What happened
On August 26, 2015, a Beechcraft King Air C90, registration OH-BEX, operated by Scanwings Oy, departed Helsinki-Vantaa Airport for a cargo flight to Örebro, Sweden. Shortly after takeoff from runway 22L, the crew heard an unusual cracking sound originating from the nose gear area. Upon attempting to retract the landing gear, the nose gear failed to lock in the up position, indicated by a warning light in the cockpit.
The crew attempted to extend the nose gear using the manual emergency method, but the gear remained unsecured. After requesting an altitude of 3,000 feet to manage the situation, the pilots determined that a landing with an unlocked nose gear was necessary. During the approach, air traffic controllers confirmed the nose gear was extended but not properly locked.
The aircraft landed on runway 15, with the crew attempting to keep the nose off the ground for as long as possible. Upon touchdown, the nose gear failed, and the aircraft's nose slid along the runway for approximately 500 meters before coming to a halt. The impact caused significant damage to the nose, nose gear doors, and propellers, though there were no injuries to the two-person crew.
The investigation
Investigators from the Finnish Safety Investigation Authority (OTKES) examined the aircraft at Helsinki-Vantaa and recovered a broken component from the nose gear actuator for detailed analysis by the NTSB in the United States. The investigation focused on the mechanical failure of the landing gear actuator, specifically the screw assembly and nut assembly.
Technical analysis revealed that the threads within the nut assembly had sheared off. Microscopic examination showed that the bronze threads had worn down to less than one-third of their original strength. The investigation also reviewed maintenance records, noting that the component had undergone approximately 900 landings, while the required inspection interval was every 1,000 landings or 30 months.
Findings
- The primary cause of the accident was the failure of the nose gear actuator due to advanced wear of the bronze threads, which eventually led to the threads shearing off during gear retraction.
- The rate of wear was exceptionally high, with the end play increasing from 0.003 inches to 0.015 inches over only 473 landings.
- Inconsistent lubrication likely accelerated the wear process, despite the presence of appropriate grease within the assembly.
- The existing maintenance interval of 1,000 landings was too infrequent to detect the rapid degradation of the threads.
- The aircraft's landing gear system lacked a fail-safe mechanism or a secondary method to lock the nose gear in the down position in the event of an actuator failure.