What happened
On July 2, 2017, a Bombardier CRJ 700, registration N796SK, operated by SkyWest Airlines, landed at Denver International Airport (DEN) after a scheduled flight from Aspen/Pitkin County Airport. During the landing roll, while the flight crew was stowing the thrust reversers, a "L ENG SRG OPEN" caution light appeared, indicating the left engine's operability bleed valve (OBV) had failed in the open position.
Approximately 75 seconds later, the crew received a "L ENG FIRE" warning. Although the crew discharged both left nacelle fire extinguishing bottles, the fire persisted. The crew subsequently shut down the left engine and stopped the aircraft on a taxiway. The cabin crew reported the smell of fuel but no smoke inside the cabin. Fire was observed inside the left inlet cowl, at the left pylon, and in a pool of fuel on the ground beneath the aircraft. All 59 occupants were not injured, though the aircraft sustained substantial damage. Airport rescue and firefighting personnel eventually extinguished the fire.
The investigation
Investigators determined that a Rosán -05 to -06 expander fitting had pulled out from the fuel supply port of the engine's OBV. Metallurgical examination of the OBV upper housing revealed advanced thread wear and fatigue damage. This damage was consistent with cyclic movement of the fitting, which eventually left insufficient threads to prevent the fitting from pulling out.
Testing revealed that two ECS support links were severely worn. Modal testing demonstrated that worn ECS links can shift the OBV's natural frequency into the engine's operating frequency range, significantly increasing loads on the fuel tube fittings. Furthermore, design evaluations showed that the specific -05 to -06 expander fitting lacked the necessary design margin to maintain preload against engine vibrations.
Findings
- The fire was caused by leaking fuel contacting hot engine surfaces within the engine core compartment.
- The nacelle drainage system was overwhelmed by a release of approximately 34 gallons of fuel during the landing roll.
- The fuel supply tube fitting pulled out due to advanced thread wear and fatigue.
- The failure was exacerbated by an undetected wear condition in the ECS support links.
- Maintenance instructions for the proper alignment of ECS link rod end bearings were missing from the base engine assembly drawing.
- There were no scheduled airplane-level or engine-level maintenance tasks required to inspect the condition of the ECS links.