C7/2002L Incident due landing flap fault near Helsinki-Malmi airport on 22 August 2002

On Thursday 22 August 2002 at 19.58 Finnish time there was an aircraft incident near Helsinki-Malmi airport, in which a pilot of a Beechcraft 95-B55 aircraft on a private flight almost lost control of the aircraft during approach. This was due to an asymmetrical trailing edge flap condition. The Accident Investigation Board Finland initiated an investigation on 27 August 2002 (decision No C 7/2002 L). Aircraft accident investigator Esko Lähteenmäki was appointed as investigator-in-charge and investigator Juhani Hipeli as member of the commission.

There were six passengers on board; three men, two women and a 3-year-old child. The weather was excellent and visual meteorological conditions prevailed. The flight went normally. As the pilot started reducing speed for approach he took the landing gear down and selected the trailing edge flap lever to extended position. When the flaps had extended some distance the left flap actuator failed and the flap stopped. The right flap extended completely and the asymmetry caused a strong left rolling moment. In this condition the control of the aircraft was extremely difficult and hindered. The aircraft flew in a left side-slip banked 15-30 degrees to the left with full rudder and aileron deflections. The aircraft had a left rolling tendency and turning to right did not seem possible. Nonetheless, the pilot managed to approach the runway 27 of Helsinki-Malmi airport which was nearly in front of him by using the slowest possible speed and reduced power setting as well as descending flight path. The pilot achieved controllability for a successful landing on the short final when he reduced the power setting which he had used symmetrically. Thus the rolling tendency, caused by the asymmetrical flap condition and propeller slip-stream, was reduced.

When the failed left flap actuator was examined it was noted that the piston thread had been worn out and disintegrated. Right actuator piston thread had also been worn out almost completely. The strength of the thread had decreased as a result of the wear so much that the thread had already bent. The premature wear of the actuators and following disintegration of the left thread was caused by the complete lack of lubricating oil inside the actuator. The actuators had been installed in the aircraft on 10 July 1999. The aircraft had flown since only 235 hours, which is approximately 12 % of the overhaul period of the actuators, 2000 hours.

The cause of the incident was the failure of the left trailing edge flap actuator because of the lack of lubricating oil inside it. The failure occurred as the flaps were moving to extended position resulting in an asymmetric flap condition and causing a serious controllability problem.

The investigation commission presents no safety recommendations.

C7/2002L Report (pdf, 0.75 Mt)