PLT OF AN SMT TWIN ENG ATX ACFT BELIEVED THAT HE WAS NOT APPROVED FOR FLT UNDER IFR SINCE THE COMPANY OPS SPECS DID NOT INCLUDE IFR.

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AFTER DROPPING OFF PAX AT THE GRAND CANYON ARPT; I TOOK OFF EMPTY (NO PAX) FOR LAS VEGAS. WX AT GCN WAS CLR; THERE WERE SCATTERED CLOUDS IN LAS VEGAS. THE USUAL RTE IS A DIRECT LINE BTWN GCN AND LAS THROUGH THE GRAND CANYON SPECIAL AIRSPACE AT A CRUISING ALT OF 10500 FT. I WAS ON THAT RTE AND ALT WHEN I SAW AN OVCST WITH TOPS AT 10000 FT. VISIBILITY WAS ABOUT 15 NM THAT DAY AND AS FAR AS I COULD SEE THE TOP OF THE CLOUDS STAYED AROUND 10000 FT. I ELECTED TO STAY ABOVE THE CLOUDS RATHER THAN BELOW BECAUSE I COULD SEE RAIN AND DETERIORATING VISIBILITY UNDERNEATH. AFTER 15 MINS OF FLT I WAS FORCED TO CLB TO STAY OUT OF THE CLOUDS. I FOUND MYSELF AT 12500 FT; STILL HAVING TO CLB AND THE OVCST SEEMED TO BE STRETCHING FOREVER WHICH I DIDN'T EXPECT. I DECIDED IT WAS TIME TO DSND AS I WAS GETTING CLOSER TO LAS VEGAS; ABOUT 60 NM AT THAT TIME. I CALLED ZLA AND REQUESTED AN IFR CLRNC TO LAS VEGAS. AFTER IDENT; THE CTLR GAVE ME A DSCNT VECTOR TO 9000 FT THEN SWITCHED ME TO THE LAS VEGAS CTLR WHO GAVE ME A DSNDING VECTOR TO 8000 FT. AT THAT POINT I BROKE OUT OF THE CLOUDS; CANCELED IFR AND PROCEEDED TO DEST VFR. THERE ARE NO WX RPTING STATIONS BTWN GCN AND LAS. GRAND CANYON TOUR OPERATOR SHOULD MAINTAIN IFR CERTIFICATES TO GIVE PLTS MORE OPTIONS IN CASE OF BAD WX. MY DECISION OF STAYING HIGH MIGHT HAVE BEEN WRONG ALTHOUGH I BELIEVE IT WAS SAFER THAN GOING BELOW THE CLOUDS AND RUNNING THE RISK OF GETTING 'BOXED' IN BAD WX CLOSE TO THE PLATEAUS AND MOUNTAINS WHERE THE CLOUDS WERE THE LOWEST.