United Airlines Flight 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 one of the three 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 NE 158th Avenue and E Burnside Street, killing 10 and seriously injuring 24 of the 189 on board.

Injuries
Of the crew members, two died, two sustained injuries classified by the National Transportation Safety Board (NTSB) as "serious", and four sustained injuries classified as "minor/none." Of the passengers, eight died, 21 had serious injuries, and 152 had minor or no injuries.

Crash investigation and report
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 of the gear damaged a microswitch so severely that it failed to complete the circuit to the cockpit green light that 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.

"The Safety Board believes that this accident exemplifies a recurring problem --a breakdown in cockpit management and teamwork during a situation involving malfunctions of aircraft systems in flight....Therefore, the Safety Board can only conclude that the flightcrew failed to relate the fuel remaining and the rate of fuel flow to the time and distance from the airport, because their attention was directed almost entirely toward diagnosing the landing gear problem."

The NTSB determined the following probable cause: "'The failure of the captain to monitor properly the aircraft's fuel state and to properly respond to the low fuel state and the crewmember's advisories regarding fuel state. This resulted in fuel exhaustion to all engines. His inattention resulted from preoccupation with a landing gear malfunction and preparations for a possible landing emergency.'"

The NTSB also determined the following contributing factor:"'The failure of the other two flight crewmembers either to fully comprehend the criticality of the fuel state or to successfully communicate their concern to the captain.'"

However, the fuel situation was very much on the minds of the pilot and crew. Transcripts of cockpit recordings (see AirDisaster.com link below) confirm this. In fact, the crew was deliberately burning off fuel to have minimum fuel on board upon landing. Media reports at the time suggested that there was a not-widely-known problem with fuel state gauges on that model aircraft. The problem was not widely known in part because commercial aircraft are expected to fly with not less than a 45 minute reserve of fuel at all times. The gauge problem is addressed, though obliquely, in one of the safety board's recommendations: "Issue an Operations Alert Bulletin to have FAA inspectors assure that crew training stresses differences in fuel-quantity measuring instruments and that crews flying with the new system are made aware of the possibility of misinterpretation of gauge readings. (Class II--Priority Action) (A-79-32)"

Aftermath
As a result of this accident, United Airlines instituted the industry's first Crew Resource Management/Cockpit Resource Management (CRM) program for pilots, in 1980. The CRM program proved to be so successful that it is now used throughout the world.

Other aircraft accidents involving faulty landing gear indicator lights were Eastern Air Lines Flight 401, which crashed while circling around the airport at Miami, on December 29, 1972, LOT Polish Airlines Flight 007, and Scandinavian Airlines Flight 933, on January 13, 1969, which crashed into the ocean during an approach to Los Angeles International Airport.

The Eastern crew became preoccupied with the nose gear indicator light problem and accidentally disconnected the autopilot, causing the aircraft to make a slow descent and crash into the Everglades. Further investigation revealed that the nose gear was down and locked. It was the same for the SAS flight, as the green light for the nose gear failed to illuminate, after the landing gear was lowered. The SAS cockpit crew became so occupied with attempting to diagnose the lack of a nose gear green light, that they allowed their rate of descent to increase, until that DC-8-62 crashed into the ocean, well short of the runway.