28 December 1978 - United 173

United Airlines Flight 173, registration N8082U, was a Douglas DC-8-61 en route from Stapleton International Airport in Denver to Portland International Airport on December 28, 1978. When the landing gear was lowered, only two of the green landing gear indicator lights came on. The plane circled in the vicinity of Portland while the crew investigated the problem. After about one hour the plane ran out of fuel and crashed in a sparsely populated area near 158th and East Burnside Street, killing 10 and seriously injuring 24 of the 189 on board.

The NTSB investigation revealed that when the landing gear was lowered, a loud thump was heard. That unusual sound was accompanied by abnormal vibration and an abnormal yaw of the aircraft. The right main landing gear retract cylinder assembly had failed, due to corrosion, and that allowed the right gear to free fall. Although it was down and locked, the rapid and abnormal free fall damaged the microswitch, which completed the circuit to the cockpit green light, which tells the pilots that gear is down and locked. It was those unusual indicators (loud noise, vibration, yaw and no green light) which led the captain to abort the landing, so that they would have time to diagnose the problem and prepare the passengers for an emergency landing. While the decision to abort the landing was correct, the accident occurred because the flight crew became so absorbed with diagnosing the problem that they failed to calculate a time when they needed to land to avoid fuel starvation.


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