WPR12FA193

An uncommanded landing gear retraction after touchdown for reasons that could not be determined because postaccident examination did not reveal any mechanical malfunctions or failures that would have precluded normal operation.

The pilot-in-command (PIC) reported that, during the visual approach to the airport, he and the second-in-command (SIC) lowered the landing gear and verified that all cockpit indications showed that the landing gear were in the down-and-locked position. The airplane touched down smoothly on the main landing gear (MLG) and then touched down on the nose landing gear (NLG). The PIC had his hand on the thrust reversers when he and the SIC heard the landing gear unsafe horn; the left MLG then collapsed followed immediately by the collapse of the right MLG and the NLG. The airplane slid on its belly for about 1,500 ft before it came to rest on the runway. Cockpit voice recorder data confirmed that the unsafe landing gear configuration warning did not occur before touchdown as reported by the PIC. Postaccident examination revealed that, although the landing gear handle was in the down-and-locked position, all of the landing gear were fully retracted and engaged in their respective uplocks. Extensive testing of the airplane's hydraulic and electrical systems did not reveal any malfunctions that would have resulted in an uncommanded gear retraction. Some small, fine particulate matter was observed within the hydraulic manifold assembly; however, testing was unable to determine if these particles contributed to the uncommanded landing gear retraction.

Review of the cockpit voice recorder (CVR) revealed that the flight crew read through the landing checklists. The recording also captured the sound of the landing gear extending and the flight crew verifying "three green." After touchdown, a thump and another thump was heard followed immediately by the gear warning horn. The airplane rolled down the runway for a few seconds and the recording captured the landing gear collapse, followed by the sound of the airplane sliding. [Additional information can be found in the CVR group chairman's report located in the public docket.] The Mahlon Sweet Field Airport (EUG) is located about 7 miles northwest of Eugene, Oregon and has a field elevation of 374 feet. The airport has two runways, runway 16R/34L and 16L/34R both of which are grooved asphalt runways and in good condition. The accident occurred on runway 16R, which is 8,009 feet long and 150 feet wide; the pilot reported that the runway was dry at the time of the accident. Examination at Mahlon Sweet Field Airport in Eugene, Oregon Initial examination of the airplane revealed the left wing sustained scrape damage, which punctured the left wing fuel tank. In addition, a communications antenna was pushed through the pressure vessel. A cockpit inspection revealed the landing gear handle was in the down position, the anti-skid was on, and the flaps were fully extended. All of the circuit breakers were engaged with the exception of the CVR and AHRS AUX BATT CHG, which were intentionally pulled after the accident as directed by the National Transportation Safety Board investigator in charge. The landing gear handle was intentionally bumped numerous times to ensure that it could not be accidentally bumped into the retract position. Examination of the landing gears revealed that all were fully retracted and engaged in their respective uplocks. The right wing skin and gear assembly sustained black circular marks. The sidewall of the tire showed a circular abrasion around the perimeter of the sidewall, which also extended slightly around the perimeter of the rim. There were no black circular marks on the left wing skin, tire, and gear assembly; however, there were linear abrasions on the rim and tire sidewall. The nose landing gear doors were damaged and would not open by hand. When the landing gear was manually released from inside the cockpit all gears dropped, but did not lock in the down position. It was noted that the nose landing gear uplock was heard releasing prior to the wheel dropping. The CVR, enhanced ground proximity warning system (EGPWS), landing gear handle, gear control printed circuit board (PCB), gear monitoring PCB, and all three landing gear actuators were removed from the airplane for further examination. The airplane was repaired to accommodate a ferry flight to the Citation Service Center in Wichita, Kansas. Examination at Honeywell Aerospace in Redmond, Washington. Review of data obtained from the EGPWS did not reveal any warnings or abnormalities prior to touchdown. [Additional information can be found in the 'Component Examination – EGPWS' document located in the public docket.] Examination at the Citation Service Center in Wichita, Kansas Once back at the Citation Service Center, the landing gear handle was tested to acceptance test procedures (ATP) standards and passed. The front knob was removed followed by the face plate. The internal components were examined and one of the three screws securing the PCB was loose; however, no anomalies were noted that would have affected the outcome of the flight. The landing gear actuators were functionally tested to ATP standards and passed. All three landing gear actuators fully extended and retracted, and there were no indications of internal or external leaks. The unlock and extend pressures were tested, and were within specification requirements. The actuators were later taken to the manufacturer for disassembly, and no anomalies were noted. [Additional information can be found in the 'Component Exam - Nabtesco Aerospace Inc' document located in the public docket.] Additional testing was conducted on the airplane; the electrical wiring was visually inspected. All lines were continuous and remained intact with the exception of some electrical wires that appeared to be chewed by a rodent. Those wires were later determined to not be associated with the landing gear system. Some aircraft wiring and connectors were found installed in the tailcone in an area that might collect water from the outside elements. These connectors were filled with tap water, and no shorts were observed within the connectors. The landing gear control PCB and gear monitoring PCB were visually inspected with no anomalies noted. Both PCB's were tested to ATP standards and passed with the exception of a hard failure on the gear monitoring PCB. The hard failure was noted on a transistor that internally shorted to ground; however, this failure was not noted during prior testing. The hydraulic lines were visually inspected; all lines were continuous and remained intact. Pressure was added to the hydraulic system and no failures were observed. The cockpit warning lights and sounds were tested and functioned normally. The landing gear handle system was tested and responded normally. Instrumentation was added to the airplane to record system inputs, outputs, and responses during testing. The airplane's hydraulically operated equipment was activated in many normal and abnormal ways. The hydraulic system operated as designed, even when the airplane received simultaneous signals from different components. [Additional information can be found in the 'Airframe Exam – Wichita, Kansas' document located in the public docket.] The check valve was removed from the airplane and functioned normally when tested. The return system filter was removed, examined, and was clear of debris. The manifold assembly, which houses a relief valve and the control valves for all hydraulic actuated systems, was removed from the airplane and taken to the manufacturer for further examination. Examination at PneuDraulics in Rancho Cucamonga, California Upon arrival at PneuDraulics, the manifold assembly was tested to ATP standards; during which, the test bench return flow was restricted to obtain appropriate pressures. Initial results showed normal indications with the exception of an internal leakage at the flap control valve. Continued testing showed that the flap valve was slow to respond. When cycled various times with the extend solenoid energized, the valve would either respond slowly or move to the retract position, which is a reverse command. During one of the reverse commands, the valve was tapped with a rubber mallet and the valve slowly transitioned from the retract to extend position. The manifold relief valve was removed and debris was noted within the fluid drained. The relief valve was tested to ATP standards and it did not pass due to lower than acceptable cracking pressure, pressure drop at rated flow, reseat pressure, and reseat leakage. The pressure relief valve was dissembled; all fluid and debris was flushed. Unusual wear marks were noted on some of the internal components; however, these components do not contact other components within the system. No other damage was noted on the seals or other internal parts. A new relief valve, which tested to ATP standards, was installed onto the manifold and the flap control valve still showed a slow response. The flap control valve was removed from the manifold for further testing and examination. The flap control valve was tested to, and passed, ATP standards with the exception of two internal leakage requirements when solenoid A (extend solenoid) was energized. Both the extend and retract control solenoids were removed for further examination. When tested, the extend solenoid passed the ATP; however a failed pressure drop test revealed too m