Role of Human Factors in Preventing Aviation Accidents: An Insight

Abstract

Flight is one of the safest modes of travel even today. However, nearly 75 percent of civil and military aviation accidents around the globe have been attributed to human errors at various levels such as design, drawing, manufacturing, assembly, maintenance, and flight operations. This paper traces the civil aviation accidents that have occurred during the last eight decades and brings out the vital factors leading to the disaster by considering a few representative cases. The concept of human factors is introduced, and the various models that have been in use to understand the root causes leading to aviation accidents are presented. An example of the application of human factors analysis and classification system (HFACS) framework is narrated. It is found that majority of recent civil aviation accidents have occurred during the landing and approach phases, and it is possible to minimize the accidents by suitably maintaining situational awareness. Considering the growth of air traffic that is expected to double in the next 10–15 years, the role of human factors in preventing aviation accidents is even more relevant. A new model for human factors is proposed. Way forward to even safer skies is presented.

1. Introduction

Man has been fascinated by the flight of birds since the time immemorial. The science of flight from dream to reality was realized by Wright brothers in the year 1903. It was the first instance in the history of aviation that a powered, sustained, and controlled flight of airplane under the control of the pilot was achieved. One can see the tremendous progress made by man in the field of aviation in a short span of about 120 years; from small two-seat ab-initio trainer aircraft such as Cessna 152, to the large wide-body aircraft such as the Airbus A380 and the Boeing 777x, capable of carrying hundreds of passengers at transonic speeds, and Concorde, the first supersonic passenger-carrying commercial airplane.

Aircraft design and production encompass diverse areas such as aerodynamics, flight mechanics and controls, material science, power plant, landing gear and hydraulics, electrical and avionics, stress analysis, vibration, manufacturing techniques, special processes, nondestructive testing, metrology, quality control, assembly, integration, static testing, flight testing, certification aspects, and so on. A set of drawings that provide unambiguous and complete information to manufacture the aircraft is the starting point. It is through these drawings that the ideas of the designer are conveyed to the manufacturer. Manufacturing drawings are further translated into a set of “process sheets” by specialists in methods engineering, and these provide a step-by-step procedure in a lucid manner to the shop floor technicians to execute the work. Likewise, airplane flight manual and quick reference handbooks are made available to the flight test crew. Aircraft maintenance manuals contain clear information about periodic procedures to be adhered to, while carrying out aircraft maintenance activities.

To err is human. It is quite possible that human errors, in some form or the other, can creep into design, manufacturing, flight operation, and maintenance phases in the aviation sector. The error may go unnoticed due to various reasons and can result in catastrophic accidents endangering precious lives of passengers and crew. Nearly 75 percent of civil and military aviation accidents around the globe have been attributed to human errors at various levels. It is, therefore, pertinent to go through the aircraft accident databases available in open literature and bring out the vital factors leading to the disaster.

Aircraft designs have evolved over a period of time, as a result of the lessons learnt from the past accidents and incidents. This is also true when we consider corresponding improvements to airport infrastructure and maintenance facilities. It was during the early 1970s that the discipline of human factors began to draw the attention of international aviation community. Pioneering work has since then been done in the field of human factors to understand why aircraft accidents happen. Human factor models have also undergone continual improvements and adaptations to suit the ever-growing needs of air traffic.

Aviation accident database available in open literature has been used in the current work. Human factor analysis models currently in use have been studied. A new model for human factors analysis is proposed, keeping in view the likely improvements in aircraft designs and potential growth in air traffic in the years to come.

This book chapter is organized as follows. To begin with, summary of civil aviation accidents that have occurred during the last eight decades is presented. This is followed by a few representative case studies to bring out the root cause of the accident. The concept of human factors is introduced, along with brief description of various models in use, to understand the root causes leading to aviation accidents. An example of the application of human factors analysis and classification system (HFACS) framework is narrated. A new “seven-segment” concept is proposed to systematically analyze human factors. Way forward to even safer skies is presented.

2. Civil aviation accidents during the last eight decades

According to Federal Aviation Administration (FAA), an aircraft accident is defined as an occurrence associated with the operation of an aircraft, which takes place between the time any person boards the aircraft with the intention of flight and all such persons have disembarked, and in which any person suffers death or serious injury, or in which the aircraft receives substantial damage [1]. Incident refers to an occurrence other than an accident, associated with the operation of an aircraft, which affects or could affect the safety of operation; occurrence is an abnormal event, other than an incident or accident. Until an event (for example, low-speed abort) can be identified as an incident, it is regarded as an occurrence. An aviation accident is defined by the Convention on International Civil Aviation Annex 13 as an occurrence associated with the operation of an aircraft, which takes place from the time any person boards the aircraft with the intention of flight until all such persons have disembarked, and in which a) a person is fatally or seriously injured, b) the aircraft sustains significant damage or structural failure, or c) the aircraft is missing or is completely inaccessible [2].

