CEN14LA127

The pilot's decision to reengage the hydraulic system following the successful extension of the landing gear using the emergency extension procedures. Contributing to the accident was the failure of the directional control valve in the gear-up position and the release of the gear down locks for undetermined reasons.

The landing gear did not extend when the copilot selected gear-down during the initial instrument approach to the destination airport. The flight crew was able to extend the landing gear using the emergency extension procedure; however, following the emergency extension, the pilot decided to reengage the hydraulic system to have power-assisted braking and nosewheel steering during landing. The pilot stated that the wheel brakes were less effective than normal and the nosewheel steering was inoperative during the landing roll, which he concluded was because the emergency landing gear selector valve handle was still extended. The pilot reported that in an attempt to resolve the braking and steering issues he reached for the emergency landing gear selector valve, but the landing gear collapsed before his hand touched the valve handle. Postaccident testing determined the directional control valve that controls the flow of hydraulic fluid to the landing gear actuators was stuck in the gear-up position. Examination of the directional control valve revealed a metallic particle trapped between one of the spool lands and the valve housing, which prevented the spool from moving into the gear-down position. The source of the trapped particle could not be conclusively determined. A teardown examination did not reveal any mechanical anomalies that would have resulted in a malfunction of the hydraulic pump package. The airplane flight manual specified that the hydraulic system be turned off before the landing gear was manually extended using the emergency procedures. The flight manual did not include a provision for the hydraulic system to be reengaged following the emergency landing gear extension. As such, the hydraulic system should have remained off during landing. Postaccident testing concluded that the landing gear would immediately retract if the emergency landing gear selector valve was in the stowed position with the hydraulic system turned on. Consequently, because the airplane landed with its gear extended, the emergency landing gear selector valve handle had to be in the extended position upon touchdown. One scenario for the unintended landing gear retraction was if the pilot repositioned the emergency landing gear selector valve to its normal (retracted) position during the landing roll. This would have allowed the unintended pressure in the normal hydraulic system lines to retract the landing gear. Although this scenario is consistent with the reported sequence-of-events, the pilot stated that he did not touch the emergency landing gear selector valve handle during the landing roll. Additionally, the emergency valve handle was found extended after the accident. Another possible scenario was if the pressure in the emergency extension line decreased below that necessary to hold the gear down locks in position. Postaccident testing completed on an exemplar airplane with a simulated failure of the directional control valve and the hydraulic system turned on, concluded that the down locks could release and allow the landing gear to retract if the line pressure in the emergency system decreased sufficiently. After considering the test data and the variables involved, the investigation was unable to conclusively determine which scenario had triggered the down locks to release during landing roll. However, the landing gear would not have retracted had the pilot not reengaged the hydraulic system following the successful extension of the landing gear using the emergency extension procedures. The hydraulic system being turned on, in combination with the failure of the directional control valve in the gear-up position, resulted in unintended pressure within the hydraulic lines associated with gear retraction and created the means for the gear to retract when the gear locks released for undetermined reasons.

