L2015-02 Nose landing gear malfunction in aircraft OH-BEX at Helsinki-Vantaa airport on 26 August 2015

An accident involving a Beechcraft King Air C90 aircraft occurred at Helsinki-Vantaa airport on 26 August 2015. The accident was caused by damage in the nose landing gear actuator screw assembly, and the lack of an alternative procedure in case of screw assembly damage was a contributing factor. The aircraft was significantly damaged in the accident. There were no injuries to persons.

The aircraft departed for a cargo flight from Helsinki-Vantaa airport to Örebro, Sweden, on 26 August 2015. It had a crew of two pilots and transported medical substances. There were no other persons on board. When departing for the flight, the aircraft was airworthy in accordance with the applicable airworthiness requirements, the pilots were appropriately trained, and their licences and qualifications were valid.

When the pilots selected gear up after take-off, they heard an abnormal crushing sound from the nose landing gear. The indicator light showed that the gear had not retracted normally. The pilots took action as instructed on the emergency checklist, but the indicator light showing that the nose landing gear was down and locked was not lit.

The pilots decided to return to the departure airport. During landing, the aircraft nose skidded on the ground for about 500 meters. The nose, landing gear doors and propellers were damaged. The airport rescue services were on standby near the runway.

The aircraft nose landing gear actuator sustained damage and became inoperative when the nut threads of the screw assembly were cut off as the gear was retracted. The failure was caused by extensive wear of the bronze threads, which finally could no longer sustain the force transmitted. In accordance with the maintenance requirements, an end play check and lubrication must be performed at an interval of 1000 landings or 30 months. In this case, about 900 landings had been recorded for the actuator before it was damaged.

Technical investigations revealed that the nut threads of the nose landing gear actuator screw assembly were significantly worn, even to less than one third of the original thickness. Typical wear of the screw assembly was accelerated by the fact that the method of lubrication used does not guarantee that the thread surfaces are evenly lubricated over time.

Due to the landing gear operating principle, the screw assembly forms a critical weakest link in the system. The alternative procedure for nose landing gear extension is only designed for situations where power is lost or the electric motor fails. Therefore the manual handle for emergency extension cannot be used to lock the gear down for landing if the screw assembly threads are cut off.

The Safety Investigation Authority, Finland, recommends that the Federal Aviation Administration (FAA) of the United States require the aircraft manufacturer to review the maintenance requirement so that the end play check interval is significantly shortened and the thread wear rate is taken into account.

The recommendation is issued to improve safety, to prevent similar accidents from recurring in the future, and to minimize any damage caused by such accidents.

L2015-02 final report (pdf, 1.32 Mt) Appendix 1. Accimap graph (pdf, 0.01 Mt)