The first fatal accident involving a powered aircraft was the crash of a Wright aircraft at Fort Myer, Virginia, in the United States on September 17, 1908, injuring its co-inventor and pilot, Orville Wright, and killing the passenger, Signal Corps Lieutenant Thomas Selfridge [3]. Orville later determined that the crash was caused by a stress crack in the propeller. The Wrights soon redesigned the Flyer to eliminate the flaws that led to this accident.

Table 1 summarizes the number of fatal civil airliner accidents from 1945 through February 2022 [4]. At 864, United States is the country with the highest number of fatal civil airliner accidents, followed by Russia, Canada, Brazil, and Colombia. At 43, Argentina has the least fatalities.

Sl. No.	Country/Region	Number of fatal Accidents
1	USA	864
2	Russia	539
3	Canada	191
4	Brazil	190
5	Colombia	184
6	UK	110
8	Indonesia	106
7	France	105
9	Mexico	102
10	India	95
11	China	76
12	Venezuela	69
13	Italy	68
14	D. R. Congo	66
15	Ukraine	64
16	Bolivia	62
18	Germany	62
17	Peru	61
19	Philippines	61
20	Spain	56
21	Australia	51
22	Atlantic Ocean	51
23	P. N. Guinea	49
24	Kazakhstan	45
25	Argentina	43

Table 1.

Countries/regions with the highest number of fatal civil airliner accidents since 1945.

Figure 1 shows the distribution pattern of the number of civil aircraft accidents from the year 1918 through 2022 [5]. Figure 2 shows the distribution pattern of the number of fatalities for the same period. A total of 28,442 aircraft accidents have resulted in 1,58,798 fatalities. Maximum peak is observed during 1940s, and there is a gradual decrease in the number of accidents from the year 1978. Considering the period between 2001 and 2022, a total of 3769 aircraft accidents have resulted in 20,172 fatalities. Fitting a linear trend line for the data between 2001 and 2022 would indicate a theoretical possibility of aircraft accidents tending to near-zero by mid 2040s.

Figure 1.

Distribution pattern of civil aircraft accidents from 1918 through 2022.

Figure 2.

Distribution pattern of civil aircraft accident fatalities from 1918 through 2022.

Apart from potential fatalities, aircraft accidents may result in partial damages to the airframe, or even complete loss of hull, making the aircraft non-airworthy. A recent example is the fatal accident of a domestic passenger aircraft Boeing 737-89P on March 21st, 2022 in China with 123 passengers and nine crew members onboard [5]. The aircraft, whose file photo is shown in Figure 3, was scheduled to fly from Kunming to Guangzhou, but plunged midflight and crashed in Wuzhou, in the Guangxi region. All the 132 persons onboard the aircraft were killed in the accident, and the aircraft fully destroyed by the impact. Accident investigation is on. Black boxes of this aircraft are found in a severely damaged condition, making data retrieval a challenge by itself. According to media reports, a preliminary assessment by the US officials has ruled out any mechanical or technical faults with the aircraft, and the aircraft is suspected to have been intentionally put into a nose-dive [6].

Figure 3.

File photo of Boeing 737-89P aircraft B-1791. (photo credits: CWong, https://www.jetphotos.com/photo/10528109).

On May 29, 2022, a Twin Otter aircraft operated by Tara Air was on a scheduled domestic flight from Pokhara Airport to Jomsom Airport, Nepal. The aircraft lost contact with air traffic controllers about 12 minutes after take-off and crashed in mountainous Mustang district of Nepal, killing all the 19 passengers and three crew members. Black box of the aircraft has been retrieved and accident investigation is ongoing.

In addition to fatalities, as many as 81 aircraft are reported to have gone missing and remain untraced till date [7]. According to a report released by Boeing [8], in the early days of flight, approximately 80 percent of accidents were caused by the machine and 20 percent were caused by human error. The trend has since then reversed, and now approximately 80 percent of the airplane accidents are caused due to human error (pilots, air traffic controllers, mechanics, etc.) and the remaining due to equipment failures. According to an analysis of 75 fatal airplane accidents carried out [9], over 70 percent of the accidents involved pilot factors mostly related to poor judgment and decision-making.

3. Case studies

Typical life cycle of an aircraft can broadly be classified into three stages. The first stage comprises activities covering design and manufacturing, signaling its “birth.” Second stage comprises “active flying years” during which the aircraft is put into operational service. The aircraft is maintained in fully airworthy condition by following approved Maintenance, Repair, and Overhaul (MRO) procedures. Once the aircraft completes its intended service life, it is eventually grounded or phased out and sent to boneyards. This corresponds to the third stage.

Some of the valuable components are dissembled from the aircraft and depending on their technical condition, are inspected, repaired or overhauled by approved facilities in the aerospace sector, prior to reuse in other aircraft. Some of the retired aircraft find their way into educational institutions or museums for display.

