DURING A NIGHT ARRIVAL, THE FLT WAS ON AN INSTRUMENT APCH TO THE GULKANA ARPT WHEN RADIO CONTACT WITH THE ACFT WAS LOST. A SEARCH WAS INITIATED & THE WRECKAGE WAS FOUND 7.4 MI NORTH OF THE TERMINAL VOR ON THE 330 DEG RADIAL. THE INBOUND COURSE FOR THE VOR RWY 14 APCH WAS ON THE 315 DEG RADIAL. AN ON-SCENE EXAM REVEALED THE ACFT WAS TRACKING INBOUND WHEN IT COLLIDED WITH TREES, CRASHED & BURNED ABOUT 2.4 MI BEFORE REACHING THE FINAL APCH FIX. BOTH ENGS WERE OPERATING DURING IMPACT & BOTH HAD INJESTED CONSIDERABLE TREE MATERIAL. NO PREIMPACT MECHANICAL FAILURE WAS EVIDENT. THE CAPTAIN & 1ST OFFICER WERE CURRENTLY RESPONSIBLE FOR FLYING BOTH THE LEARJET & DHC-7 ACFT. THIS WAS THE CAPTAIN'S 1ST DAY OF WORK AFTER RETURNING FROM A VACATION TRIP TO HAWAII (APRX 1-1/2 WEEKS). THE 1ST OFFICER HAD AN ADEQUATE PERIOD FOR CREW REST PRIOR TO REPORTING FOR WORK; HOWEVER, HE HAD PREVIOUSLY BEEN WORKING A VARIED SCHEDULE. THE OPERATOR REPEATEDLY SHIFTED PLTS FROM 14 CFR 135 REST/DUTY REQUIREMENTS TO THOSE OF 14 CFR 121.