THE PLT HAD JUST TAKEN OFF AT NIGHT ON A FLT TO MEXICO WHEN HE DECLARED AN INTENT TO RETURN & LAND WITH SOME UNSPECIFIED EMERGENCY. WHILE HE WAS USING HIS RADIO, THE SOUND OF BOTH ENGS, RUNNING AT HI POWER, WAS AUDIBLE. WHEN ASKED IF HE REQUIRED ASSISTANCE, HE GAVE AN AFFIRMATIVE REPLY, BUT WHEN ASKED IF HE REQUIRED A CRASH CREW, HE GAVE A NEGATIVE REPLY. SHORTLY AFTER THAT, THE ACFT COLLIDED WITH POWER LINES ABOUT 1 MI FROM THE RWY, THEN IMPACTED THE GROUND & WAS DEMOLISHED BY FIRE. IMPACT WITH THE POWER LINES OCCURRED AT ABOUT 55 FT AGL. A WIRE IMPACT MARK ON THE NOSE DOOR INDICATED THE ACFT WAS IN A VERTICAL BANK WHEN THE COLLISION OCCURRED. AN EXAM OF THE WRECKAGE REVEALED NO EVIDENCE OF A PREIMPACT/MECHANICAL MALFUNCTION OR FAILURE; HOWEVER, THERE WAS EXTENSIVE DAMAGE FROM FIRE. THERE WAS EVIDENCE THAT THE ACFT WAS LOADED BEYOND ITS MAX CERTIFICATED GROSS WT. THE PLT HAD A TEMPORARY U.S. LICENSE BASED ON HIS CANADIAN LICENSE.