Aircraft accidents can be attributed to one or more of a combination of causes identifiable in the above three stages and usually comprise human factors and technical failures. Generally, occurrences due to experimental test flights, terrorism, hijacking, sabotage, and direct military action are not considered for analysis of airplane accidents. Analysis of the data from aviation accidents throws light on several vital factors that eventually resulted in respective accidents. Let us look at a few typical examples of aircraft accidents from history. Statistical analysis and summary of commercial aviation accidents for the last 60 years are documented in [10, 11].

3.1 Crash of a Boeing 707-321C airplane

A Boeing 707-321C airplane crashed on May 14th, 1977, in Zambia, Africa, [12]. The accident occurred in daylight and in good weather, and on approach to landing, killing all the six people onboard. The complete right-hand horizontal stabilizer and elevator assembly of the aircraft had separated from the aircraft during flight and was found 200 meters behind the aircraft wreckage. Subsequent investigation of the fractured horizontal stabilizer revealed that the accident was caused by a loss of pitch control following the in-flight separation of the right-hand horizontal stabilizer and elevator as a result of a combination of metal fatigue and inadequate failsafe design in the rear spar structure. Shortcomings in design assessment, certification, and inspection procedures were contributory factors.

3.2 Crash of a Boeing 747: 200 freighter

A Boeing Model 747–200 freighter (El Al Flight 1862) crashed on October 4th, 1992 during climb out from Schiphol Airport, Amsterdam, Netherlands [13]. All four people onboard as well 43 others on the ground were killed in the accident. The aircraft experienced the separation of both engines from the right wing (inboard engine 3 and outboard engine 4), resulting in loss of control of the aircraft and subsequent crash. Subsequent investigation revealed that the inboard engine and strut (engine 3) had separated from the wing and impacted the outboard engine (engine 4). The lessons learnt from this crash led to changes in the strut design philosophy across the aerospace industry.

3.3 Crash of an airbus 380: 842 airplane

An Airbus 380–842 Qantas Flight 32 suffered significant damages due to engine failure on November 4th, 2010, minutes after taking off from Changi Airport, Singapore, on a scheduled passenger flight to Sydney, Australia [14]. The flight carried 469 passengers and crew. Pilots managed to land the aircraft back at Changi Airport and there were no reported injuries to the passengers and crew onboard. Accident investigation revealed that the probable cause of engine failure was a manufacturing error involving an internal oil feed pipe. The oil feed pipe was manufactured with a reduced wall thickness that eventually cracked and resulted in the accident.

3.4 Crash of a Boeing 747 SR-100 aircraft

A Boeing 747 SR-100 flight 123 operated by Japan Airlines crashed on August 12th, 1985 when flying from Tokyo-Haneda to Osaka [15]. In total, 520 out of 524 people on board the aircraft were killed in what is dubbed as the worst ever single aircraft accident in the history of aviation. When the aircraft was cruising at an altitude of about 24,000 ft., decompression occurred due to rupturing of the rear pressure bulkhead, causing serious damage to the rear of the plane. The airplane became uncontrollable and struck a ridge, bursting into flames. Investigations revealed that the probable reason for the initiation and propagation of fatigue cracks in the rear pressure bulkhead was attributable to improper repairs of this bulkhead and lapses in maintenance inspection.

3.5 Crash of an Airbus model 330: 243 aircraft

On August 24th, 2001, Air Transat Flight TSC236, an Airbus Model 330–243 aircraft, was on a scheduled flight from Toronto, Canada, to Lisbon, Portugal [16]. A fuel leak in the right engine was not detected by the flight crew. Right engine eventually flamed out, followed by the left engine too. The flight crew initiated a diversion from the planned route for a landing at Lajes Airport on Terceira Island in the Azores. Assisted by radar vectors from Lajes air traffic control, the crew carried out an all-engine out visual approach and landing at night, in good visual weather conditions. Sixteen passengers and two cabin-crew members received injuries during the emergency evacuation. The aircraft suffered structural damage to the fuselage and to the main landing gear. The accident investigators determined that the fuel leak was caused by fuel line cracking that resulted from interference between the fuel line and a hydraulic line on the right engine. The interference was caused by an incomplete service bulletin incorporation creating a mismatch between the fuel and hydraulic lines during replacement of the right engine.

3.6 Collision between an Illushin IL-76 and a Boeing 747

On November 12th, 1996, Kazakhstan Airlines Flight 1907, an Ilyushin Il-76 coming from Shymkent, Kazakhstan, was to land in Delhi [17]. In the meantime, Saudi Arabian Airlines Flight SV763, Boeing 747, had departed from Delhi for a passenger flight to Dhahran. Both aircraft collided, plummeted down in flames, and crashed in an arid farming area, resulting in 312 fatalities. The root and approximate cause of the collision was the unauthorized descending by the Kazak aircraft to FL140 and failure to maintain the assigned FL150. This is attributed to inadequate knowledge of English language of Kazak pilot, resulting in wrong interpretations of ATC instructions; poor airmanship and lack of proper CRM (Crew Resource Management) skill on the part of Pilot-in-Command, compounded by leadership quality lacking in him; casual attitude of the crew and lack of coordination in the performance of their respective duties by crew of Kazak aircraft; and absence of standard callouts from any crew member.

