DCA15IA089

The nose landing gear’s failure to fully extend due to missing and damaged fastening hardware on the drive arm assembly.

The flight crew had flown the incident airplane on the day before the incident and heard a sound, which the captain described as a “bang,” when extending the landing gear on approach. The first officer inspected the nose gear after landing and indicated that it looked like the data plate had come loose on the strut. The captain sent photographs of the nose landing gear to company maintenance control and then entered “placard strap bent” in the airplane logbook. He did not write up the “bang” sound because of his conversation with maintenance control. Maintenance personnel repaired the damaged placard that night but did not do any additional inspections. If the captain had been more descriptive in his writeup, additional maintenance would likely have been completed that may have found the mechanical failure in nose gear system. During the incident flight, the crew observed a GEAR DISAGREE caution message during their approach and conducted a go-around to troubleshoot the problem. The first officer, as pilot monitoring, began performing the GEAR DISAGREE checklist, however, he misstated several items in the checklist and did not perform the last item that directed him to perform another checklist. Although operational issues were identified during this investigation related to checklist usage and company procedures, they did not relate or contribute to the cause of the incident. The flight was subsequently cleared to land, and the airplane touched down without the nose gear extended, which resulted in minor damage to the forward fuselage. Nose Landing Gear Failure The opening and closing of the landing gear doors were accomplished through the driven link, lower link, and upper link assemblies, articulated by the drive link assembly attached to the drive arm assembly. Postincident examination of the nose landing gear revealed that two areas on the lower link assembly had damage from contacting the upper link assembly and the drive link assembly. The spherical bearing attached to the drive link assembly had fractured away from the lower link assembly. The drive arm assembly that attached to the lower lock link assembly using two D head pin/nut combinations had only one pin/nut still attached; the bolt and washer were heavily distorted. The portion of the hole in the lower lock link assembly that facilitated one of the two bolts/nuts to fasten the drive arm assembly was found sheared from the lug. Examination of the primed mating surfaces of the drive arm assembly and lower lock link assembly revealed missing paint adjacent to the bolt holes, indicating back-and-forth movement between the two components. Given that the end of the protruding threads on the sheared pin did not show any physical damage from the threads interacting with the nut, it is likely that one of the two nuts on the lower D head pins securing and providing anti-rotation between the drive arm assembly to the lower lock link assembly lost preload and backed off. With the nut missing from the D head pin, the drive arm assembly was allowed to have side-to-side rotational movement as well as a gap with the lower lock link assembly during the extension and retraction of the gear. Given the absence of the nut and the resulting gap between components, loads were transferred to the upper D head pin, which caused the aluminum lug to fail in tensile overload.  With the upper D head pin no longer attached, the entire load was transferred through the lower D head pin, and the subsequent loads applied resulted in fracture of the lower pin in shear. Because the drive arm was no longer tethered to the lock link, the drive arm was free to rotate, and, thus, the door links were able to go over center, farther than designed, allowing the drive rod to contact the lower link. When the nose landing gear was commanded to extend, the drive link could not pull the upper/lower link out of lock and, thus, as the nose landing gear continued to traverse, the rod end of the driven link contacted the lower link and subsequently failed in bending. As a result, the door linkage was no longer connected to the kinematics of the landing gear; the air loads on the forward doors held the upper and lower linkage in the over center condition, and the nose landing gear subsequently rested on the forward doors. Thus, the nose landing gear failed to fully extend when the lower link, upper link, and driven link assemblies articulated over center, mechanically immobilizing those components and the attached door rods, preventing them from fully extending. The over centering of the linkage occurred due to the overtravel of the drive link assembly, which was the result of missing and damaged fastening hardware on the drive arm assembly. After the incident, GoJet Airlines inspected its fleet of 47 CRJ 700 airplanes to ensure that the nose landing gear drive arm attaching hardware was intact. As a result of the detailed inspection, three airplanes were found with loose hardware, which was replaced in accordance with the applicable maintenance manual. In addition, Goodrich published a Service Bulletin that directed operators to remove the drive arm pin nuts, replace them with nuts with a new part number, and install them with Loctite at a higher torque value. Bombardier also published a Service Bulletin, which recommended operators accomplish the Goodrich Service Bulletin within 1,320 flight hours from the bulletin release date.

