Scandinavian Airlines System Flight 933

Investigation

Because the crash took place in international waters, the investigation was carried out in accordance with the Convention on International Civil Aviation. The Government of Norway requested that the investigation be carried out by the United States' National Transportation Safety Board. The maintenance records were investigated by Norway's Aviation Accident Commission. The final report from the board was issued on July 1, 1970, after 534 days of investigation.

Flight 933 was the 20th hull loss to a DC-8; it was at the time the tenth-deadliest accident of the type and remains the twentieth-deadliest. It was SAS's third fatal crash, but the airline would not experience another until the 2001 Linate Airport runway collision.

All navigational aid systems at LAX were checked and found to be working at the time of the accident. The flight recorder was recovered using a remotely operated underwater vehicle and found to be intact. Flights and simulator tests were carried out by SAS, confirming that the recorded data could be simulated in an appropriate manner on schedule. As the aircraft was found airworthy and able to be flown, the bulk of the work of the investigation commission focused on operational procedures.

Cause

The accident was caused through a series of events which, although not in themselves sufficient to cause the crash, combined to create a breakdown in crew resource management. The flight experienced two delays (de-icing at Seattle–Tacoma and holding at Bakersfield), which along with wind speeds increased the flight time by nearly three hours. This caused the captain to consider diverting to Las Vegas. The first pilot error occurred when the first officer incorrectly set his altimeter when the descent started. The difference between his and the captain's altimeter was never noticed.

Upon receiving clearance, a non-standard terminology was used by air-traffic control. As he did not have authorization to use a localizer back-course approach, the captain should have requested a different approach. Instead, the crew opted to conduct a VOR approach without informing air traffic control. Neither pilot had carried out instrument approach and landing at runway 07R making them less familiar with this than their commonly used Runway 25. Another factor was that the SAS aircraft was forced to operate at the lowest-permissible safe speeds while closing in on the Cessna.

The commission interpreted several of these actions as taking shortcuts to avoid further delays on an already severely delayed flight. They regarded the decision to descend at 5 meters per second (1,000 fpm) as reasonable given the conditions. However, as the first officer focused on the nose gear issue, the aircraft actually experienced a descent of 10.0 meters per second (1,960 fpm) for 26 seconds, zero descent for 16 seconds, and then an average descent of 8.6 meters per second (1,720 fpm) until impact. The first officer was distracted by the captain's dealings with the landing-gear issues, hindering him from primary task: flying the aircraft. The cycling of the landing gear and delay in extending the flaps made speed and altitude control more difficult. The captain also failed to inform the first officer when the flaps were fully extended.

Both the landing-gear issue and the concerns regarding speed made the captain focus on the possibility of a missed approach and the major inconvenience of diverting to Las Vegas. It was the commission's impression that the captain failed to properly monitor the approach, and crew resource management broke down. He failed to give proper instructions to the first officer and failed to carry out instructions from the first officer which moved the first officer's attention away from his task of monitoring the flight instruments. The situation was worsened by the crew attempting to fly at 126 kn when the aircraft was not configured for that speed. These factors created a situation in which neither pilot was monitoring the altitude. There was also a shortcoming in the approach chart which did not display a minimum altitude at Del Rey Intersection. This would have given the pilots an opportunity to correct the aircraft's altitude.

The commission classified the accident as survivable because the impact forces varied along the fuselage. The tail-first impact was caused by the first officer's last-second attempt at raising the aircraft. Most of the fatalities resulted from people having been trapped in the sinking sections which was caused by the collapsing of the structure after impact. The collapse was caused by the compromise of the tubular integrity which was dependent on the keel beam that had been torn off on impact.

The nose-gear light indicators were designed to be fail safe by having two separate light bulbs. This proved to be inadequate, as it was impossible to look through the cover to check whether one of the bulbs had been compromised, meaning that a failure of one bulb would not be detected until both bulbs malfunctioned. The first bulb was thus presumed to have gone inoperative some time before the day of the flight, while the second bulb broke during Flight 933. The NTSB therefore advised the Federal Aviation Administration to articulate means to avoid similar compromised fail-safe designs in the future. Both pilots had minimum-descent altitude light warnings, which were presumed to have given a visual warning, but because of the work overload, neither pilot directed his attention to these alerts.

The investigation commission produced the following conclusion:

Two similar accidents occurred in the following decade. Eastern Air Lines Flight 401 was a watershed incident in airline safety: on December 29, 1972, its entire flight crew became preoccupied with a burnt-out landing-gear indicator light and failed to notice that the autopilot had inadvertently been disconnected. As a result, the aircraft gradually lost altitude and eventually crashed. A similar incident occurred on December 28, 1978, when the captain of United Airlines Flight 173 was distracted by a landing-gear issue and did not heed his crewmembers' concerns about the aircraft's fuel level, resulting in an exhaustion of fuel to all engines and a subsequent crash.