3.7 Crash of a Boeing 757: 200 airplane

On October 2nd, 1996, Boeing 757–200 aircraft departed from Lima-Jorge Chávez Airport on an international regular service to Santiago de Chile, carrying 61 passengers and a crew of nine [18]. After 29 minutes of flight, it impacted with the sea 48 nautical miles from the airport, with the total loss of the aircraft and all of its occupants. Investigation revealed that the maintenance staff had not removed the protective adhesive tape from the static ports. This tape was not detected during the various phases of the aircraft’s release to the line mechanic, its transfer to the passenger boarding apron and, lastly, the inspection by the crew responsible for the flight (the walk-around or pre-flight check), which was carried out by the pilot-in-command. Both the pilot-in-command and the copilot had made errors by not complying with the ground proximity alarms.

3.8 Crash of Convair 580

As many as 10 recent air crashes, including the American Airlines disaster in New York in November 2001, could have been linked to a newly uncovered scam by which old and faulty aircraft parts were sold as new [19]. Parts illegally salvaged from crashes, counterfeit parts, and other substandard components regularly find their way into the world’s air fleets, sold at bargain prices, often with falsified documents about their origin or composition. The worst confirmed accident occurred on Sept. 8, 1989, when at 22,000 feet over the North Sea, the tail section of a Convair 580 turboprop plane began vibrating violently and tore loose [20]. The charter aircraft, carrying 55 people from Oslo to Hamburg, splattered over 3 1/2 miles of sea. Everyone onboard died. Norwegian investigators painstakingly dredged up 90% of the 36-year-old plane and found the cause: bogus bolts, bushings, and brackets.

3.9 Twin accidents involving Boeing 737 max

A Boeing 737-Max airplane (Lion Air Flight 610) took off from Jakarta, Indonesia, on October 28th, 2018 and crashed into the Java Sea soon after takeoff, killing all 189 passengers and crew. Five months down the line, another Boeing 737-Max airplane (Ethiopian Airlines Flight 302) took off from Addis Ababa, Ethiopia, on March 10th, 2019 and crashed near the town of Bishoftu in Ethiopia, killing all 157 people in the aircraft. The aircraft contained a new feature called the Maneuvering Characteristics Augmentation System (MCAS) in its flight control computer [21]. Crash of these two Boeing 737max airplanes raised eyebrows about the technical design flaws, faulty assumptions about pilot responses, and management failures by both Boeing and the FAA.

4. Human factors

Case studies in the preceding section have shown that human errors can cause catastrophic aircraft accidents. We have heard of the popular saying, “to err is human,” meaning that it is normal for people to make mistakes. Analogous to manufacturing tolerances that are used to control the inherent variations in aircraft parts to ensure greater consistency, interchangeability, and intended performance, we can think of human error as a deviation from an intended action that does not lead to undesirable consequence. An intentional deviation would amount to violation. In this section, we will introduce the concept of human factors and the various models that have been in use to understand their role in aviation accidents.

In the FAA, Human Factors are defined as a “multi-disciplinary effort to generate and compile information about human capabilities and limitations and apply that information to equipment, systems, facilities, procedures, jobs, environments, training, staffing, and personnel management for safe, comfortable, and effective human performance” [22].

The concept of human factors can be understood by referring to the SHEL model, developed by Edwards and modified by Hawkins as shown in Figure 4. The SHEL model is represented by five blocks whose edges are not simple and straight. The most critical and flexible component is in the center of this model and corresponds to Liveware (human). This is flanked by four blocks corresponding to Software, Hardware, Environment, and Liveware. Achieving proper matching of each of these peripheral blocks with that in the center is vital, as any mismatch can be a source of human error [23]. Table 2 lists the relevance of blocks in the SHEL model.

Figure 4.

SHEL model as modified by Hawkins.

Block	Relevance
Software	Rules, procedures, written documents etc., which are part of the Standard Operating Procedures
Hardware	Air Traffic Control suites, their configuration, controls and surfaces, displays and functional systems
Environment	the situation in which the L-H-S system must function, the social and economic climate as well as the natural environment
Liveware	the human beings – the controller with other controllers, flight crews, engineers and maintenance personnel, management and administration people – within in the system

Table 2.

Relevance of blocks in the SHEL model.

The PEAR Model is one of the popular concepts related to the science and practice of Human Factors, especially for developing aviation maintenance activities. This mnemonic comprises four key elements, namely People who do the job; Environment in which they work; Actions that they perform; Resources that are necessary to carry out the job [24]. Table 3 lists the parameters addressed in the PEAR model.

