Clay Lacy Aviation Learjet 55 accident

The airplane was about 500-600 feet above ground level (agl) on approach to landing when the emergency pressurization and right bleed air annunciator lights illuminated simultaneously. There was no change in the cabin pressurization, and the crew decided to continue and land. About 100-200 feet agl, the left bleed air light illuminated and the cabin pressure went to emergency. The crew switched the cabin air off. They landed, taxied quickly to their parking area, and turned the left and right bleed air switches off prior to engine shutdown. The flight line crew notified the flight crew that smoke was coming out of the ram air vent in the dorsal fin area. The pilot indicated that no smoke or smells entered the cabin or cockpit. He also pointed out that the air conditioner was blowing hot air and its operation appeared to be intermittent; the passengers had complained about the high temperature in the cabin. External visual examination revealed heat discoloration and wavy skin on the aft fuselage skin starting at station 512.39, frame 39, which extended aft to station 544.39, frame 41. Internal examination of the empennage in the tail cone access door revealed that frames 39, 39a, and 41 were scorched and sooty. In the aft E & E compartment in the tail cone, investigators observed fire damage to the windshield heat shutoff valve, left and right mixing valve control boxes and associated wiring, landing light/taxi light/auxiliary hydraulic pump relay box, upper side of the flow control valve, and the windshield heat shutoff valve and wiring. There was evidence of leakage and blow-by on the outflow flange of the windshield heat modulating valve; however, the observed damage was not as severe as would be expected if bleed air was the initiator of the fire. The evidence was inclusive for an electrical origin for the fire.

Verbatim NTSB analysis and probable cause from the NTSB dataset