ICAO Aviation/Aircraft Maintenance SHELL Model

Different industrial sectors can benefit from the SHELL Model

The ICAO SHELL Model, described in the International Civil Aviation Organization (ICAO) Safety Management Manual, is a conceptual tool used to analyze the interaction of multiple system components. It was created in the early 1970s as a result of the intensive work of a group of scientists in the aircraft maintenance discipline. It is a concept often referred to in the effective and safe management of aircraft maintenance. Developed by the Civil Aviation Organization, this model has been adopted, developed and used extensively by military aviation/maintenance organizations.

The SHELL Model is strictly a systems engineering approach, but the sciences of psychology and sociology were heavily utilized in its preparation.

In fact, it can be applied not only in aircraft maintenance but also in many other industrial sector groups.

Concept (the name is derived from the initials of its components), 

Anything that cannot be handled by hand, software-software, 

Software - rules, procedures, written documents, etc. that are part of standard operating procedures.

Everything that is held by hand (Hardware - hardware), 

Equipment - Aircraft, aircraft systems, aircraft ground support equipment, materials, tools, hardware, etc. 

Environment,

Environment - the situation in which the L-H-S system must operate, the social and economic climate and the natural environment.

Human as an individual-liveware1),

Human1, human as an individual shows different behaviors.

As a group, human-liveware2,

Liveware1- people as a group - other aircraft maintenance specialists in the aircraft maintenance system, flight crews, engineers and maintenance personnel, management and administrative people, etc.

A practical diagram to illustrate this conceptual model uses blocks to represent the different components of Human Factors. This building block diagram is intended to be a basic aid to understanding the key factors. The SHELL model is a model that serves the main concept of HFIM (Human Factors in Maintenance). 

According to the SHELL Model, the mismatch between Liveware and the other four components contributes to human error. Therefore, these interactions must be evaluated and considered in all sectors of the aviation system.

Human (Liveware)

The critical focus of the model is the human participant or living being, i.e. aircraft maintenance personnel, who are the most critical and at the same time the most resilient component in the system. The edges of this block are not simple and straight, and so the other components of the system need to be carefully fitted to them if stress and eventual failure in the system are to be avoided.

However, of all the dimensions in the model, the least predictable are the internal (hunger, fatigue, motivation, etc.) and external (temperature, light, noise, workload, etc.) changes.

Human-Human (Liveware-Liveware) Interaction 

(Interface between humans and other humans)

I will never forget many years ago, after a Fenerbahçe match in Kadıköy, a group of Fenerbahçe fans burned down a gas station in Kadıköy. The people who carried out this attack were caught, their professions surprised Turkey, they were lawyers, doctors, engineers, etc., highly educated and career people, people who could not individually commit such a lawlessness, such a mistake. However, when they were involved in fan group fanaticism, they behaved differently.

Human Error is often seen as the negative consequence of the live (aircraft maintenance personnel) dimension in this interactive system. Sometimes there is no point in trying to remove errors from human performance to address and prevent error, they are independent of training or in other words humans are error prone, so in risky situations the human should be removed from the decision making process and replaced by computer controlled devices. The 4th and later generation jet fighters pay attention to this and the safety modules of the systems are designed in this context. Civil air transportation aircraft of the modern era also take important safety measures in this context.

This is the interface between people (the interface of relationships between individual-individual, individual-group, group-group). At this interface we are interested in leadership, cooperation, teamwork and personality interactions. For example, the concept and model of Crew Resources Management (CRM) in the aircraft maintenance system emerged and developed in the context of the SHELL Model.

Human-Software

(Interface between humans and software)

Software is a collective term that refers to all laws, rules, regulations, orders, standard operating procedures, customs and contracts, and the normal way things are done. Increasingly, software also refers to computer-based programs developed to run automated systems.

To achieve safe and effective operation between live software and software, it is important to ensure that the software can be implemented, especially if it relates to rules and procedures. It is also important to beware of error-prone, confusing or overly complex wording. Simple, clear and as concise as possible software will help to avoid human misperceptions.

Human-Hardware

(Interface between humans and hardware)

Another interactive component of the SHELL model is the interface between the living (aircraft maintenance personnel) and the hardware (every piece of hardware in the system, especially the aircraft). This interface is the one most often considered when talking about human-machine systems: Seat design to suit the seating characteristics of the human body, displays to suit the sensory and information processing characteristics of the user, controls that move appropriately, coding and location (i.e. Human Factors Engineering-Ergonomics) are all seriously and thoroughly scrutinized.

Hardware, e.g. troubleshooting, refers to the physical features of the control environment, especially those related to workstations. For example, the engine emergency silence button/switch is a hardware component that interfaces with the live, i.e. aircraft maintainer. When this button is pressed, a human interacts with the hardware and software and immediately disables the engine or engines. Similarly, the tens/hundreds of switches on aircraft and ground support equipment should be located in locations that are easily accessible to the relevant aircraft maintenance specialists in a variety of situations, and manipulation of the equipment should not interfere with the reading of displayed information or other devices that may need to be used at the same time.

Human - Environment

(Interface between people and the environment)

Live (Aircraft Maintenance Personnel) - environment interface refers to interactions that may be outside the direct control of Humans, i.e. the physical environment - temperature, weather, etc., but in which the aircraft operates. Much of the human factors development in this area has been concerned with designing ways in which people or equipment can be protected, developing protective systems for lights, noise and radiation. The proper matching of living things - environmental interactions involves a wide range of different disciplines, from environmental studies, physiology, psychology to physics and engineering.

Conclusion:

The SHELL Model is a practical, effective and useful model that puts the human (aircraft maintenance personnel) at the center of the interactive interaction between human, software, hardware, working environment and other people.