Key element	Parameters addressed
People	Physical (physical size, gender, age, strength, sensory limitations), Physiological (nutritional, health, lifestyle, fatigue, dependency on chemical), Psychological (workload, experience, knowledge, training, attitude, mental or emotional state) and Psychosocial factors (interpersonal conflicts, personal loss, financial hardships, recent divorce)
Environment	Physical (Weather, location inside/ outside work space, shift, lighting, sound level, safety) and Organizational (personnel, supervision, labour-management relations, pressures, crew structure, company size, profitability, morale, corporate culture)
Actions	Steps to perform a task, sequence of activity, number of people involved, communication requirements, information control requirements, knowledge requirements, skill requirements, attitude requirements, certification requirements, inspection requirements
Resources	Procedures/ work cards, technical manuals, other people, test equipment, tools, computers/ software, paperwork/ signoffs, ground handling equipment, work stands and lifts, fixtures, materials, task lighting, training, quality systems

Table 3.

Parameters addressed in the PEAR model.

While there are over 300 conditions that can result in human error [25], the aerospace industry very frequently uses a set of 12 (called the “dirty dozen”) when discussing human factors. The elements of this set are “lack of communication,” “complacency,” “lack of knowledge,” “distraction,” “lack of teamwork,” “fatigue,” “lack of resources,” “pressure,’ “lack of assertiveness,” “stress,” “lack of awareness,” and “norms” [26]. Although primarily used in aircraft maintenance programs, this concept has been extended to all areas of the aviation industry and has enhanced aviation safety. Factors associated with pilot errors have been discussed in [27, 28].

The Swiss-Cheese model of accident causation, originally proposed by James Reason, likens human system defenses to a series of slices of randomly holed Swiss-cheese arranged vertically and parallel to each other with gaps in-between each slice [29, 30]. The Swiss-cheese model, adapted from [29], is shown schematically in Figure 5. Each slice of cheese represents the defense of an organization against failure. Holes in each slice represent individual weaknesses in individual parts of the system and have a dynamic nature. It means that the holes vary continually in both size and position on the slice. A limited window of accident opportunity is created when the holes in all the slices align momentarily. This in turn facilitates a hazard to pass through all the holes leading to an accident.

Figure 5.

Concept of Swiss-cheese model developed by reason.

Boeing has used procedural event analysis tool (PEAT) to help the airline industry effectively manage the risks associated with flight crew procedural deviations. Similarly, Boeing has used maintenance error decision aid (MEDA) to help airlines shift from blaming maintenance personnel for making errors to systematically investigating and understanding contributing causes [31]. A variety of operators have witnessed substantial safety improvements, and some have also experienced significant economic benefits because of reduced maintenance errors.

5. Human factors analysis and classification system framework

5.1 HFACS basics

Although the concept of Swiss-cheese model changed the way aviation and other accident investigators view human error, it did not provide the level of detail necessary to apply in the real world. There was a need to have a comprehensive tool for investigating and analyzing human error associated with accidents and incidents. This necessitated the development of the Human Factors Analysis and Classification System (HFACS) [32].

The HFACS framework adapted from [33] is shown in Figure 6. The HFACS framework has a total of 19 causal categories identified within the four levels of human failure. By using the HFACS framework for accident investigation, organizations are able to identify the breakdowns within the entire system that allowed an accident to occur. HFACS can also be used proactively by analyzing historical events to identify recurring trends in human performance and system deficiencies. Both of these methods will allow organizations to identify weak areas and implement targeted, data-driven interventions that will ultimately reduce accident and injury rates.

Figure 6.

The HFACS framework.

Most accidents can be traced to one or more levels of failure related to organizational influences (three causal categories), unsafe supervision (four causal categories), preconditions for unsafe acts (seven causal categories), and the unsafe acts themselves (five causal categories).

5.2 Application of human factors knowledge to aviation accidents

HFACS provides a structure to review and analyze historical accident and safety data. By breaking down the human contribution to performance, it enables the analyst to identify the underlying factors that are associated with an unsafe act. The HFACS framework may also be useful as a tool for guiding future accident investigations in the field and for developing better accident databases, both of which would improve the overall quality and accessibility of human factors accident data. Common trends within an organization can be derived from comparisons of psychological origins of the unsafe acts or from the latent conditions that allowed these acts within the organization. Identifying those common trends supports the identification and prioritization of where intervention is needed within an organization.

By using HFACS, an organization can identify where hazards have arisen historically and implement procedures to prevent these hazards resulting in improved human performance and decreased accident and injury rates. The HFACS framework has been successfully applied to analyze accidents in military, commercial, and general aviation sectors [33, 34, 35, 36, 37, 38, 39].

An example of analysis using HFACS is shown in Figure 7 from the data available in [36], considering a total of 1020 aircraft (181 carrier aircraft and 839 commuter aircraft) accidents involving aircrew or supervisory error for the period 1990–2002. Accidents classified as having “undetermined causes” and those that were attributed to sabotage, suicide, or criminal activity are not included in the analysis. Commercial aviation accident data obtained from the NTSB databases were used in the analysis. Numbers 1 through 18 on the abscissa represent the 19 causal categories in the HFACS framework. Note that routine and exceptional violations have not been shown separately, but as a single number represented as 18 on the abscissa.

