N796SK

HISTORY OF FLIGHT On July 2, 2017, about 1319 mountain daylight time, a SkyWest Airlines CL-600-2C10 (CRJ 700) airplane, N796SK, experienced a left engine undercowl fire after landing at Denver International Airport, Denver, Colorado (DEN). None of the 59 occupants aboard the airplane were injured, and the airplane sustained substantial damage. The airplane was operating under Title 14 Code of Federal Regulations Part 121 as a scheduled flight from Aspen/Pitkin County Airport, Aspen, Colorado, to DEN.

After the airplane touched down and while the thrust reversers were being stowed, the flight crew observed a “L ENG SRG OPEN” caution (indicating that the left engine’s operability bleed valve [OBV] failed while open) as the airplane exited the runway. The flight crew reduced engine power to idle and continued to taxi. The flight crew observed a “L ENG FIRE” warning (indicating a fire in the left engine) 1 minute 15 seconds later. The flight crew discharged both left nacelle fire extinguishing bottles, but the fire did not extinguish. The flight crew stopped the airplane on the taxiway and shut down the left engine. The cabin crew reported the odor but not the presence of smoke in the cabin and observed fire inside the left inlet cowl, at the left pylon, and in a pool of fuel on the ground beneath the airplane. The passengers evacuated the airplane through the main cabin door. The fire was extinguished by airport rescue and firefighting personnel.

AIRCRAFT INFORMATIONEach of the airplane’s General Electric (GE) CF34-8C5B1 turbofan engines was equipped with a Parker Aerospace OBV, part number 4123T71P04, which is a fuel-actuated, electrically controlled butterfly valve that increases an engine’s operability margin by unloading the highpressure compressor during engine starts and at higher power settings. The valve opens to bleed stage 10 compressor air into the fan bypass air stream. Three Rosán-type expander fittings are installed in the OBV upper housing (housing cap). (Rosán was the trade name for this two-piece, ring-lock-type fluid fitting, which is now simply referred to as a rosan fitting.) The three fittings are for the supply, return, and drain ports. Fuel from the engine highpressure pump is routed to the supply port and returned to the pump inlet through the return port. Residual fuel is drained through the drain port. The supply and return fittings, which are part of a high-pressure valve actuation circuit, have rosan expanders identified as 05 to -06.

The engine core is enclosed by the forward core cowls (core compartment). The OBV, which is part of each engine’s environmental control system (ECS), extends away from the engine core to mate with an air exhaust port in the forward upper core cowl. The assembly is stabilized by four adjustable support link rod assemblies (ECS support links) that are installed between the 10th stage duct and bleed air manifold and the engine combustion chamber frame flanges.

The engine core compartment is a designated fire zone because CF34-8C engine case temperatures are capable of igniting fuel. High rates of ventilation through the core compartment cool the engine and remove flammable fluids and gases to prevent ignition due to contact with hot engine surfaces. Core ventilation rates decrease with engine power reduction. Fluids that are not removed with the ventilation airflow collect at the bottom of the core compartment and drain into the fan bypass duct (fan duct) through three holes in the lower core cowl that are 0.25 inch in diameter. Drained fluids are removed from the fan duct with the fan discharge airstream exiting the nacelle. With the engine shut down, fuel draining from the core compartment is no longer removed by fan exhaust; this fuel accumulates in the fan duct and is routed overboard. The forward lower core cowl includes a pressure relief door that was designed to open to relieve excess pressure from the core compartment/fire zone.

Review of the operator’s maintenance records found that the left and right engines and the left and right OBVs were original to the airplane and had not been removed. Both engines and OBVs had accumulated 22,419 total hours and 13,157 total flight cycles.

Meteorological Information and Flight Plan

Conditions at Accident Site:

Unknown

Condition of Light:

Day

Observation Facility, Elevation:

Distance from Accident Site:

Observation Time:

Direction from Accident Site:

Lowest Cloud Condition:

Visibility

Lowest Ceiling:

Visibility (RVR):

Wind Speed/Gusts:

/

Turbulence Type Forecast/Actual:

/

Wind Direction:

Turbulence Severity Forecast/Actual:

/

Altimeter Setting:

Temperature/Dew Point:

Precipitation and Obscuration:

Departure Point:

Aspen, CO (KASE)

Type of Flight Plan Filed:

