Bombardier Inc CL 600 2C10 N796SK accident near Denver, Colorado

Probable cause

The fuel supply tube fitting pulling out of the left engine’s operability bleed valve (OBV) during the landing rollout, allowing fuel to leak and contact hot engine cases, which ignited a fire that caused thermal damage to the engine pylon. Contributing to the fitting pullout from the OBV was an undetected progressive environmental control system (ECS) support link wear condition that allowed excessive OBV movement relative to the engine, and the lack of alignment instructions in the base engine assembly drawing and the lack of maintenance tasks to assess the operational condition of the ECS links.

NTSB analysis

After the scheduled air carrier flight touched down on the runway, a left engine undercowl fire occurred. The fire occurred after leaked fuel accumulating inside the engine core compartment (a fire zone) ignited from contact with a hot engine surface. The fuel accumulated inside the core compartment because the nacelle drainage system was overwhelmed by the release of about 34 gallons of fuel during the landing roll, at which time the high rates of ventilation through the nacelle were reduced.

The leak occurred during landing when the removal of fuel leakage depends in part on gravity drainage. A nacelle drainage capability assessment found that the core compartment contained about 20 gallons of fuel at the time of ignition. After engine shutdown, fuel draining from the core compartment was no longer carried overboard by the fan exhaust, and the fuel pooled in the fan duct. Some of this fuel drained to the ground through the transcowl drain holes. The fire became uncontrolled when it escaped the fire zone and spread to areas where expelled/drained fuel had accumulated in the nacelle and on the ground. The draining fuel led to a fire on the ground and on the underside of the left pylon. The fire was extinguished by airport rescue and firefighting personnel.

On-site investigation found that a rosan -05 to -06 expander fitting had pulled out from the fuel supply port of the operability bleed valve (OBV), which is part of the left engine’s environmental control system (ECS). Also, two ECS support links (which attach and support the ECS hardware, including the OBV, to the engine) were found severely worn.

Metallurgical examination of the OBV upper housing fuel supply port found evidence of advanced thread wear and fatigue damage, consistent with side-to-side fitting cyclic movement and insufficient remaining threads to prevent the fitting in the upper housing from pulling out. Modal testing demonstrated that ECS link wear can shift the OBV natural operating frequency into the engine operating frequency range, significantly increasing the fuel tube loads into the fitting. OBV design evaluation testing found that fitting joint preload loss permits relative motion to occur between the fitting and upper housing port threads, initiating upper housing thread wear and leading to upper housing thread failure and fitting pullout. The rosan -05 to 06 expander fitting with the cast aluminum housing lacked the design margin to maintain sufficient preload against engine vibration loads and fitting pullout.

A review of CF34-8 maintenance engineering sheets and field maintenance documents found that rod end alignment instructions critical to preventing ECS support link wear were not provided in the base engine assembly drawing and that the ECS support links were not part of any scheduled airplane- or engine-level maintenance. As a result, maintenance personnel did not inspect for wear caused by poor rod end alignment. Worn or misaligned ECS support links increase fuel tube fitting loads and can result in fitting pullouts.

Text from the NTSB. Open the NTSB docket for DCA17LA182 for the full record.