What happened
On February 6, 1998, at 11:39 UTC, a Gulfstream G III, registration VP-BLN, was performing an instrument approach to runway 18 at Chambéry. The flight originated from Geneva, Switzerland, as a private passenger flight.
During the final approach, the pilot decided to deviate from the established ILS glide slope. The intention was to fly a shallower descent profile to reach the runway threshold at a lower speed, thereby reducing the need for heavy braking upon landing. However, as the aircraft descended below the normal glide path, the pilot became disoriented. The aircraft's vertical speed increased, and the crew lost visual contact with the runway. Approximately 0.6 NM from the start of the runway, the aircraft struck the surface of the water in Lac du Bourget.
Following the impact, the occupants evacuated the sinking aircraft. They used cushions as makeshift flotation devices until they were rescued by nearby fishermen. One occupant, a flight attendant, sustained a broken arm, and the crew suffered from hypothermia. The aircraft was destroyed.
The investigation
The investigation focused on the flight path, meteorological conditions, and crew actions. Investigators examined the aircraft wreckage, which had been recovered from the lake bed. While the tail section was found, the cockpit voice recorder (CVR) could not be located because it was not equipped with an automatic underwater locator beacon.
Meteorological analysis revealed that while the sky was clear, a layer of dense mist was present over the lake. Because the air over the water warmed more slowly than the surrounding land, visibility was significantly reduced horizontally and obliquely. Furthermore, the sun's position caused intense glare, which, combined with the lack of surface ripples due to calm winds, eliminated visual depth perception and contrast.
Findings
- The crew voluntarily descended below the ILS glide slope to attempt a flatter approach.
- The loss of visual references due to intense solar glare and dense mist over the lake prevented the crew from maintaining situational awareness.
- The pilot did not initiate a go-around despite losing visual contact with the runway.
- The lack of a visual reference for the water surface, caused by the uniform appearance of the lake and the mist, made it impossible to estimate the aircraft's altitude.
- The crew failed to monitor the radio altimeter to trigger an immediate missed approach as the altitude approached zero.