RED Air Flight 203

NTSB narrative

The collapse of the left main landing gear during the landing roll resulted in a runway excursion due to a loss of controllability on the runway, during which the aircraft impacted a small equipment building, breaching the right-wing fuel tank and causing a post-crash fire. The performance of the crew was thoroughly evaluated during this investigation and found to be appropriate for the circumstances of the accident. The focus of this analysis is the cause of the left main landing gear collapse. Particularly (1) the left shimmy damper’s failure to adequately dampen vibration during landing and (2) the failure of the left main gear downlock mechanism due to excessive vibration. Figure 1 is a representation of how vibrations are typically dampened by a fully functioning shimmy damper. Figure 2 shows the rubber tire transfer markings that indicate the left main landing gear was shimmying down the runway.

Figure 1. The blue line is a representation of vibrations from a normal landing dampened by normally operating shimmy dampers when both main landing gear are down and locked. Note: Figure is not to scale nor depicting actual data.

Figure 2. Photos showing the left landing main landing gear tire transfer marks indicating shimming of the landing gear. Left Shimmy Damper Failure The left shimmy damper was examined and found to be assembled correctly. However, the check valve was found to be leaking and the cap for the check valve service port was missing. The damper requires sufficient hydraulic fluid to adequately dampen normal vibrations in the system. For a significant amount of fluid to leave the damper, there would have to be failure of both the check valve and the cap for the service port. If the leak in the check valve existed prior to the accident flight, there was potential for it to be discovered by maintenance when servicing the shimmy damper with hydraulic fluid. When a damper is serviced, documentation of that service must be made in the maintenance logs. The investigation’s review of maintenance logs revealed that the last time the left shimmy damper was serviced was in June of 2021, 12 months prior to the accident. Damage to check valves typically occur during servicing and any leaks would occur following a servicing event. Considering the last documented servicing of the shimmy damper occurred 1 year prior to the accident, it is likely that the check valve leak had silently developed and was sitting latent until a problem with the service port cap occurred. The shimmy damper was checked the day before the accident per the “S” check. There was no record of the damper being serviced during this “S” check suggesting that the service port cap was present and hydraulic fluid levels were above minimums. Therefore, the investigation deduced that the cap went missing or was compromised at some point in the 2 flights and 24 hours between the last “S” check and the accident touchdown. After the service port cap failed, sufficient fluid likely leaked from the damper through the leaking check valve and the uncovered service port in the hours prior to the accident, ultimately compromising the functionality of the shimmy damper. Unfortunately, the physical breach of the reservoir inflicted during the gear collapse resulted in the release of hydraulic fluid and precluded evaluation of whether the damper was properly serviced. This limited the investigation’s ability to definitively determine the root causes behind the apparent check valve and service port cap failure. The investigation explored potential contributions of operator organizational oversight and regulatory oversight of operator’s maintenance, however insufficient evidence existed to establish a direct link between any of these factors and the accident. Left Main Gear Downlock Failure Facing severe undampened axial vibration on rollout, the left main gear lower torque link (downlock) failed due to overload. When the downlock failed, the side braces were free to, and did, fold in the opposite direction from normal as the gear folded inboard and collapsed.

Verbatim NTSB analysis and probable cause from the NTSB dataset