On April 15, 2008, about 1350 eastern daylight time, a Cessna 560XL, N613QS, was substantially damaged during an uncommanded gear retraction while landing at the Westchester County Airport (HPN), White Plains, New York. The two certificated airline transport pilots and two passengers were not injured. Visual meteorological conditions prevailed and an instrument flight rules flight plan had been filed for the flight that departed Palm Beach International Airport (PBI), West Palm Beach, Florida, about 1115. The fractional ownership flight was conducted under Title 14 Code of Federal Regulations Part 91, Subpart K. The flight crew reported that the flight from PBI was uneventful and the airplane was on a normal visual approach to runway 34, a 6,548-foot-long, 150-foot-wide, asphalt runway at HPN. The landing gear was extended and both crewmembers confirmed that indications in the cockpit were that the landing gear was in the down and locked position. After a normal touchdown, the copilot deployed the speed brakes, and the captain deployed the thrust reversers. At that time, the landing gear warning horn was heard, and the left main landing gear collapsed, which was followed by the collapse nose gear, and the right main landing gear. An approximate 960-foot-long ground scar was observed on the runway between taxiways "H" and "G." The airplane slid to a stop on the runway, near taxiway "C," and all occupants exited the airplane from the right over-wing emergency exit. It was noted that because the airplane was resting on its fuselage, the main cabin door could not be opened. The airplane was not, nor was it required to be equipped with a "crash-axe." According to a Cessna representative, crash axes were not installed as standard equipment on any production airplanes; however, one may be installed as optional equipment based on customer or regulatory needs. Initial examination of the airplane was conducted under the supervision of a Federal Aviation Administration (FAA) inspector. The left wing sustained scraping damage, which resulted in a fuel leak. In addition, two ribs, and a stringer in the left wing were damaged and a communications antenna was pushed through the pressure vessel. A cockpit inspection revealed that the landing gear handle was in the down position. The airplane was raised off the ground using straps and both the right main and nose gear were found to be fully retracted and engaged in their respective uplocks. The left main landing gear was also fully retracted; however, the left landing gear uplock was damaged. The landing gear was manually extended, pneumatically blown down, and locked before the airplane was towed off the runway to the ramp. In addition, inspection of the hydraulic reservoir indicated that the hydraulic fluid level was within the normal range. The airplane was equipped with a cockpit voice recorder (CVR) and an enhanced ground proximity warning system (EGPWS), which were removed for download. Data was also obtained from an Altair Aircraft Data Acquisition System (ADAS). All three landing gear actuators and the landing gear hydraulic loading valve were removed for further examination. In addition, the airplane was retained for further examination, pending a ferry flight to the Cessna Service Center, Wichita, Kansas. Subsequent postaccident tests and examinations were conducted on several components and subcomponents of the airplane's hydraulic and electrical systems. The testing was performed under the supervision of an FAA inspector and/or NTSB investigator, with participation from the parties to the investigation. None of the testing revealed any malfunctions that would have resulted in an uncommanded gear retraction. It was noted that the landing gear extend hydraulic line located in the left wing between the landing gear control valve and the left main landing gear actuator was compromised consistent with an overpressure burst. The line was also common to the nose and right main landing gear actuators. According to Cessna, the design working pressure for the hydraulic line was 1,500 pounds per square inch (psi) and the proof pressure was 2,250 psi. In addition, if the gear control valve had been commanding extend, as would have been expected, the entire volume of hydraulic fluid would have been expelled through the compromised hydraulic line. Review of the CVR did not reveal any unsafe landing gear configuration warnings prior to touchdown. Both crewmembers indicated that the landing gear were down and locked during completion of the before landing checklist. Approximately 3 seconds after touchdown, the copilot indicated that the speedbrakes were deployed. About that time, a sound similar to the landing gear warning horn began. [Additional information can be found in the CVR Group Chairman's Factual Report located in the public docket.] Review of data obtained from the Altair ADAS system indicated that the landing gear was down and locked from about 5 minutes before touchdown, until about 3 seconds after touchdown. The airplane touched down at an airspeed of approximately 108 knots, and the "weight-on-wheels" (WOW) signal went from air to ground. Approximately 11 seconds later, the WOW signal switched back from ground to air. Review of data obtained from the EGPWS did not reveal any warnings or abnormalities prior to touchdown. Airport surveillance cameras captured images of the airplane from just prior, to just after the touchdown. The video footage depicted the gear in an extended position; however, it could not be determined if the gear was in the fully down and locked position. Review of the Cessna 560XL Maintenance Manual revealed that the landing gear control valve was a four-way valve, operated by a three-position solenoid (extend, off, and retract), by two electrical signals (extend and retract). With electrical power removed, the landing gear control valve was spring loaded to the center (off) position. As designed, voltage would not be applied to the retract and extend inputs simultaneously. During post accident testing, it was noted that if voltage was applied to the retract and extend inputs simultaneously, the landing gear control valve would move to, and remain in the center (off) position. Review of wiring schematics revealed that the landing gear and speedbrake systems were independent; however, wiring from both systems were located on five sets of common connectors. Examination of the wires and connector pins within wiring bundles common to the speedbrake and landing gear system did not reveal any evidence of electrical faults. The airplane, serial number 560-5599, was manufactured in 2005, and was maintained under an approved aircraft inspection program. At the time of the accident, the airplane had been operated for approximately 2,496 hours and had accumulated 1,932 cycles since new. The captain held an airline transport pilot certificate with ratings that included airplane single and multiengine land, and instrument airplane. At the time of the accident, he had accumulated about 8,660 hours of total flight experience, which included 1,651 hours in the same make and model as the accident airplane. His most recent FAA first-class-medical certificate was issued on March 27, 2008. The copilot held an airline transport pilot certificate with ratings that included airplane single and multiengine land, and instrument airplane. At the time of the accident, he had accumulated about 23,000 hours of total flight experience, which included 43 hours in the same make and model as the accident airplane. His most recent FAA first-class-medical certificate was issued on November 27, 2007. A weather observation taken at HPN, at 1356, reported: winds from 290 degrees at 11 knots, visibility 10 statute miles, clear skies, temperature 13 degrees Celsius (C), dew point -9 degrees C, altimeter 30.14 inches of mercury. On September 26, 2005, a Citation 560XL, N699QS, was substantially damaged when the nose landing gear collapsed while taxiing for takeoff, at the Port Columbus International Airport, Columbus, Ohio [NTSB Accident Number CHI05LA277]. The subsequent investigation was not able to determine the cause of the gear collapse. It was noted that the landing gear extend hydraulic line was compromised consistent with overpressure, and that the hydraulic fluid reservoir was not emptied.