Figure 7.

Example of HFACS analysis applied to 1020 commercial aviation accidents.

The HFACS framework has been applied by several researchers to analyze and understand the cause of accidents in aviation and a number of other domains such as rail, maritime, construction, mining, and nuclear power. A consolidated review is available in [40], where the authors conclude that HFCAS can help in analysis to identify both latent and active factors underpinning accidents. The development history of 29 different accident causation models and a new 24Model that discusses linear as well as nonlinear accident causation methods is presented in [41]. In 24Model, the cause of the accident is attributed at individual and organizational levels to immediate cause, indirect cause, radical cause, and root cause.

6. A relook at human factors

In this section, we try to look at human factors and their role in the prevention of aviation accidents from a different perspective. Referring to Figure 8, the total environment within which aviation accidents happen is marked by the largest circle and represents the overall domain. The seven smaller circles within this domain represent seven possible sources from where the “holes in the cheese” can get triggered or generated. Each of these seven segments also needs to have closely knit coordination with rest of the segments to ensure that human errors are minimized and do not propagate through the system resulting in accidents and fatalities. A brief description of the seven segments is provided in the following paragraphs.

Figure 8.

The seven-segment model.

6.1 The seven-segment model

6.1.1 Design organization

This segment covers all the activities commencing from conceptual design of an aircraft till its certification. Design drivers are based on market demand, customer preferences, requirement of reduction in carbon emissions, increase in traffic growth, profit margins, and design modifications to existing aircraft as well. Design organization coordinates and interacts mainly with regulatory body and production organization in achieving its objectives.

6.1.2 Production organization

This segment covers all the activities connected with manufacturing of aircraft commencing from conceptual design to certification of aircraft, including both prototype and production versions. It also includes Original Equipment Manufacturers (OEMs) connected with engines, propellers, and line replaceable units (LRUs). Production organization coordinates and interacts with design organization, OEMs, sub-contract vendors, and regulatory body in achieving its objectives.

6.1.3 Maintenance organization

This segment covers all the activities connected with maintenance, repair, and overhaul of aircraft in order to maintain the aircraft in airworthy condition. Maintenance organization coordinates and interacts with airline operators, production organization, design organization, and regulatory body in achieving its objectives.

6.1.4 Airline operators

This segment covers all the relevant activities carried out by companies providing air transport service for transport of people and freight through air navigation. Airline operators coordinate and interact with airports, regulatory bodies, and maintenance organizations in achieving their objectives.

6.1.5 Air space (flight operations, pax and ferry)

This segment covers all the activities connected with low-speed and high-speed taxy checks of aircraft, test flights for certification of aircraft, commercial operations of aircraft for transporting people and freight. It also covers rest of the air space. These include the space populated by helicopters, other commercial aircraft, drones, trainer aircraft, cargo aircraft, flying cars, birds, and natural phenomena such as thunderstorms, lightning, heavy rains, poor visibility, and so on. It is here that the pilots and crew members play their role and interact with air traffic controller, design organization, and regulatory body.

6.1.6 Airport and air traffic controller (ATC)

This segment covers all the activities connected with management of airport operations, right from fueling the aircraft, pre-flight inspection, clearance for taxying out to the runway, clearance for flight take off, continual monitoring of hundreds of aircraft that are cruising at various flight altitudes, clearance for landings including emergency landings, etc. The ATC coordinates and interacts with the flight crew, design organization, and regulatory body as necessary in achieving its objectives.

6.1.7 Regulatory body

This is an extremely important segment and covers regulation of transport services from source to destination, enforces civil aviation regulations, air safety, and airworthiness standards. It also plays a key role in auditing of design, production, and maintenance organizations, participates in the certification process of aircraft, maintains an aircraft register, and issues certificate of registration to aircraft. It also plays a key role in accident investigation and subsequent issue of advisory circulars/airworthiness directives as applicable to prevent future recurrence.

6.2 Man and environment

Aircraft accidents happen essentially due to a mismatch in the interaction between man and the environment. This means that there must be an issue either with the man or the environment, or a combination of both. For the purpose of discussion, environment is defined to include all such factors outside of the human system. The word “man” is not gender-specific, but is retained for generic description of personnel. The concept is shown in Figure 9 illustrating a safe zone, two “potentially unsafe” zones and an unsafe zone. These four zones represent four possible combinations arising out of man-environment interaction. These are elucidated in the following paragraphs.

Figure 9.

Safe, potentially unsafe, and unsafe zones.

6.2.1 Safe zone

This is the target zone that every safety-conscious organization strives for and would like to remain in this zone forever. The requirement to reach and sustain in this zone requires fully fit personnel and an ideal/flawless environmental condition. Fitness of the personnel includes physical, physiological, psychological, and psychosocial factors and his interaction with the environment. Ideal environment includes adequate funding, infrastructure, resources, feasible time schedules, healthy work environment, and so on.