The airplane was equipped with a CVR and a flight data recorder (FDR). More information about the CVR and FDR can be found in the public docket for this accident. Postincident Actions On September 11, 2015, the nose landing gear manufacturer (Goodrich) published Service Bulletin 52200-32-88, which directed operators to remove the drive arm pin nuts, replace them with nuts with a new part number, and install them with Loctite at a higher torque value. The bulletin indicated that "[o]perators have reported loose drive arm pins PN [part number] 52621-1. Loose drive arm pin nuts could lead to damage of the drag brace jury strut attachment flange." On October 2, 2015, the airplane manufacturer (Bombardier) published Service Bulletin 670BA-32-055, which provided operators with instructions for Goodrich Service Bulletin 52200-32-88. The bulletin indicated that "[i]t is possible for the drive arm of the NLG [nose landing gear] drag brace to be disconnected from the lower lock-link assembly, if one or both nuts for the attachment pins come off. This can cause the NLG to not extend." Bombardier recommended that operators accomplish the service bulletin within no more than 1,320 flight hours from the bulletin release date, unless otherwise directed by the operator. In addition, GoJet Airlines initiated a campaign to inspect all CRJ 700 airplanes within its fleet to ensure that the nose landing gear drive arm attaching hardware was intact. The campaign included a detailed visual inspection to check for the presence of the two attaching pins, two flat washers, and two self-locking nuts. In addition, the inspection checked to ensure that threads were protruding from the self-locking nuts. Three of the 47 airplanes inspected were found with loose hardware, which was replaced in accordance with the applicable maintenance manual. Postincident examination of the nose landing gear revealed that two areas on the lower link assembly displayed damage from contacting the upper link assembly and the drive link assembly. The spherical bearing attached to the drive link assembly was found to have fractured away from the lower link assembly. The drive arm assembly that attached to the lower lock link assembly using two D pin/MS21042L4 nut combinations was recovered with only one pin/nut still attached; the bolt and washer were heavily distorted. The portion of the hole in the lower lock link assembly that facilitated one of the two bolts/nuts to fasten the drive arm assembly was found sheared from the lug. Examination of the primed mating surfaces of the drive arm assembly and lower lock link assembly revealed missing paint 180° from each other adjacent to the bolt holes, indicating back-and-forth movement between the two components. The incident airplane’s nose landing gear was a double wheel, forward retracting type with two spin down assemblies. The nose landing gear consisted of a twin wheel shock strut and a folding drag brace that incorporated a mechanical downlock/uplock. In normal operation, the nose landing gear drag brace assembly folded and positioned the shock strut relative to the nose of the fuselage in both extend and retract positions. It consisted of an upper drag brace, lower drag brace, over center lock link assembly (jury strut), and two tension springs. The upper drag brace assembly connected the lower drag brace assembly and the jury strut assembly to the airframe attachment point. The lower drag brace assembly connected the upper drag brace and the jury strut assembly to a lug integral to the forward side of the outer cylinder. The jury strut assembly, comprised of lower and upper lock link assemblies, connected the upper and lower drag brace assemblies to the airplane attachment point, over center springs, and the lock actuator. Dual acting springs assisted the lock link assembly arms and were designed to ensure over center and mechanical locking of the jury strut. The nose landing gear doors were driven by the nose landing gear and were open when the nose landing gear was down and closed when the gear was up. The nose landing gear doors opened and closed with landing gear motion through mechanical means. The forward door mechanism assembly was comprised of two door rods, a driven link assembly, upper and lower locks link assemblies, a drive link assembly, and a drive arm assembly. The door rods, each with rod ends and spherical bearings at each end, attached to lugs integral to their respective door assembly. Both door rods then connected to lugs integral to the driven link assembly, which had a clevis at the opposite end for pinning to a lug integral to gear bay structure. The driven link assembly was acted upon by an upper link assembly, which connected to a lower link assembly. The lower link assembly was pinned to the upper drag brace and acted upon by the drive link assembly. The drive link assembly was pinned to the drive arm assembly, which was rigidly attached to the lower lock link of the jury strut. When nose landing gear extension would begin, hydraulic pressure would be directed to the lock actuators, which would break the over center lock of the lock link assembly. The nose landing gear would be brought down and locked in the extended position by the retract actuator and two tension springs. The lock actuator and the tension springs completed a solid over center lock of the lock link assembly. To retract the nose landing gear, the hydraulic pressure would be directed to the lock actuator to break the downlock and to the retract actuator to raise the nose landing gear. The nose landing gear would retract into the wheel well and be locked into position by the two tension springs. Maintenance At the time of the accident, the GoJet Airlines CRJ 700 maintenance program was controlled by its Continuous Airworthiness Maintenance Program document, revision 41, dated December 19, 2014. The maintenance program consisted of line checks, work packages, and block checks, progressing from lighter to heavier maintenance requirements. The investigation revealed that all required maintenance had been performed. The nose landing gear and drag brace were due for overhaul in early 2016. On March 14, 2015, about 1521 central daylight time, GoJet Airlines, dba United Express, flight 3645, a Bombardier CL-600-2C10, N157GJ, landed with the nose landing gear not extended on runway 32R at Chicago O'Hare International Airport (ORD), Chicago, Illinois. None of the 44 passengers and crew on board were injured, and the airplane sustained minor damage. The flight was operating under the provisions of Title 14 Code of Federal Regulations Part 121 as a regularly scheduled passenger flight from Gerald R. Ford International Airport (GRR), Grand Rapids, Michigan, to ORD. On the previous day, the incident flight crew had flown the incident airplane to GRR and heard a sound when extending the landing gear on approach; all three landing gear extended normally. In a postincident interview, the captain described the sound as a "bang" that he thought was odd and that he had not heard before. The first officer indicated that, after landing, he inspected the nose gear, and it looked like the data plate had come loose on the strut. The captain stated that he sent photographs of the nose landing gear to company maintenance control personnel, who indicated that the problem appeared to be with the placard strap. The captain entered “placard strap bent” in the airplane logbook; he stated that he did not write up the “bang” sound but based his logbook entry on his conversation with maintenance control. Maintenance was performed on the airplane overnight to repair the damaged placard. The next day, another crew flew the incident airplane on two legs, from GRR to Denver and back to GRR. The first officer of the other crew stated that they heard a "slight noise" when extending the gear for landing, which that captain described as a “little click,