Swiss investigators must determine how a seasoned Alitalia crew flew their DC-9 into the ground just a few miles short of a Zurich runway.
Alitalia Flight 404 (AZ404/AZA404) was an international passenger flight scheduled to fly from Linate Airport in Milan, Italy, to Zürich Airport in Zürich, Switzerland, which crashed on 14 November 1990. The Douglas DC-9-32, operated by Alitalia, crashed into the woodlands of Weiach as it approached Zurich Airport, killing all 46 people on board.
A Swiss investigation concluded that the crash was caused by a short circuit, which led to the failure of the aircraft's NAV receiver. The malfunction went unnoticed by the crew, who likely believed they were on the correct flight path until the crash. Swiss authorities also blamed inadequate crew resource management, exemplified when the captain vetoed the first officer's attempted go-around, along with the absence of lighting on Stadlerberg Mountain and a known problem with errors in reading the drum pointer altimeter of the aircraft.
The final report by the Federal Aircraft Accidents Inquiry Board requested several major changes and made further recommendations.
Accident
During the approach to runway 14 of Zürich International Airport, the pilot's instrument landing system (ILS) display gave incorrect values due to a faulty receiver. It falsely indicated that the aircraft was about 1,000 feet higher than it actually was, which also prevented the Ground proximity warning system (GPWS) from reading the situation and sounding a warning. The ILS receiver of the first officer was working correctly and displayed the dangerously low approach. Without thoroughly examining which value was correct, the pilot decided to ignore the second device, aborting a go-around maneuver initiated by the first officer. Shortly afterward, at 20:11 CET, the plane struck Stadlerberg Mountain at 1,660 feet (510 m), killing all 40 passengers and six crew.
The aircraft's first impact was with trees on the right side of the aircraft, causing several essential parts on the right side of the aircraft, such as the right-wing flaps and the outer right wing, to detach. As a result, the aircraft produced an asymmetric lift force and began to roll to the right, finally slamming into the mountain in a nearly inverted position.
Firefighters and police were immediately dispatched to the crash site but the fire was so intense that it took an entire day to put out. Eyewitnesses stated that "the plane was burning like a volcano." Linate Airport immediately set up a crisis center to handle the relatives of the victims aboard the flight.
Aircraft
The aircraft involved in the crash was a Douglas DC-9-32, built-in 1974 with serial number 47641 and the registration number I-ATJA. The aircraft was first delivered to Aero Trasporti Italiani,a subsidiary of Alitalia and was transferred to Alitalia in October 1988. According to investigators, the aircraft had accumulated more than 43,400 cycles and also stated that the aircraft had been inspected 10 days prior to the accident.
Investigation
Swiss investigators were informed about the accident one hour after the crash. The investigation involved at least 80 Swiss investigators. Additionally, Italian Interior Minister Vincenzo Scotti sent a team of investigators to the crash site along with the United States' NTSB and FAA. Both the Flight Data Recorder and the Cockpit Voice Recorder were recovered from the crash site.
In the initial hours after the accident, several witnesses claimed that the aircraft was on fire before it hit Stadlerberg while others claimed that the aircraft had exploded before it impacted terrain. Josef Meier, a spokesman for Swiss air traffic control authorities stated in a news conference that all possible causes, including terrorism, were being evaluated by the investigators.
Italian news media reported that the mountain was covered with cloud and that there was heavy rain in the area, while Swiss investigators claimed that visibility was good during the accident with light showers occurring at the airport.
On 16 November, an official from Swiss investigation team stated that the aircraft was flying at a lower altitude than it should have been: "The plane was 300 metres (980 ft) too low on its approach, and was slightly off course. [The captain] was not following the radio beacon signals." The statement did not say why the plane was too low.
Fatal decision
The first officer had initiated a go-around but the captain aborted it. He said, "Hold the glide slope, can you hold it?". The first officer replied, "Yes". Investigators working with McDonnell Douglas concluded that, if the captain had not interrupted the go-around and had a situation awareness as the first officer had, the disaster would have been averted. Investigators believe that the reason for the bad call was that the captain had no situation awareness at all and was completely dissatisfied with the first officer's performance during the flight. As a result, the captain showed a lack of trust in his first officer.
Stadlerberg Mountain
As Stadlerberg Mountain is relatively far from the runway, per International Civil Aviation Organization (ICAO) standards, obstruction lighting was not required. The 2,090-foot (640 m) mountain could not be seen during the night, and thus could endanger flights. This issue had been discussed between Swissair, Swiss airport authorities, and the Swiss Federal Office of Civil Aviation (FOCA) since 1976. Analysis of the cockpit voice recorder did not give any indications that the crew was looking for the cloud-capped mountain during the approach.
NAV Receiver failure
Recordings from the black boxes showed that the navigation instrument, the ADI/HSI, had apparently captured the glideslope. At the time when the glide slope was captured, the aircraft was flying about 1,300 feet below the glide path. Examining the receiver with a microscope, the investigation revealed that during the impact, the glide path indicator had been positioned just above the center "on glide" position, which means that the glide path had been captured. Investigators also revealed that all NAV instruments were following the glide path, although the aircraft was flying below the nominal glide path. At the time, investigators didn't find any defects on the instrument.
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