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.