Bombardier CL 600 2C10 N796SK

Summary

On 2 July 2017 at about 19:19 local time, a Bombardier CL 600 2C10 registered N796SK, operated by Skywest Airlines Inc, was involved in an accident near Denver, Colorado, United States. 59 people were on board and nobody was injured. The aircraft was substantially damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

2 July 2017 at 19:19

Classification

Accident

Location

Denver, Colorado, United States

Nearest airport

Denver International Airport (KDEN)

Coordinates

39.8617, -104.6731

Aircraft

Bombardier CL 600 2C10

Registration

N796SK

Category

Airplane

Year built

2010

Engines

2

Operator

Skywest Airlines Inc

Operating rule

Part 121: Air Carrier

Phase of flight

Not recorded

Route

Aspen → Denver

Aircraft damage

Substantial

Weather

Unk

Light

Day

NTSB number

DCA17LA182

Nobody was injured.

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.  

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full NTSB narrative

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.  

Quoted verbatim from the NTSB record.