BSAS 320 Module 7.2 Discussion
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Dec 6, 2023
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The SHELL model is a human factors framework used to analyze and understand the interactions
between humans and their environment. SHELL stands for software, hardware, environment, and
liveware. Software refers to the rules or physical processes of the operation. Hardware includes the
equipment, tools, and technology used in the system. Environment is the external factors and conditions
that influence the system. Finally, liveware represents the humans who operate within the system
(Skybrary, n.d.).
For this discussion, the SHELL model will be applied to American Airlines Flight 191, a DC-10 aircraft,
which crashed shortly after takeoff from Chicago O'Hare International Airport on May 25, 1979. During
takeoff from the runway its left engine detached from the wing, hydraulic fluid poured out of the
severed lines, and electrical power to cockpit warning systems was cut off. The pilots could not see what
happened and assumed an engine failure had occurred. They decreased airspeed from 165 to 153 knots
according to standard procedure for engine failure during takeoff. As a result, hydraulic system #1 was
the only mechanism that locked the left-wing slats in an extended position and the loss of hydraulic fluid
caused the leading-edge slats on the left wing to retract. With leading-edge slats retracted, the left wing
could not generate lift at the reduced speed, and the aircraft entered an uncontrollable roll to the left.
Then the aircraft plummeted into an open field, killing all 271 people on board. Investigations revealed
that during scheduled maintenance the left engine was removed and installed using procedures not
recommended by the manufacturer to cut maintenance time by 200 hours. This shortcut caused extra
stress and fractures on the pylon that holds the engine to the aircraft resulting in the departure of the
engine from the wing (Dark Records, 2021). When applying the SHELL model, the software for this
incident is the improper maintenance procedures performed recently that caused fatigue cracking and
fractures on the engine pylon. Hardware was the engine and pylon assembly that was torn loose from
the airplane and the hydraulic line that was damaged during engine separation. The environment there
were no external factors like weather only the internal environment for the company performing the
maintenance elected to save time and money versus following the manufacturer's directions. Liveware
addresses the crew's training, experience, and familiarity with the DC-10 but for this incident that is
most likely not a factor. However, the maintenance procedures by the maintenance personnel that did
not follow the guidance and regulations and used a forklift to hold the engine up during pylon and
engine removal that caused the fracture that led to the engine separation can be cited as a liveware
hazard.
The strengths of SHELL are its ability to emphasize human performance and how it is influenced by
interactions with hardware, software, and the environment. It allows for a comprehensive analysis of
factors outside the individual's control, such as equipment design and external conditions. The
weaknesses of SHELL are that Identifying the interactions and interdependencies between software,
hardware, environment, and liveware can be challenging. HFACS and SHELL offer individual approaches
to understanding aviation accidents from a human factor perspective. HFACS is more focused on the
sequence of human actions, while SHELL takes a broader approach, considering the entire enterprise like
the maintenance facility cutting corners to save money. I believe the choice between the two models
depends on the specific needs of an investigation and the complexity of the accident.
References
Dark Records. (2021, October 4).
The American Airlines Flight 191 (Disaster Documentary)
[Video].
Youtube.
https://www.youtube.com/watch?v=gEAl-ryvo2U
Skybrary. (n.d.). ICAO SHELL Model.
https://skybrary.aero/articles/icao-shell-model
Benjamin,
I agree with your evaluation that the SHELL model offers a different perspective and can be more
dynamic and adaptable in certain situations compared to the HFACS model. The SHELL model tends to
have a more holistic approach. It looks at more aspects of the details. However, I would not stop using
HFACS as it has some key benefits especially when a linear approach is needed. I would use a blended
system if I were attempting to identify the cause in a safety analysis. The choice between them would be
guided by the nature of the incident or accident under investigation and the desired level of detail and
perspective required.
Aramayis,
Your analysis of the SHELL model's application to the accident scenario involving Wagner's leader is
thorough and well-structured. I wish there was more information available to apply the SHELL model to
this scenario. Both the HFACS and SHELL models offer valuable perspectives for aviation safety analysis,
with their own sets of strengths and weaknesses. However, I refuse to believe that this particular
scenario was caused by any human factor outside of a Russian missile impact or detonated explosive
explicitly based on the political nature of this flight and its passengers. It will be interesting to see if the
International Civil Aviation Organization will investigate it and what their findings will be.
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