The Weekly Reflektion 18/2024

In many industries we are relient on computers, PC’s andautomated systems for a safe and efficient operation. We are then dependent on the way people interact with these and how the human machine interfaces (HMI) are set up.

Are your human-machine interfaces designed to help or hinder?

Air Inter Flight 148 was a scheduled flight from Lyon to Strasbourg. On 20th January 1992, the Airbus A320 crashed into the Vosges Mountains killing 87 of the 96 people on board. The captain and first officer were both experienced pilots, however they only had 162 hours and 61 hours respectively on the A320. Both had flown into Strasbourg on many occasions so were familiar with the airport and the approaches.

As the flight neared Strasbourg Airport, the crew requested anILS (Instrument landing system) approach for Runway 23 until the airport was in sight, followed by a visual approach onto Runway 05. This type of approach was common at Strasbourg Airport due to the mountains and high terrain.Runway 05 was not equipped with ILS since the high terrain would interfere with the glide slope signal the ILS transmitted. The flight controller denied this approach due to conflicts with several aircraft departing on Runway 05. An alternative approach was suggested that used radio signals and automatic distance measuring, termed a VOR/DEM approach, which the crew accepted. The flight was cleared to descend to 5,000ft and to a waypoint 11 nautical miles from Strasbourg Airport. As the aircraft passed the waypoint, the flight controller thencleared the aircraft onto the final approach turning left onto the runway heading and continuing to descend as per the approach chart. Since Runway 05 did not have an ILS approach then the crew had to calculate the angle of descent manually. During the descent the crew noticed they were travelling too fast and applied the speed breaks to slow down. The Vosges mountains were in clouds above 2000 ft and the tops, up to 6400 feet, were obscured. The aircraft crashed at about 2700 feet.

The investigation found that the crashed occurred because the crew had the autopilot set in Vertical Speed Mode instead of Flight Path Angle Mode. They set “33” for what they believed was “3.3° descent angle” equivalent to a descent rate of about 800ft per minute. In the Vertical Speed Mode ‘33’ resulted in a descent rate of 3,300 ft per minute. The vertical speed selector and flight angle mode were both on the same window on the control panel and both used the same selector knob. When the flight path angle was activated, it showed ‘-3.3’. The vertical speed selector when activated showed ‘-33’. It is likely that the similar displays, the similarity of the numbers and the crews limited experience with the A320 aircraft confused them. The pilots had no warning of the imminent impact because Air Inter had not equipped its aircraft with a ground proximity warning system (GPWS). It was speculated that this was because Air Inter may have encouraged its pilots to fly fast at low level up to 350 knots (650 km/hr) below 10,000 ft, while other airlines generally do not exceed 250 knots [460 km/h), and GPWS systems gave too many nuisance warnings. Competition with France’s TGV high-speed trains may have been the reason for this practice. The investigators also found that the airline did not conduct extensive training with non-precision approaches with the A320.

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