Baker City, OR, USA
N9KG
ROCKWELL 690B
The airplane sustained substantial damage following a right main landing gear collapse on its first landing after a 150-hour maintenance inspection. The collapse was due to a fracture and separation of the right main landing gear's housing boss from the inboard hydraulic cylinder clevis. The 150-hour inspection included the installation of a new clevis on the right main landing gear's inboard hydraulic actuator. A postaccident examination by a Federal Aviation Administration inspector revealed that the inboard hydraulic rod bearing attach bolt was screwed all the way in, which was contrary to instructions specified in the maintenance manual. While the manual states that the attach bolt should be backed off three complete turns in order to maintain proper rigging, the manufacturer reported that improper preload adjustment of the right main landing gear actuator cylinder could not create a large enough force to induce the failure of the right main landing gear outer body boss and clevis fitting. A metallurgical examination of the inboard hydraulic cylinder clevis and associated clevis pin housing boss showed that the housing boss had fractured from the main body of the strut and that the fracture was consistent with overstress. There was no evidence of wear or fatigue cracking on either the clevis or the housing boss. The postaccident examination of the airplane, the landing gear system, and its associated components failed to reveal the initiating event that subsequently resulted in the overstress failure and landing gear collapse.
On June 25, 2008, about 1600 Pacific daylight time, a Rockwell International 690B, N9KG, sustained substantial damage following a right main landing gear collapse during landing roll at the Baker City Municipal Airport (BKE), Baker City, Oregon. The commercial pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed for the personal cross-country flight, which was operated in accordance with 14 Code of Federal Regulations (CFR) Part 91, and an instrument flight rules (IFR) flight plan was filed. The flight departed the Portland-Hillsboro Airport (HIO), Portland, Oregon, about 1500, with its destination being BKE. In a written statement provided to the Safety Board investigator-in-charge (IIC), the pilot reported that the airplane had undergone a 150-hour inspection at the facilities of Aero Air, LLC, located at HIO, and that he was to return the airplane to BKE. The pilot stated that upon arriving at BKE he lowered the [landing] gear and verified that the gear was down and locked with three green lights. The pilot reported that he then lowered the first notch of flaps about midway on the downwind leg, added more flaps while turning onto base leg, and added full flaps on final approach. The pilot further reported that he confirmed the landing gear was down and locked with three green lights and then moved the condition levers to HI RPM. The pilot stated that after a normal landing, he brought the condition levers to ground (GND) idle, then over the gate to BETA. The pilot further stated that about midway down the runway he applied brakes slowly, which was followed by the collapse of the right main landing gear. A Federal Aviation Administration (FAA) airworthiness inspector assigned to the Boise Flight Standards District Office (FSDO), Boise, Idaho, examined the airplane. The inspector reported that the inboard housing boss had fractured from the main body of the right main landing gear strut. The inspector stated that the right main landing gear retract cylinder was disconnected from the upper gear casting, and that the rod end was fastened to the associated clevis in the proper manner. The inspector further noted that the rod end jam nut was loose and that no threads were exposed between the rod end and the jam nut. The inspector stated that when he inspected the auxiliary cylinder on the right main landing gear, there were 3 threads exposed between the rod end and the jam nut. The strut body, inboard hydraulic cylinder clevis and housing boss for the right main landing gear were sent to the Safety Board's Office of Research and Engineering, Materials Laboratory Division, Washington, D.C., for examination and analysis. A Senior Metallurgist reported that the results of his examination revealed that the housing boss had fractured from the main body of the strut, and that the fracture surface was consistent with overstress. There was no other evidence of wear or fatigue cracking on either the clevis or the housing boss. In a report submitted to the IIC by the Twin Commander Engineering Manager, the manager stated, "It's been suggested that inadequate maintenance was a significant factor contributing to the aircraft gear failure. It is Twin Commander Aircraft LLC's position that in this particular aircraft incident, inadequate maintenance could not be a contributing factor. Support of this is based on the reported aircraft landing conditions, main landing gear design, and the landing gear system operations." The Engineering Manager further stated "…the retraction and extension of the main landing gear is accomplished through hydraulic actuation of the main gear and emergency gear actuating cylinders. By design the landing gear actuating cylinders (main and emergency) react on the landing gear drag brace assembly, forcing the assembly hinge point over center during extension. This over center position provides positive down locking of the drag brace. Should hydraulic actuator pressure be lost to the main cylinder or if the main cylinder malfunctions, pneumatic pressure is available at the emergency actuating cylinder allowing gear deployment. Bungees, which are attached to the lower drag brace assembly and strut outer body also force action on the drag assembly, keeping the hinge point over center and in the locked position." The report continued by noting that the landing gear failure did not occur during the touch down, indicating that the drag brace hinge point was over center and positively locked at that time. Additionally, a failure occurring at the clevis attach point of the right main hydraulic actuator would not prevent the drag brace assembly from locking. The report concludes that inadequate maintenance could not be a contributing factor because the gear failure occurred during rollout and after landing. According to Section VI of the Gulfstream Commander (Twin Commander) 690A/690B Maintenance Manual, both landing gear actuating cylinders are to be adjusted and preloaded by removing the rod end bearing attachment bolt, then backing off the piston rod end bearing three turns and tightening the check nut. According to maintenance records provided by Aero Air, the following work was conducted on the accident airplane's right main landing gear during the 150-hour inspection: 1. Performed Landing Gear and Landing Gear Actuator Inspection per Major Guide III and IV. 2. C/W SL 376 Landing gear clevis and drag brace inspection. (Airworthiness issue.) Installed new clevises on inboard actuators and NDT inspected clevises on outboard actuators. Trunnion bores are within limits. Fluorescent penetrant inspections of trunnions and drag braces checked okay. Next SL376 inspection due in 500 hours. 3. Left and right main landing gear scissors center bushings worn. Replaced bushings with new p/n 111060 and 11061.
The landing gear collapse during the landing roll due to the fracture of the inboard hydraulic cylinder clevis pin housing boss for undetermined reasons.
Source: NTSB Aviation Accident Database
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