What happened
On October 14, 2008, a Saab 340A operating a passenger flight from Laage to Munich experienced a significant malfunction in its flight controls. The aircraft was carrying three crew members and 17 passengers. During the descent toward Munich, at approximately 07:40 local time, the crew noticed a disturbance in the left aileron system, characterized by the left control column rotating approximately 45 degrees to the left, while the co-pilot's control column moved slightly to the right.
Upon identifying the issue, the crew notified air traffic control and maintained a flight level of 120. Following the "Aileront System Jammed" emergency checklist, the crew utilized the "Roll Disconnect" lever to separate the left and right aileron control linkages. This allowed the co-pilot to control the aircraft from the right side. The aircraft landed safely at Munich Airport on runway 26R at 08:00 local time with no injuries and no damage to the aircraft.
The investigation
The BFU investigation focused on the mechanical state of the aileron control system and the Roll Disconnect Unit. Investigators examined the left aileron and the area between the wings, where the control linkages are connected. Upon inspection of the Roll Disconnect Unit, investigators found that the connection between the left and right aileron control strands had been disconnected. They discovered that the switch intended to reset the actuator and reconnect the strands was non-functional.
Further examination of the control box, which houses the disconnect lever, revealed that a critical guide pin—which was designed to secure the lever against excessive rotation and limit its travel—was missing. The pin had been attached to the lever shaft with a screw, but the internal threads in the shaft showed no signs of damage or remaining threads. The guide pin and its securing screw could not be located within the cockpit floor area.
Findings
- The missing guide pin left the disconnect lever unsecured, allowing for unintended rotation and pulling movements.
- The metal bracket responsible for actuating the microswitch was severely deformed due to the uncontrolled movement of the lever.
- The deformation of the bracket rendered the microswitch inoperable, resulting in a state where the aileron control strands remained permanently disconnected.