The airplane was equipped with an L-3/Fairchild model FA2100-1020 CVR, serial number 851267. The CVR recording contained about 2 hours 4 minutes of digital audio, which was stored in solid-state memory modules. The CVR was not damaged during the accident and the audio information was extracted from the recorder normally. The recording consisted of four channels of audio information, ranging from good to excellent quality. The first 5 minutes of the recording were from the previous flight. The accident flight recording began about 1119 when the airplane, operated as Foothills 70, received its instrument clearance from MSN clearance delivery. The flight departed MSN about 1156 and climbed to flight level 270. About 1222, the flight received automatic terminal information service (ATIS) information at SPI. After receiving the ATIS information, the flight crew briefed the ILS approach to runway 4 at SPI. By 1239, the flight began receiving air traffic control (ATC) radar vectors toward the final approach course for ILS runway 4 at SPI. At 1243, the airplane had descended to 2,300 ft msl. At 1247, the flight was cleared for the first ILS runway 4 approach to SPI. A full transcript was created for the flight from 1249:57 until 1313:39 as the passengers and flight crew members evacuated the airplane and electrical power was removed from the CVR. The transcript of the CVR audio information is included with the docket materials associated with the investigation. The airplane was not equipped with a flight data recorder, nor was it required to be so equipped. Abraham Lincoln Capital Airport (SPI), a public airport located about 3 miles northwest of Springfield, Illinois, was owned and operated by the Springfield Airport Authority. The airport field elevation was 598 ft msl. The airport was served by three runways, runway 4/22 (8,001 ft by 150 ft), runway 13/31 (7,400 ft by 150 ft), and runway 18/36 (5,300 ft by 150 ft). The airport was equipped with an air traffic control tower that was operational at the time of the accident. A follow-up examination was completed by Federal Aviation Administration (FAA) maintenance inspectors with assistance from the airplane manufacturer, airplane operator, and maintenance personnel. The airplane was placed on jackstands to test the landing gear extension/retraction system. An electrical continuity test confirmed the correct operation of the landing gear selector handle (up/down) to the solenoid-operated directional control valve. Before testing continued, the landing gear selector handle was confirmed to be in the gear-down position, the emergency landing gear selector valve handle was confirmed to be extended, and the electric master switch was turned on. When the hydraulic system was momentarily powered on, the nose and right main landing gear immediately showed an unsafe condition and the right main landing gear retracted slightly. Hydraulic power was immediately turned off and the emergency landing gear extension valve handle was pushed down to return the system to a normal configuration. Although the landing gear selector handle remained in the gear-down position and the emergency landing gear extension valve was stowed, all three landing gear retracted when the hydraulic system was turned back on. The emergency extension procedures were then used to return the landing gear to a down-and-locked position. The electrical connection to the solenoid-operated directional control valve was removed to ensure the solenoid remained deenergized (gear down). The landing gear still retracted when hydraulic power was turned back on, despite the landing gear selector handle in the gear-down position, the solenoid deenergized, and the emergency landing gear extension valve stowed. Based on the testing, it was determined that the solenoid-operated directional control valve was stuck in the gear-up position. No mechanical anomalies were identified with the emergency landing gear extension valve during several pull-force tests. The hydraulic pump package, which included the solenoid-operated directional control valve, and the emergency landing gear extension valve were removed from the airplane for additional testing. The hydraulic pump package, solenoid-operated directional control valve, and emergency landing gear selector valve underwent x-ray computed tomography (CT) scanning to document their internal conditions. The results of the CT scanning revealed a single particle located between one of the spool lands and the directional control valve housing. The position of the trapped particle prevented the spool from returning to the default (deenergized) position for normal landing gear extension. Further review established that trapped particle and jammed spool would allow hydraulic flow to the landing gear retraction lines. Additional CT scanning did not identify any anomalies with the remaining components of the hydraulic pump package or the emergency landing gear selector valve. The solenoid-operated directional control valve was examined at the manufacturer to extract the trapped particle. Disassembly of the directional control valve confirmed the spool was jammed and required the spool to be forced out by hand. There was a minor impact mark on the edge of the spool where it had been in contact with the trapped particle. Metallurgical examinations of the trapped particle indicated that it measured about 2.9 mm long, 0.98 mm wide, and 0.5 mm thick. According to the directional control valve manufacturer, the maximum aperture opening in any spool configuration is 0.470 mm. Energy dispersive x-ray spectroscopy determined the composition of the trapped particle was about 94% iron, 3% nickel, 2% chromium, and 0.5% manganese (consistent with 9300 or 3300 series steel). A review of materials used in the directional control valve did not match the material composition of the trapped particle. Trace amounts of aluminum, magnesium, and silicon were also identified during testing, which were attributed to incidental contact with an aluminum component within the directional control valve. A review of hydraulic system schematics indicated that the fluid returning to the hydraulic pump package flowed through the directional control valve before reaching the pump filter. Additionally, according to the hydraulic pump package manufacturer, based on spool orientation and the clearances within the directional control valve, it was possible for debris to migrate, due to pressure pulsations and vibrations, into the central chamber of the spool when the actuator was at the end of its travel and the pump was in a zero-flow condition. A teardown examination of the hydraulic pump package did not reveal any mechanical anomalies that would have resulted in a malfunction of the pump. However, during removal of the actuator sleeve the stainless-steel threaded insert pulled-out with the threaded section of the actuator sleeve. A visual examination established that the threaded insert had jumped a thread and was deformed. Additionally, there was damage to the actuator sleeve thread and corresponding female thread in the valve plate subassembly. The actuator sleeve was a machined steel component. Although the actuator sleeve had a similar material composition to the trapped particle that was recovered from the directional control valve, a further review did not provide a positive match between the particle and the materials used in the hydraulic pump package. The airplane manufacturer performed testing on an exemplar airplane to determine if they could duplicate an uncommanded landing gear retraction with a simulated failure of the directional control valve in the gear-up position. Without the hydraulic system on, the landing gear was extended using the emergency landing gear extension procedure. The test results indicted the emergency line pressure was 1,800-2,700 psi after receiving a gear safe indication and significant force was required to actu