6.2.2 Potentially unsafe zone with flawed environment

This zone represents situations wherein fully fit personnel perform their work in environments that are deficient of certain factors and are hence “flawed.” An example for this situation is personnel being pressurized to perform aircraft maintenance within a short notice, although it is practically not feasible. This may trigger personnel to take a few shortcuts in the maintenance procedures and has potential to create “holes in the cheese,” which may remain latent till the bubble bursts. Sometimes it may so happen that the personnel are able to meet the short notice requirements without any error (for example, by resorting to overtime work); however, there is a danger of setting a precedent for the future and the management may demand even more output from the personnel working in a flawed environment. This may gradually result in stretching the capacity of the personnel beyond the normal limits and may lead to errors and violations in subsequent work that they are assigned.

6.2.3 Potentially unsafe zone with ideal environment

This zone represents situations wherein the management has provided ideal environment for the personnel to carry out their duties, but the fitness of the personnel who are carrying out the tasks is far from satisfactory. Hence the personnel may not be able to carry out the duties assigned to them, without committing errors. The reasons for errors or violations committed in this zone are internal to the person, attributable to his/her physical, physiological, psychological, or psychosocial condition. An example is that of a person who had some serious argument with his family members before coming to work, or a person who did not have adequate rest before coming to work, and so on. The work carried out under such a situation can lead the output to potentially unsafe zone.

6.2.4 Potentially unsafe zone with ideal environment

This is a very dangerous zone that every safety-conscious organization tries not to enter. If personnel, not adequately fit to undertake the activity, are made to work under flawed environments, the end result is a disaster, resulting in loss of precious lives, or hull, or both. All the aviation accidents that have hitherto taken place can be attributed a combination of human errors and environmental factors. Pilots alone cannot be blamed for the accidents, but the latent errors that have crept into the system from the potentially unsafe zones will also have to be considered.

6.3 Modified HFACS model

The HFACS framework adapted from [33] and shown in Figure 6 is suitably modified by adding additional layer to account for external influences such as market demand, economic and profit motives, inter-segment coordination, and political pressures. The modified HFACS framework is shown in Figure 10. To cite an example, due to an increased market demand, aircraft manufacturers may hike up the production rate of aircraft without adequately enhancing the resources required to achieve the targeted production. This will result in additional work pressure and can result in human errors. In the process of increased competition from other aircraft manufacturers, aircraft design agencies may resort to suggesting quick modifications to an already existing design in order to save time and money. Many a times, there may be lack of coordination between the various segments in the seven-segment model. There may also be situations where a VIP overrules the suggestions given by the pilot and pressurizes him to fly the aircraft. In all these cases, holes are added to the cheese in the system, thus increasing the probability of accidents.

Figure 10.

Modified HFACS framework.

Figure 11 shows schematic of modified HFACS framework for a sample segment 1 and its mutual interaction with rest of the segments 2 through 7. The same concept is applicable to every other segment, and the mutual interaction for the complete domain is depicted in Figure 12. It is necessary to cover all the segments during aircraft accident investigation in order to converge onto the root cause(s) for the accident.

Figure 11.

Modified HFACS framework for a sample segment.

Figure 12.

Mutual interaction paths in the seven-segment model.

7. Way forward for even safer skies

Newer technologies as well as materials available today and in the years to come will result in novel designs of new aircraft configurations as we march towards the zero-emission target. The air space is going to get even more populated with thousands of newer flying machines. This will mean that the way to safer skies begins right from the commencement of the new airplane configuration design. It is right from this stage that the required human factor elements should be embedded into the aircraft projects/programs. Human-to-machine interface designs such as flight deck design will require a relook in view of the new technologies. Also, the concept of design for maintainability and in-service support should be kept in mind. Customer inputs are also to be considered in areas such as passenger cabin design and flight deck design. Human factor experts should review the whole system before the project is taken up. The assessment ideally requires human factors specialists to assess the vulnerability to human error of the response tasks.

In order to achieve and maintain the goal of zero fatalities in commercial operations over the next couple of decades, five specific high-risk categories (HRCs) of occurrences have been identified by ICAO, based on analysis of hitherto aircraft accidents and incidents [42]. These five are Runway excursion (RE), Midair collision (MAC), Loss of control in-flight (LOC-I), Controlled flight into terrain (CFIT), and Runway incursion (RI). Reduction of operational safety risks is a must in order to achieve the goal of zero fatalities, and top management commitment to safety plays an important role in this direction. Continually evolving regulatory procedures must be respected and followed at all times.

Top managements must establish a conducive work environment to prevent the staff of the organization from getting into physical or mental tension. It is not uncommon for anyone subjected to emotional factors such as pressure, distraction, fatigue, and stress to suffer from making judgment errors in the work they carry out. Management should encourage error reporting and honesty among working staff. Many staff may not disclose unintentional errors that have been committed by them for the fear of losing their jobs.

