What happened
On August 22, 2002, a private flight involving a Beechcraft 95-B55 (registration OH-BBX) experienced a serious loss of control while approaching Helsinki-Malmi Airport. The aircraft was carrying the pilot and six passengers, including a three-year-old child.
During the approach, the pilot extended the landing gear and selected the flaps to the down position. As the flaps were deploying, the left flap actuator failed, causing the left flap to stop moving while the right flap continued to extend fully. This created a significant asymmetric flap condition, resulting in a powerful left-rolling moment and a left side-slip. The aircraft banked between 15 and 30 degrees, and the pilot found it extremely difficult to maintain control, noting that turning to the right seemed nearly impossible.
To manage the situation, the pilot utilized a low-speed, high-drag approach, eventually reducing engine power to minimize the combined effect of the asymmetric flaps and propeller slipstream. This reduction in power allowed the pilot to regain enough controllability to land successfully on runway 27.
The investigation
The investigation focused on the mechanical failure of the flap system and the contributing environmental and operational factors. Investigators examined the failed left flap actuator and found that the piston thread had worn down and disintegrated. The right actuator showed similar extreme wear, with the threads having bent due to weakened structural integrity.
Technical analysis revealed that the actuators had been installed only 235 flight hours prior to the incident, far short of their 2,000-hour overhaul interval. The investigation also noted that the left actuator had been installed upside down, and both actuators had been covered with plastic tape to prevent oil leaks during transit, which the mechanic failed to remove before installation. Furthermore, the investigation found that the aircraft was operating at a high mass, near its maximum allowable limit, with a center of gravity located behind the aft limit.
Findings
- The primary cause of the incident was the failure of the left flap actuator due to a lack of lubricating oil.
- The lack of lubrication was caused by the fact that the actuator's vent hole was pointing downward due to incorrect installation, and the vent hole had been blocked by adhesive tape left in place from storage.
- The extreme wear on both actuators was accelerated by the absence of lubrication and the missing bearing bushings on the left actuator.
- The aircraft's high mass and aft center of gravity increased the difficulty of maintaining control during the asymmetric flap condition.
- The pilot did not utilize asymmetric engine power (reducing power on the right engine while increasing it on the left) to counteract the rolling moment.