IFR

AIRPORT INFORMATIONEach of the airplane’s General Electric (GE) CF34-8C5B1 turbofan engines was equipped with a Parker Aerospace OBV, part number 4123T71P04, which is a fuel-actuated, electrically controlled butterfly valve that increases an engine’s operability margin by unloading the highpressure compressor during engine starts and at higher power settings. The valve opens to bleed stage 10 compressor air into the fan bypass air stream. Three Rosán-type expander fittings are installed in the OBV upper housing (housing cap). (Rosán was the trade name for this two-piece, ring-lock-type fluid fitting, which is now simply referred to as a rosan fitting.) The three fittings are for the supply, return, and drain ports. Fuel from the engine highpressure pump is routed to the supply port and returned to the pump inlet through the return port. Residual fuel is drained through the drain port. The supply and return fittings, which are part of a high-pressure valve actuation circuit, have rosan expanders identified as 05 to -06.

The engine core is enclosed by the forward core cowls (core compartment). The OBV, which is part of each engine’s environmental control system (ECS), extends away from the engine core to mate with an air exhaust port in the forward upper core cowl. The assembly is stabilized by four adjustable support link rod assemblies (ECS support links) that are installed between the 10th stage duct and bleed air manifold and the engine combustion chamber frame flanges.

The engine core compartment is a designated fire zone because CF34-8C engine case temperatures are capable of igniting fuel. High rates of ventilation through the core compartment cool the engine and remove flammable fluids and gases to prevent ignition due to contact with hot engine surfaces. Core ventilation rates decrease with engine power reduction. Fluids that are not removed with the ventilation airflow collect at the bottom of the core compartment and drain into the fan bypass duct (fan duct) through three holes in the lower core cowl that are 0.25 inch in diameter. Drained fluids are removed from the fan duct with the fan discharge airstream exiting the nacelle. With the engine shut down, fuel draining from the core compartment is no longer removed by fan exhaust; this fuel accumulates in the fan duct and is routed overboard. The forward lower core cowl includes a pressure relief door that was designed to open to relieve excess pressure from the core compartment/fire zone.

Review of the operator’s maintenance records found that the left and right engines and the left and right OBVs were original to the airplane and had not been removed. Both engines and OBVs had accumulated 22,419 total hours and 13,157 total flight cycles.

Meteorological Information and Flight Plan

Conditions at Accident Site:

Unknown

Condition of Light:

Day

Observation Facility, Elevation:

Distance from Accident Site:

Observation Time:

Direction from Accident Site:

Lowest Cloud Condition:

Visibility

Lowest Ceiling:

Visibility (RVR):

Wind Speed/Gusts:

/

Turbulence Type Forecast/Actual:

/

Wind Direction:

Turbulence Severity Forecast/Actual:

/

Altimeter Setting:

Temperature/Dew Point:

Precipitation and Obscuration:

Departure Point:

Aspen, CO (KASE)

Type of Flight Plan Filed:

IFR

WRECKAGE AND IMPACT INFORMATIONEnvironmental Control System Link Alignment

Bombardier Shorts stated that ECS link rod end bearings must be properly aligned and securely clamped between the fork bushings to prevent lateral movement and premature wear. Bombardier Shorts found that the CF34-8 engine maintenance engineering sheets did not include rod end alignment procedures required to prevent lateral movement and premature wear and that the CRJ700/900/1000 Aircraft Maintenance Manual did not include any controls for ECS link condition. GE confirmed that instructions to align the ECS links during installation were missing from the base engine assembly drawing and that the ECS links were not part of any scheduled airplane-level maintenance.

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In August 2017 and January 2018, GE published Service Bulletin (SB) 8C 75-0019 and SB 8E 75-0016, respectively. These SBs required a one-time inspection of the operability bleed valve (OBV) supply, return, and drain fuel fittings and ECS link rod assemblies and replacement of OBVs or related hardware that fail inspection. In November 2017, the FAA issued emergency Airworthiness Directive (AD) 2017-23-06 to mandate compliance with 8C 75-0019, which was then superseded in November 2019 by AD 2019-22-05, to require a more detailed inspection and mandate repetitive inspections of the OBV fuel tubes, ECS link rod assemblies, and OBV fuel fittings. In January 2020, a 16,000 flight hour ECS link special inspection was made an 8C Engine Manual Chapter 5 Table 803 Airworthiness Limitation. In February 2020, an improved-design P/N 4123T71P06 (P06) OBV was released to service by SB 8E 75-0020 and SB 8C 75-0026. In December 2021, the FAA published a notice of proposed rulemaking (NPRM) to supersede AD 2019-22-05 that would apply to both CF34-8C and CF34-8E engines and includes initial and repetitive inspections of OBV fuel tubes, ECS link rod assemblies, an...