THE ACFT TO 8000 FT MSL AND PROCEEDED TO DEST. THE INADVERTENT ALT EXCURSION WAS CAUSED BY SPATIAL DISORIENTATION, DUE TO THE RISING CLOUD FORMATION, THE TURNING ACFT, AND BY MY LOOKING OUTSIDE DURING THE TURN. WHEN I RETURNED TO MY INST XCHK, THE SIT CLRED UP IMMEDIATELY. SUPPLEMENTAL INFO FROM ACN 310670: THE FAST RISING CLOUD FORMATIONS GAVE A SENSE OF THE ACFT BEING IN A STEEP DSCNT. CAPT BECAME SPATIALLY DISORIENTED, AND ACCIDENTALLY DISCONNECTED THE AUTOPLT. HE IMMEDIATELY PUT ACFT INTO STEEP CLB AND 30 DEG L BANK. I ADVISED HIM OF 30 DEG BANK ANGLE AND CLB. I IMMEDIATELY CALLED DEP CTL AND ADVISED THEM THAT WE WERE CLBING TO 9000 FT TO AVOID WX.

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.