On August 27, 1995, at 2003 mountain standard time, a Cessna 421C, N745CA, overran the departure end of the runway while landing at Chinle, Arizona, and collided with ground obstructions and terrain. The aircraft was owned and operated by Critical Air Medicine, Inc., of San Diego, California, and was on a positioning flight returning from an aeromedical patient evacuation mission. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft sustained substantial damage. The certificated airline transport pilot and the flight nurse, the sole occupants, were not injured. The flight originated at Phoenix, Arizona, on the day of the accident at 1845. According to information obtained from the Federal Aviation Administration (FAA), the operator and the Navajo Nation authorities, the aircraft is based at Chinle to provide medical evacuation capability for the Navajo community. The pilot and nurse had taken a patient to Phoenix and were returning to Chinle in a second company aircraft which had completed a maintenance inspection at Phoenix. The pilot landed on runway 17 to avoid obstacles on the approach end of runway 35 during the night landing. The pilot told local Navajo authorities immediately after the accident that after touchdown as the aircraft passed the hangar he observed the windsock "sticking straight out indicating a tailwind." The pilot said he applied maximum braking, but could not stop prior to over running the departure end of the runway. The aircraft traveled down an embankment, through a ditch, and collided with two steel irrigation posts before coming to rest. The Navajo authorities said the winds were southerly at about 30 knots when the aircraft attempted to land. The windsock is not illuminated. FAA operations and airworthiness inspectors from the Scottsdale, Arizona, Flight Standards District Office, traveled to the accident site on August 28, 1995. Both the runway and the aircraft were examined. The airport does not have any wind indicators which would be visible during the hours of darkness. The touchdown point for the aircraft was identified as about the midpoint of the runway. Tire marks were apparent from that point to where the aircraft left the departure end of the runway and went through a cattle guard. A ditch was associated with the cattle guard. According to the operations inspector, the ground distance traveled from the touchdown point was about the normal ground roll for the aircraft under the reported 20- to 30-knot tailwind condition. The aircraft was observed in the company hangar at the airport. The landing gear had collapsed. Examination of the landing gear and the ditch associated with the cattle guard revealed evidence and ground scars that the gear collapsed as a result of overload when the aircraft went through the ditch. The encounter with the ditch and the resultant collapse disrupted the brake hydraulic lines. No discrepancies were identified with the aircraft.