What happened
On November 26, 2008, a Learjet 40 XR, registration OE-GVA, was performing a non-scheduled positioning flight from Paris to Stockholm/Bromma airport. While under radar vectoring for an ILS approach to runway 30, the aircraft deviated from its assigned heading and altitude.
During the approach, the crew attempted to execute a left turn via the autopilot, but the aircraft failed to respond to the new heading selection, continuing straight on its original path. Upon realizing the autopilot was not executing the turn, the commander disconnected the system and took manual control. During this period of manual maneuvering, the aircraft began an unintended descent. The aircraft's altitude dropped from a cleared 2,500 feet to a minimum of approximately 650 feet, bringing it dangerously close to radio and TV broadcasting antennas located in Nacka. Air traffic control (ATC) intervened, warning the crew of the obstacles and instructing them to turn and climb. The crew subsequently performed a go-around and eventually landed safely at Bromma airport.
The investigation
The Swedish Accident Investigation Authority (SHK) examined radar data, ATC communications, and interviews with the flight crew. The investigation focused on why the aircraft failed to follow the heading change and why the descent occurred. Investigators also analyzed the configuration of the aircraft's altimeters and the crew's management of the complex flight management systems.
Findings
- The aircraft's autopilot was likely in a basic mode that prevented the heading change from being executed despite the selection on the panel.
- The crew experienced a significant inappropriate prioritisation and allocation of workload while attempting to troubleshoot the automation.
- At least one altimeter was set to standard pressure (STD) rather than the local pressure (QNH), which caused the pilots to miscalculate their true altitude by approximately 500 feet.
- The transition of control between the commander and co-pilot was not performed using standard phraseology, leading to potential confusion regarding who was flying the aircraft.
- The high level of system complexity in the Learjet 40 XR contributed to the crew spending excessive time "head down" managing electronic instruments during a critical phase of flight.