PLT IN IMC EXPERIENCES VERTIGO.

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I WAS A CREW MEMBER IN THE CA HWY PATROL AIRPLANE. OFFICER X (PLT) AND I STARTED OUR SHIFT AT AM00 HRS. WE PLANNED A LCL NIGHT INST (IFR) FLT. THE WX WAS GETTING FOGGY AS WE PULLED THE AIRPLANE OUT OF THE HANGAR AND PREPARED FOR TKOF. THE GND VISIBILITY WAS GOOD; AS I COULD SEE THE LENGTH OF THE RWY; BUT IT SEEMED THAT THE FOG WAS GETTING LOWER. APPROX AM45 HRS; OFFICER X ASKED AND WAS CLRED FOR AN INST FLT TO THE SCAGGS ISLAND NAVAID (VOR) AND THEN ON TO AN INST APCH TO APC. WE TOOK OFF FROM RWY 24 WHICH IS ALMOST IN DIRECT LINE WITH THE COURSE REQUIRED TO TAKE US TO THE VOR. I HAD BEEN LOOKING OUTSIDE DURING TKOF AND SAW THAT WE ENTERED INST FLT CONDITIONS (IMC) SHORTLY AFTER TKOF. I ALSO FELT THE ACFT START A R TURN. I DID A QUICK SCAN OF THE ACFT INSTS AND SAW THAT WE WERE IN A 20 DEG BANKED R TURN. I ALSO SAW THAT WE WERE OFF COURSE; OUR ALT WAS LEVELING OFF AT 500 FT AND OUR AIRSPD WAS INCREASING. ADDITIONALLY; I NOTED THAT THE AUTOPLT WAS ON THE WING LEVEL OR STABILIZE MODE. I KNEW FROM A PREVIOUS DISCUSSION WITH THE OTHER UNIT PLT; OFFICER Y; THAT THE AUTOPLT WAS UNRELIABLE IN THAT MODE. THE AUTOPLT WOULD NOT MAINTAIN WINGS-LEVEL BUT WOULD START A R TURN. I ASKED OFFICER X WHERE WE WERE HEADED. HE CONFIRMED WE WERE ENRTE TO THE VOR. I SAID THAT WE WEREN'T GOING TO GET THERE ON OUR PRESENT HDG; APPROX 60 DEGS OFF COURSE. OFFICER X MADE NO MOVE TO CORRECT OUR HDG SO I ASKED HIM IF HE WAS ALL RIGHT. HE SAID THAT HE HAD VERTIGO. AT THAT TIME; I TOLD HIM THAT I HAD CTL OF THE AIRPLANE. I INSTITUTED A CLBING L TURN TO GET BACK ON COURSE AND GAIN ALT. UPON REACHING THE VOR; WE WERE IN VFR BUT BECAUSE OF THE CLOUD COVER BELOW US AND THE DARK SKY; WE WERE STILL NAVING ON INSTS. I GAVE CTL OF THE ACFT BACK TO OFFICER X AND HE BEGAN A HOLDING PATTERN. ON THE INBOUND LEG OF THE HOLDING PATTERN; TO THE VOR; HE CALLED ZOA AND ASKED TO BE CLRED FOR THE APCH BACK TO APC. ZOA GAVE HIM THE CLRNC. AS WE PASSED OVER THE VOR; OFFICER X FAILED TO TURN BACK TO THE HOLDING PATTERN COURSE OR TURN OUTBOUND TO BEGIN THE INST APCH. AS I WAS QUESTIONING HIM ON THIS; ZOA ASKED WHAT OUR INTENTIONS WERE SINCE WE WERE 4 MI SW OF COURSE. OFFICER X SAID THAT WE WOULD BE RETURNING TO THE HOLDING PATTERN. AT THAT TIME; I SAID THAT WE SHOULD CANCEL OUR INST FLT PLAN AND CONTINUE VFR TO SOLANO COUNTY AREA WHERE THE WX WAS CLR AND THE GND VISIBILITY WAS UNOBSTRUCTED. HE READILY AGREED AND THE REST OF OUR FLT WAS UNEVENTFUL. AT THE TIME; I ACTED AS PIC; I WAS RATED AS A COMMERCIAL/INST FIXED WING PLT WITH A CURRENT CLASS 2 MEDICAL. I DID LACK A CURRENT BIENNIAL FLT REVIEW AND INST CURRENCY CHK. I BELIEVE SEVERAL THINGS CONTRIBUTED TO THE SIT. FIRSTLY; THE PLT HAD SET IN HIS MIND TO DO NIGHT VFR FLYING. IT WASN'T UNTIL WE PULLED THE AIRPLANE OUT OF THE HANGAR THAT VFR FLYING WAS OUT OF THE QUESTION. THE PLT WAS NOT MENTALLY PREPARED FOR ENTERING IMC. SECONDLY; THE PULSE LIGHTS ON THE ACFT WINGTIPS WERE ON. THIRDLY; LACK OF RECENT NIGHT AND ESPECIALLY NIGHT IFR EXPERIENCE. I SUGGEST THAT THE PLTS RECEIVE REGULAR DUAL NIGHT TRAINING AND THAT ALL CREW MEMBERS HAVE BEEN TRAINED TO RECOGNIZE VERTIGO AND WHAT TO DO WHEN IT OCCURS.