History of Flight
On July 2, 2017, at about 1319 mountain daylight time, a SkyWest Airlines CL-600-2C10 (CRJ700) airplane, registration N796SK, experienced a left engine undercowl fire after landing at Denver International Airport (DEN). The airplane was operating as a scheduled passenger flight under 14 CFR Part 121 from Aspen/Pitkin County Airport (ASE) to DEN. None of the 59 occupants were injured, but the airplane sustained substantial damage.
After touchdown, while the thrust reversers were being stowed, the flight crew received a "L ENG SRG OPEN" caution, indicating that the left engine's operability bleed valve (OBV) had failed open. The crew reduced engine power to idle and continued taxiing. About 1 minute 15 seconds later, a "L ENG FIRE" warning illuminated. The crew discharged both left nacelle fire extinguishing bottles, but the fire did not extinguish. They stopped the airplane on a taxiway and shut down the left engine. The cabin crew reported an odor but no visible smoke in the cabin; they observed fire inside the left inlet cowl, at the left pylon, and in a pool of fuel on the ground beneath the airplane. Passengers evacuated via the main cabin door, and airport rescue and firefighting personnel extinguished the fire.
Aircraft Information
The airplane was equipped with two General Electric CF34-8C5B1 turbofan engines. Each engine had a Parker Aerospace OBV (part number 4123T71P04), a fuel-actuated, electrically controlled butterfly valve that bleeds stage 10 compressor air into the fan bypass airstream. The OBV upper housing contained three Rosán-type expander fittings for supply, return, and drain ports. The supply and return fittings were part of a high-pressure valve actuation circuit and used rosan expanders identified as -05 to -06.
The engine core was enclosed by forward core cowls (core compartment). The OBV extended from the core to mate with an air exhaust port in the forward upper core cowl, stabilized by four adjustable support link rod assemblies (ECS support links). The core compartment was a designated fire zone due to high engine case temperatures capable of igniting fuel. Ventilation rates decreased with power reduction. Fluids not removed by ventilation collected at the bottom and drained into the fan bypass duct through three 0.25-inch holes. With the engine shut down, drained fuel accumulated in the fan duct and was routed overboard. The forward lower core cowl included a pressure relief door.
Maintenance records showed that the left and right engines and OBVs were original to the airplane, with 22,419 total hours and 13,157 total flight cycles.
Additional Findings
Bombardier Shorts stated that ECS link rod end bearings must be properly aligned and clamped to prevent lateral movement and premature wear. They found that the CF34-8 engine maintenance engineering sheets lacked rod end alignment procedures, and the CRJ700/900/1000 Aircraft Maintenance Manual had no controls for ECS link condition. GE confirmed that alignment instructions were missing from the base engine assembly drawing and that ECS links were not part of any scheduled airplane-level maintenance.
GE tested worn ECS links from a January 2017 fitting pullout event (similar wear to the accident links). Installation of worn links shifted the OBV natural frequency into the engine operating frequency range. GE also performed rotary flex and torque tension tests on the rosan expander fitting joint, determining that the -05 to -06 expander fitting was susceptible to wear at engine vibration levels, while a -06 to -06 standard fitting had substantially more resistance.
In response, GE released a new-design OBV (part number 4123T71P05) with larger -06 to -06 standard fittings in December 2018. Further tests showed that increasing installation torque could eliminate port thread wear, prompting an upgrade to forged aluminum upper housing and more durable fittings (part number 4123T71P06) introduced in February 2020. These changes resulted in a 21% average fitting preload increase.
GE’s nacelle drainage capability assessment for the accident scenario estimated that about 34 gallons of fuel were released into the core compartment over 2 minutes 6 seconds at 16.2 gallons per minute, with fuel contacting a hot surface and igniting about 1 minute 15 seconds after the fitting failure. After engine shutdown, fuel pooled in the fan duct and some drained to the ground.
Subsequent airworthiness actions included FAA emergency AD 2017-23-06 (November 2017), superseded by AD 2019-22-05, which mandated inspections of OBV fuel tubes, ECS link rod assemblies, and OBV fittings. In January 2020, a 16,000 flight hour ECS link inspection became an airworthiness limitation. In December 2021, the FAA proposed a new AD requiring initial and repetitive inspections and replacement of certain OBVs with the improved P06 design.