A WESTWIND ON VISUAL APCH TO LEB DSNDED BELOW ASSIGNED ALT BECAUSE OF COCKPIT DISTR.

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FROM PWM TO LEB; PVT FLT BY ACR OPERATING CORPORATE ACFT. 2 CAPTS ASSIGNED; I WAS PIC FLYING IN L SEAT. CRUISE ALT WAS 16000 FT; CLRED BY ZBW TO 10000 FT THEN 6000 FT. LEBANON IS SURROUNDED BY HILLS; DIFFICULT TO SEE AND THE OPTIONS WERE A FULL ILS WITH PROC TURN OR A VISUAL. I HAD ONLY BEEN IN THERE ONCE BEFORE; UNDER SIMILAR LIGHTING CONDITIONS; BUT THE OTHER PLT HAD BEEN IN MANY TIMES SO I DECIDED TO LET HIM TALK ME THROUGH A VISUAL APCH. OUR ALT ALERTER WAS INTERMITTENT; SOMETIMES SHOWING A FLAG AND SOMETIMES NOT. WE GOT INTO A DISCUSSION OF ALTS ON THE APCH; AND FOUND OURSELVES AT 4700 FT. WE CALLED BOSTON AND THEY SWITCHED US OVER; DIDN'T SAY ANYTHING. THE OTHER PLT IS LEAVING THE COMPANY IN A FEW WKS AND WILL BE SOMEONE ELSE'S PROB. I HATE TO BLAME IT ON HIM BUT HE IS COMPLETELY UNRELIABLE AND HAS LET ME DOWN BEFORE. AS MUCH AS I TRY TO CONCENTRATE AND I KNOW HE'S DANGEROUS -- HE SOMEHOW PULLS ME DOWN TO HIS LEVEL. I SHOULD HAVE REALIZED THE 'DICEYNESS' OF THE SITUATION AND REQUESTED THE FULL ILS APCH; THEN I COULD HAVE JUST WORRIED ABOUT ONE ALT AT A TIME. 3 PROBS: UNFAMILIAR ARPT IN MOUNTAINS AT NIGHT; BROKEN EQUIP; AND UNRELIABLE CREW MEMBER ARE TWO TOO MANY.