Landing gear collapse shows how tiny defects and documentation gaps can undermine compliance
On 14 April 2023, a Northwestern Air Lease Ltd. British Aerospace Jetstream 3112 departed Fort Smith Airport (CYSM), Northwest Territories, on a day VFR training flight with only the captain and a trainee first officer on board. The program included a series of standard exercises, culminating in a VOR Runway 12 circling approach to Runway 30 with a simulated single‑engine landing.
Routine training, sudden gear collapse
Weather was good, both pilots were properly qualified, and the aircraft was within weight and balance limits. On final, the aircraft ballooned slightly before the captain corrected the flight path and the first officer completed the landing on Runway 30. During the rollout, the left main landing gear (MLG) unit collapsed, and the aircraft veered off the left side of the runway, striking a runway edge light before coming to rest on grass. There were no injuries, but the left MLG, engine, propeller, wingtip and belly cargo pod were substantially damaged.
Stress corrosion cracking below NDT thresholds
The Transportation Safety Board of Canada (TSB) traced the collapse to the spigot housing that attaches the left MLG to the wing. The forward end of this housing fractured into three pieces, with a critical crack at the 12 o’clock position on the forward face. Microscopic analysis found a small overstress anomaly and several minute corrosion pits that developed into intergranular cracking consistent with stress corrosion cracking (SCC) in the DTD 5094 aluminum.
SCC requires a susceptible material, tensile stress and a corrosive environment. According to the Board, “several cracks in the spigot housing of the left main landing gear unit grew, to the point where the structure of the main landing gear unit failed.” The alloy is known to be vulnerable, the housing carried service and residual stresses, and chemical analysis confirmed a corrosive environment at the crack sites.
All required eddy current inspections had been carried out in accordance with European Union Aviation Safety Agency Airworthiness Directive 2016‑0224 and associated service bulletins. However, the approved technique was developed to detect cracks at least 1.27 mm long, or corrosion pits of 1.27 mm by 0.13 mm. The defects that triggered failure were significantly smaller, which meant they could progress undetected until the remaining structure could no longer sustain landing loads. The TSB concluded that “the required eddy current inspection method… was unable to detect the defects… and contributed to the failure of the spigot housing.”
Coating practices, documentation and safety lessons
Investigators also examined surface protection and documentation. On the spigot housing’s forward face, they found randomly distributed wear marks and determined that the surface did not meet the roughness specification in the overhaul manual. A chromium‑based chemical conversion coating was present over most of the face but absent in a repaired area at the 6 o’clock position, contrary to a referenced service bulletin.
While that missing coating did not cause the 12 o’clock fracture, the TSB warned that “if repair instructions contained within a service bulletin are not explicitly followed, there is a risk that the repair will be ineffective and the respective part will fail prematurely.” It also noted that maintainers must consult multiple documents to piece together specific requirements and that “if service bulletins do not provide specific details… the issue identified in the SB may not be adequately addressed.”
In response, BAE Systems issued new guidance on corrosion protection and maintenance of the Jetstream 31 MLG forward pintle end face and revised key bulletins to clarify finish and re‑protection requirements—changes that speak directly to occupational health and safety concerns around aging landing gear systems.
