A MECHANIC, WHO WAS NOT RATED AS A PLT, TOOK THE ACFT WITHOUT PERMISSION & ELECTED TO TAKEOFF AT NGT. A WITNESS SAW THE ACFT DEP AT ABOUT 0250 PDT. SUBSEQUENTLY, IT CRASHED APRX 3 MI ENE OF THE ARPT IN AN OPEN UNDEVELOPED AREA. THE TIME OF THE ACDNT WAS NOT DETERMINED, BUT A CLOCK IN THE WRECKAGE HAD STOPPED AT 0355. THERE WAS EVIDENCE THE ACFT HAD IMPACTED IN A NOSE DOWN, INVERTED ATTITUDE WHILE ON A NNE HDG. THE ACFT MOVED ONLY 28' AFTER IMPACT & CAME TO REST INVERTED WITH THE GEAR EXTENDED & THE FLAPS RETRACTED. NO PREIMPACT MECHANICAL PROBLEM WAS FND. TOXICOLOGY TESTS SHOWED THE PLT HAD A BLOOD/ALCOHOL LVL OF 0.37% & A VITREOUS/ALCOHOL LVL OF 0.33%. NO RECORD WAS FND TO INDC THAT HE HAD ENGAGED IN ANY PREVIOUS FORMAL FLT TRAINING. THERE WAS EVIDENCE THE PLT HAD BEEN UNDER RECENT STRESS. A FRIEND RPRTD THE PLT & HIS WIFE WERE 'BREAKING UP.' THE PLT'S WIFE RPRTD HE HAD A DRINKING PROBLEM' WHICH WAS THE REASON FOR THEIR SEPN.