JACKSON, WY, USA
N27518
Piper PA-31
During a normal landing, the aircraft's nose gear collapsed. The pilots stated that three green indicator lights were observed during the final landing checklist. In postaccident troubleshooting, an instrument panel mounting screw which was too long was discovered. This screw was obstructing the movement of the landing gear selector handle to a full down position, allowing it to return prematurely to neutral (which removes hydraulic pressure from the gear system.) The nose gear was thus not hydraulically moved over center and was unlocked, even though the nose gear indicated down and locked in this configuration. Per the aircraft service manual, the nose gear linkage incorporates a downlock spring, and both gear linkage overcenter travel and gear down limit switch position are adjustable. It was not determined whether, or when, these items were checked or adjusted before the accident. After the proper screw was installed in the instrument panel, the gear operated properly in retraction tests.
On July 2, 1996, approximately 0730 mountain daylight time, a Piper PA-31, N27518, registered to Satellite Aero of Jackson, Wyoming and operating with the call sign of SXX1826, was substantially damaged when its nose landing gear collapsed during landing at Jackson Hole Airport in Jackson. Neither the airline transport pilot-in-command nor a pilot-rated passenger were injured. The flight operated under 14 CFR 135 as a United Parcel Service feeder cargo flight and had originated at Salt Lake City International Airport, Salt Lake City, Utah. Visual meteorological conditions prevailed and the flight was on a visual flight rules flight plan. The pilot submitted a written statement to the NTSB investigator in which he stated that he observed three green lights during the final landing checklist. He also stated that at least two individuals on the ground, whom he named in his statement, observed the gear in the down position. The passenger, in a written statement, stated that the airplane was at 100 knots over the runway 36 threshold fence which he stated "is perfect for that airplane" and that the main gear touched down "just past the numbers." The pilot stated: "The final approach was a stabilized landing with the indicated airspeed approx[imately] 90 [knots] in the flare." Both occupants stated that at main gear touchdown, a warning horn sounded which they thought was the stall warning horn, and that the nose then contacted the runway and the airplane slid to a stop. Winds at the time of the event were reported as calm. The load manifest indicated that the airplane was loaded with 1,030 pounds of cargo and was within its maximum gross weight limit at the time of the accident. In a letter to the FAA inspector assigned to the accident, dated February 22, 1997, Jackson Hole Aviation's director of maintenance reported on the findings of post-accident troubleshooting of the landing gear system. The director of maintenance reported that during post-accident troubleshooting, an instrument panel mount screw which was too long was discovered. The time at which the improper screw was installed was not determined. According to the director of maintenance, the improper screw obstructed the landing gear handle from moving to a full down position, permitting the gear handle to return to the neutral position with "very little hydraulic pressure" (the three-position gear selector switch is designed to return hydraulically to the center "neutral" or "off" position upon completion of gear extension or retraction). The director of maintenance stated that with the gear handle in neutral, the gear system would not pressurize and the nose gear drag brace was not hydraulically over-centered; the nose gear was thus not locked down. The director of maintenance reported that the nose gear position indicator on the accident aircraft displayed a green light with the landing gear in this configuration (however, the nose gear down limit switch position is adjustable, and should be adjusted to illuminate the green light with the nose gear in a proper down and locked position, according to the aircraft service manual). The director of maintenance reported to the FAA that after the improper screw was replaced with the proper part, a number of gear retraction tests were completed on the accident aircraft without any problems. In examination of nose landing gear assembly diagrams in the PA-31 service manual, it was noted that the nose landing gear assembly design incorporates a downlock spring which exerts tension on the linkage to move it in a down-and-locked direction. The PA-31 service manual also contains checking and adjustment procedures to ensure proper overcenter travel of the nose gear linkage. It was not determined whether these procedures had been performed on the accident aircraft at any time prior to the accident. However, 14 CFR 43, Appendix D, requires landing gear system linkages, trusses, and members to be inspected for undue or excessive wear, fatigue, and distortion, and that gear retracting and locking mechanisms be checked for improper operation, on annual and 100-hour inspections. According to copies of the aircraft logs supplied by the operator, the aircraft received an annual/100-hour inspection on May 4, 1996. The operator reported that the aircraft had flown 93.1 hours since the last inspection.
An improper screw installation by an unknown individual into the instrument panel, which restricted movement of the gear handle resulting in premature disengagement of the normal landing gear extension system. A factor was one or more improperly adjusted nose gear system components, which resulted in an unlocked nose gear that falsely indicated down and locked.
Source: NTSB Aviation Accident Database
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