MCDONNELL DOUGLAS HELI CO 369E near Kahuku, HI — 2009-11-20

Probable cause

The fatigue failure of the landing gear strut and maintenance personnel’s failure to perform an adequate inspection of the landing gear.

NTSB narrative

The helicopter was positioned on a landing platform. Four passengers had boarded the helicopter for a sightseeing tour. The pilot brought the engine up to flight idle, preparing for takeoff, when the left rear landing skid strut collapsed. The helicopter tilted to the left rear, and the tail skid and vertical stabilizer struck the ground and sustained damage. The tail rotor blades also struck the ground. The pilot shut down the engine and stopped the rotor using the rotor brake. A postaccident examination of the left rear landing gear strut revealed that the strut fractured at the bolt hole where the strut was attached to the drag brace. The struts had been inspected 13 days (30.2 flight hours) prior to the accident as part of a 100-hour inspection in accordance with the manufacturer’s maintenance inspection program. Examination of the broken strut (left rear) revealed that it fractured due to a fatigue crack that initiated from the circumference of one of the bolt holes used to secure the strut to the drag brace. The intact strut (right rear) was also examined and found to have fatigue cracks growing outwards from the circumference of the two bolt holes used to secure the strut to the drag brace. Additionally, the diameters of the bolt holes were larger than the maximum allowed as outlined in the maintenance manual. The right strut was installed on an exemplar airframe in order to determine if any play could be identified in the strut. The 100-hour landing gear inspection was performed and the strut exhibited play, which according to the manufacturer’s maintenance manual requires removal and additional inspections.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Inspection of landing gear strut prior to takeoff could have prevented failure.

NTSB coding

• Aircraft structural failure · Standing-engine(s) operating
• Maintenance

Evidence available

• Photos
• 10 docket documents

Docket documents10

• Helicopter Maintenance Information From the Operatorform
• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Statement of Party Representatives to NTSB Investigationinterview
• 100-Hour Inspection (Extract)other
• MD 369 Maintenance Manual- Strut Inspection & Installation (Extract)other
• MD Permission to use Maintenance Manual Extractother
• Strut Functional Exam Notesother
• Photosphotos
• Materials Laboratory 15 - Factual Reportreport
• Release of Aircraft Wreckage-Landing Gear Strutwreckage