On January 20, 1992, Flight 148, a French domestic flight, took off from Lyon Satolas Airport at 17:20, commanded by Captain Christian Hecquet (42) and First Officer Joël Cherubin (37). There were 90 passengers and four other crew members on board. The flight was operated on an Airbus A320, which had been recently introduced at the time.
The Swiss Cheese Model
Introduced in the early 1990s, the Swiss Cheese Model is a scientific principle used to evaluate risks in various fields, including healthcare and engineering. In aviation safety, it is often applied when analyzing air incidents and accidents.
Each layer of the Swiss Cheese represents the different factors and groups of people that are involved in the industry: the airline, pilots, technical crew, rules and regulations, and so on. Each layer is bound to have shortcomings, such as poor design and procedures, which are represented by “holes” in each layer.
In the fatal crash of Air Inter Flight 148, the holes of the Swiss Cheese lined up in a way that seemed to be an unlikely stroke of tragic luck. Let’s take a deeper dive.
Hole #1: Landing at Strasbourg Airport
Flight 148 was headed for Strasbourg Airport, which had two runways: Runway 05 extended southwest to northeast while Runway 23 ran in the opposite direction. Due to high terrain, only Runway 23 was equipped with an Instrument Landing System (ILS).
At the time, the active landing runway was Runway 05, which meant the pilots had to execute a non-precision approach by manually working out the descent profile. This proved to be an obstacle for Hecquet, who was not comfortable carrying out such an approach (pilots at Air Inter rarely flew non-precision approaches).
Hole #2: A lack of communication
As a workaround, Hecquet devised a plan to approach the airport as if he were landing on Runway 23 so that he could use the ILS, then loop around to land on Runway 05 visually. Unfortunately, he failed to convey his intentions to the First Officer and the air traffic controller.
Only after Cherubin configured the flight systems for a direct landing on Runway 05 did Hecquet inform Cherubin about his plans and overrode the First Officer’s inputs. Cherubin questioned Hecquet, who responded that it would take an extra ten minutes to execute such an approach. This was, in fact, untrue.
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Hole #3: Imprecise flight path
The air traffic controller, who was still under the impression that Flight 148 was landing straight onto Runway 05, gave instructions for the flight crew to line up with waypoint ANDLO to commence landing. The A320 was flying too high to comply, and thus Cherubin stated their actual intention.
The controller advised that if the flight crew were to go ahead with their plan, they would be holding off traffic that was waiting to take off. So, Hecquet was forced to abandon his plan and revert to a non-precision approach directly to Runway 05, as initially suggested by Cherubin and the controller.
Flight 148 began a loop around to fly over ANDLO and commence landing. Instead of overflying ANDLO, Hecquet completed the turn too early and passed ANDLO on the left. Cherubin alerted Hecquet, who didn’t respond. The first officer then repeated himself, to which Hecquet was heard replying, “Yeah, yeah” before correcting the path. However, Hecquet remained flying slightly left of the approach path, taking the A320 over the 2,700-foot (823-meter) Mont Sainte-Odile, which was covered in thick clouds.
Hole #4: Poor design and procedures
The state-of-the-art A320 was a significant upgrade – the first fully fly-by-wire aircraft – to Air Inter’s previous fleet of aircraft from the 1960s. The type boasted never-before-seen features, such as side-stick controls and several autopilot modes.
The flight computer had two navigation modes: Heading/Vertical Speed (HDG/VS) and Track/Flight Path Angle (TRK/FPA). Using the first mode, pilots could control the aircraft by entering a compass heading and the desired rate of descent. The latter mode allowed pilots to create a specific track to fly along.
Hecquet had been using the HDG/VS mode to steer the A320 onto the various headings relayed by the controller. Now, as he neared Runway 05, he calculated that a flight path angle of -3.3° would be needed to land. He entered this into the computer without switching the mode to TRK/FPA. In Hecquet’s defense, it was easy to forget as the mode change button was not located near the associated display.
Thus, instead of telling the aircraft to descend at an angle of -3.3°, his input of “-33” was understood by the computer as a descent. This seemingly minor input led to a catastrophic outcome: the aircraft began descending at an alarming rate of -3,300 feet (1,006 meters) per minute, before crashing into the snow-covered mountainside.
Holes #5 and #6: Insufficient training and regulations
While both Captain Hecquet and First Officer Cherubin were experienced pilots, they were new to the A320: Hecquet had 162 hours on the type, and Cherubin had only 61 hours. Furthermore, training was insufficient as Air Intel trained its pilots in large groups. Due to inexperience and poor training, neither Hecquet nor Cherubin noticed the differences between the HDG/VS and TRK/FPA modes.
Yet, there were other layers of the Swiss Cheese that could have prevented the crash – such as a Ground Proximity Warning System (GPWS). At the time of the accident, France only just started mandating that all aircraft should be fitted with GPWS technology, and Air Inter had not begun the process of installing such systems.
The aftermath
Only nine of the 96 people on board survived. They were found hours after the crash, freezing in the sub-zero conditions of the mountain.
Recommendations in the final investigation report presented changes that would fortify the layers of the Swiss Cheese. These included improvements to training requirements, systems and equipment refinements, and certification rules around cockpit design.
The crash of Flight 148 ultimately highlighted issues surrounding how humans interact with the environment under stress, the importance of ergonomics and intuitive design, and the need for strict enforcement of regulations.
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Airbus
- Stock Code:
- AIR
- Date Founded:
- 1970-12-18
- CEO:
- Guillaume Faury
- Headquarters Location:
- Toulouse, France
- Key Product Lines:
- Airbus A220, Airbus A320, Airbus A330, Airbus A340, Airbus A350, Airbus A380
- Business Type:
- Planemaker
