Technological progress is fundamental in a field like aviation: improvements and innovations capable of bringing a leap in quality. But sometimes half a second is enough to question everything and to turn the spotlight on the human component, which remains central. The case of the is emblematic Air Inter flight 148 which on 20 January 1992 crashed in the Vosges mountains, near Mont Sainte-Odile, in France, while waiting to land at Strasbourg-Entzheim airport. Tragic toll 87 dead and 9 injured. Among the most complex ever carried out, the investigations revealed a disconcerting truth.
The crash of Air Inter Flight 148
The aircraft of Air Inter flight 148 is an Airbus A320-111, delivered to the Air Inter company four years earlier. It is one of the most technologically advanced commercial aircraft in the world, the last frontier of innovation. The flight commander is Christian Hecquet42 years old, while the co-pilot is Joel Cherubin37 years old: two very experienced pilots, over 12 thousand hours of flight, but just 223 on board an Airbus A320. The plane takes off from Lyon-Satolas, destination Strasbourg-Entzheim, in the mountainous region of Alsace.
The Air Inter company is famous for its punctuality, mainly businessmen travel there. The two pilots engage autopilot but are forced to change plans due to the bad weather: instead of landing on the planned runway – 23 – they will have to land on 05, surrounded by a mountainous area that could interrupt radio signals. Hecquet decides to leave the autopilot programmed, and then take control and make a visual landing.
The Strasbourg control tower approves the plan, but it doesn’t end there. Air Inter flight 148 has to deal with a new unexpected event, this time linked to air traffic. Thanks to the great nervousness of the two pilots, in particular the captain, the controller offers assistance and proposes to give radar indications to bring the plane to Andlo at an altitude of 1500, i.e. the navigation coordinate on the route to runway 05 which helps the pilots to align the plane for landing. On that runway it is not possible to use automatic pilot and for this reason the pilots must calculate the angle of descent, 3.3 degrees on this occasion.
The control tower helps the pilots align on the runway, but the first officer notices that the Airbus A-320 is slightly off course. The controller also notices the offset course, but nevertheless authorizes the landing. Hecquet begins the appropriate maneuvers, but realizes that the plane has gained too much speed and decides to activate the airbrakes. After a few seconds, the first officer focuses on the horizontal position with respect to the runway but before being able to set the course, the terrible crash against Mont Sainte-Odile.
The research
Air Inter Flight 148 immediately disappears from radar and does not respond to contact requests from the Strasbourg control tower. Taken immediatelyalarmthe investigators start looking for the accident site but the road is uphill: the radar signals are not recorded, no alarms went off from the plane and no one saw it crash. In the meantime, some survivors exit the aircraft, confident of immediate help.
The hours pass, but there is no clue on the position of Air Inter flight 148. The survivors, however, have to deal with sub-zero temperatures. The investigations are entrusted to chief investigator Jean Paries, who leads the research team made up of a thousand people. There turning point arrives by chance: looking for help, one of the survivors crosses paths with a TF1 television crew who, after initial doubts, decide to follow him to the accident site, where they find eight other survivors. After four and a half hours of searches, help finally arrives: the toll is 87 deaths among crew (5) and passengers (82). Nothing to do for the two pilots.
The investigations
Bob Macintosh of the Air Inter flight 148 also arrives at the site of the disaster.NTSB American. The transalpine authorities – BEA (Bureau d’enquêtes et d’analyses pour la sécurité de l’aviation civil) in the lead – in fact immediately understood the international scope of the tragedy and invited a group of foreign investigators to participate in the investigation. Attention is immediately drawn to black boxeswhich could be damaged due to the fire that broke out on the Airbus A320. Nothing to be done, however, for the emergency locator, destroyed in the impact.
Excluding engine problems, investigators have to deal with some bad news: the recorder cannot be used because it is too damaged. Excessive exposure to heat is fatal. The last hopes are linked to the cockpit device that allows access to the on-board computer: the investigators find it but the last centimeters of the tape are damaged. Hence the decision to adopt an experimental technique to recover those data that seem irreversibly lost, but it takes time before the outcome.
For this reason investigators focus on voice recorder in the cabin and the captain’s anxiety emerges forcefully already during the flight. Hecquet highlights concern about landing on runway 05 and the reason is very simple: Air Inter’s pilot training history reveals that many are not very prepared for non-precision landings on the new A-320s. The investigators therefore decide to ask the company for a detailed list of the pilot’s landings and make an interesting discovery: the captain landed in Strasbourg many times but had never made a non-precision landing with the A320. Furthermore, the two pilots were not very familiar with the advanced technology of the aircraft.
The pilots of Air Inter flight 148 did not have sufficient information and preparations, but this was not enough to explain the crash. The focus shifts to control tower and a surprising error emerges: the controller’s directions were wrong, instead of towards the runway he directed the pilots towards the mountains. The pilots were off course due to the indications received from the control tower: for this reason the investigators will suggest no longer using terms such as “right” or “left”, which could cause confusion, but to use the cardinal points. However, investigators believe that is not the only cause of the accident.
The team of experts reconstructs the path of the plane and another alarming detail emerges: Air Inter flight 148, after having circled the mountain, begins a steep descent. But why? By studying the trajectory, it is determined that the descent began sixty seconds before the crash. But there is no indication on the tape that the descent seems intentional: why didn’t the pilots notice? The altimeter is an extremely reliable indicator, yet nothing. The only reference in the conversation between the two pilots comes sixteen seconds before the crash, after having extended the flaps. Then the first officer changes the subject and focuses on the lateral situation rather than the vertical one. In other words, neither of them realized the gravity of the situation.
The turning point
One question captures the attention of investigators: what caused that deadly descent? The answer comes a few weeks later, when all the data from Air Inter flight 148 is recovered. The numbers leave no room for doubt: shortly before the crash the plane headed towards the ground at very high speed, one kilometer per minute. But not only that: the data certifies that the descent angle is dangerously steep, more than the 3.3 degrees calculated by the captain. And here’s the twist, linked to the suspicious similarity between two key numbers, vertical speed (3,300 feet) and descent angle (3.3 degrees).
The investigators use a flight simulator to prove his theory and here’s the key: the two parameters are displayed on the same one display and selected with the same knob. In other words, confusing the two parameters appears rather easy: the captain could have selected the vertical speed instead of the descent angle. Reality confirms everything: two months after the tragedy of Air Inter flight 148, another Airbus A320 faces a steep descent for the same reason but fortunately the pilots manage to regain control before the crash.
But it doesn’t end there, because the simulator states that despite inverting the data, the crash into the mountain cannot be explained. Several theories are ruled out, until investigators turn to the plane’s manufacturer. The designer provides an explanation on a little-known element of the autopilot: In emergency situations where you need to change direction quickly, the autopilot is programmed to reverse course at double the normal speed.
This is not a random element. Shortly before the crash, turbulence caused Air Inter Flight 148 to rise slightly: this caused a vertical speed of 180 meters per minute for about half a second. During the same half second, the pilots ordered the airplane to descend: hence the autopilot interpreted the command as an emergency situation requiring a rapid descent. A lethal combination between the confusion with the display, the turbulence and the safety measure of the plane.
The consequences
The investigators draw up a list with 35 recommendations to avoid tragedies like that of Air Inter 148.
First of all, the modification of the display of the Airbus on-board instrument: the descent speed display goes from two to four digits, while the descent angle remains at two digits, so as to avoid confusion. Furthermore, the GPWS ground proximity warning system is adopted and obviously it is indicated to improve pilot training and to build more heat-resistant black boxes.