Unknown Failure Mode Cause of Failure?

By Paula Hollywood

Industry Trends

Was an unknown failure mode the root cause of the Southwest Airlines engine failure of April 17. 2018?  Shortly following the accident, Col. Stephen T. Ganyard, USMC (Ret) and ABC News contributor for military and aviation issues, asked in this piece, was it a maintenance issue with Southwest, a design failure at the manufacturing level, or is it a new failure mode that comes with age?  According to NTSB investigators, early indications are metal fatigue may have played a role, but final determination is months away.

It is not the purpose of this blog to draw any conclusions, but rather to reinforce the criticality of that last part of Col. Ganyard’s question regarding failure mode.  In fact, I recently wrote about the importance of knowing asset failure modes in this Industrial IoT/Industry 4.0 Viewpoints blog.  What is fair to say is that all parties involved the design, operation and maintenance of the engine will be involved in the investigation and root cause analysis of this uncontained engine failure. 

Commercial Aviation Is Highly Reliable

As the birthplace of reliability centered maintenance (RCM), the aviation industry is extremely reliable, so much so that the flying public has come to take airline reliability for granted because the risk of an accident is minuscule.  But there is risk and we expressly accept it each and every time we board an aircraft.  As horrific as it was, it was the first US airline fatality in nine years.  However, this one is similar to a 2016 incident that landed safely and that has the attention of the FAA.  As of this writing, the agency has stated it will issue an airworthiness directive within the next two weeks requiring fan blades of engines known as CFM56-7B to be reviewed when they reach a certain number of takeoffs and landings.

Commercial aviation only makes money when planes are flying.  Way back in 1960, the industry recognized the way to keep planes in the air was to improve equipment reliability while containing the cost of maintenance giving birth to reliability centered maintenance (RCM).  RCM is an analysis to determine appropriate actions to ensure that a physical asset continues to operate as expected in its present operating context.  Although RCM has evolved over time, it is still practiced by the aviation industry as well as migrated to many others.  The Nowlan and Heap research at United Airlines at the time concluded that age-related asset failures account for only about 20 percent of all asset failures.  This research has stood the test of time and has been confirmed by other studies.  The ability of the industry to better understand failure lead to a reduction in the number and consequences of commercial aviation accidents.  Other factors in age-related failures include length of service, stress fatigue, erosion/corrosion, and wear.  In this case, the high frequency, short-flight business model employed by Southwest may have contributed to additional engine stress and accelerated potential fan failure. 

RCM 7 Questions

In the 1980’s, John Moubray translated commercial aviation RCM into a universal technique, RCM2 applicable to all industries.  Moubray distilled it down to seven basic questions that are the essence of RCM. 



What are the functions and associated performance standards of the asset in its present operating context?

Identifies asset functions and performance standards

In what ways does it fail to fulfill its functions?

Defines asset unfitness of asset to perform function to expected and acceptable performance standards

What causes each functional failure?

All events within reason likely to cause each failed state

What happens when each failure occurs?

Description of what happens when asset fails

In what way does each failure matter?

Assessment of failure consequences

What can be done to predict or prevent each failure?

Determines proactive tasks to predict or prevent failure

What should be done if a suitable proactive task cannot be found?

Identifies default actions in absence of known proactive tasks

Seven Basic RCM Questions

In this as in every civil aviation accident, the NTSB will be looking at operations, structure, powerplants, systems, air traffic control, weather, human performance, and survival factors to determine the probable cause.  If the results of the investigation indicate a previously unknown failure mode as the root cause, no doubt investigators will be asking these very same questions as it works to ensure the high reliability of the commercial aviation industry. 

RCM in Manufacturing

In the manufacturing industry, knowing how assets fail is critical to developing effective maintenance strategies.  RCM provides a structured approach to detecting, eliminating, and reducing the frequency of as well as the consequences of each failure mode with a mitigating task.  It also addresses what should be done in the absence of a previously identified mitigating task.  The focus of RCM is the maintenance activities that have the greatest effect on plant performance.  Benefits include improved asset available and reliability, reduced operating and maintenance costs, and great safety.  A better understanding of how plant equipment works is priceless. 

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