Top managements must identify and implement independent and multiple inspections in critical areas covering design, drawing, raw material inward inspection and storage, fabrication, assembly, integration, aircraft inspection, ground testing, flight testing, servicing of Line Replaceable Units (LRUs), and so on. Design, manufacturing, and flight testing agencies must work with mutual coordination. Human factors will play a key role in every aspect of aircraft life cycle from drawing board till the end of its service life. Organizations that take up aircraft improvisation programs and/or new aircraft development programs will need to first do their homework regarding the feasibility of achieving the project goals within the committed time and funding. Ambitious and impractical goals may land the organization itself into a swirl of external pressures from the stakeholders and funding agencies and result in human errors creeping into the design, development, and manufacturing stages of the program, leading to costly and irreversible consequences.

Pilot familiarization and refresher training including simulator and CRM is an essential element for ensuring flight safety. Training of pilots in competencies such as communication, aircraft flight path management (both manual and automated), leadership and team work, problem-solving and decision making, application of procedures, work load management, emergency procedures, situational awareness and knowledge, fatigue and sleep duration, intoxication, checklist verification is a must on continual basis.

Aircraft maintenance activities will increase in the next few decades due to the increase in the number of aircraft as well as number of flights. It is important to integrate human factors into Safety Management System (SMS) in MROs. Some of the bare minimum aspects to be considered in this process are hazard identification, risk assessment and mitigation, management of change, design of systems and equipment, training of operational personnel, job and task design, safety reporting and data analysis, incident/accident investigation. Bullying by the bosses may have significant adverse consequences and may jeopardize the safety of flight. Figure 13 depicts a typical scenario wherein an aircraft technician is instructed by the top management, to keep the aircraft ready for flight at short notice.

Figure 13.

Aircraft technician under undue time pressure from top management. (illustration credits; Vidya Kamalesh).

Uniformity should be maintained in all final reports pertaining to aviation accidents especially with respect to various human factors and root causes leading to the accident. This will facilitate use of powerful tools such as Watson Analytics and Cognos Analytics for analysis of airplane crash patterns and also to find possible solutions to make continual improvements in flight safety [43]. An advanced method of reporting system for integrating human factors into safety management system in aviation maintenance is described in [44, 45]. Trust, communication, and transparency are at the heart of an appropriately human-centered design process and, in combination, can have a powerful impact on the successful and safe use of advanced automated technologies [46].

8. Conclusions

The dream of man to fly became a reality with the successful flight of the Wright Flyer in 1903. In a short span of about 120 years, aircraft and power-plant designs have undergone continual improvements to cater to the ever-growing air traffic requirements. Innovations in design and analysis tools, material sciences, manufacturing processes, electronics, and communication have made flying easier than ever before and reduced the pilot workload. However, the possibility of human error creeping its way into the aircraft cannot be ruled out. If the human error is left unnoticed, it may result in fatal accidents. Vital factors leading to civil aviation accidents have been brought out by quoting a few examples. Various models used for the analysis of human factors have been discussed. A seven-segment model for identifying causal factors based on modified HFACS has been presented.

Most of the accidents in the last two decades have occurred during approach and landing phases of flight. Human-machine interface designs such as flight deck design should be revisited in light of the new technologies. Uniformity should be maintained in all final reports pertaining to aviation accidents especially with respect to various human factors and root causes leading to the accident. This will facilitate use of powerful tools such as Watson Analytics and Cognos Analytics for analysis of airplane crash patterns and also to find possible solutions to make continual improvements in flight safety. Safety Culture has to be established in the complete aviation industry setup, covering aircraft design, production, flight operations, and maintenance. This is even more important in view of the expected increase in air traffic in the coming years. Commitment of top management to safety is very important. Documented procedures should not just remain on paper, but shall be followed in both letter and spirit. Use of Integrated Vehicle Health Management (IVHM) concepts in aircraft design will result in improving the overall safety of the aircraft, while reducing both operation and maintenance costs. Also, the concept of design for manufacturing, maintainability, and in-service support should be kept in mind. CRM refresher at regular intervals including simulator training to handle flight exigencies is a must.

Air traffic will continue to grow increasing airspace congestion and pose challenges for bringing down the number of aircraft accidents. Use of Artificial Intelligence and Big Data can help improve utilization of airspace safely and efficiently. Human factors will continue to play an even more important role in striving towards an accident-free air travel. At the 77th IATA Annual General Meeting in Boston, USA, on October 4, 2021, a resolution was passed by IATA member airlines committing them to achieving net-zero carbon emissions from their operations by 2050. To succeed, it will require the coordinated efforts of the entire industry (airlines, airports, air navigation service providers, manufacturers) and significant government support. A similar commitment is in order to bring down the number of aircraft accidents and fatalities to zero.

Acknowledgments

The authors would like to place on record the valuable suggestions and feedback provided by Prof. Zain Anwar Ali, academic editor, in bringing out this book chapter.

Conflict of interest

The authors declare no conflict of interest.

Notes/thanks/other declarations

The authors are grateful to Vidya Kamalesh for the neat illustration, reflecting the work pressure from top management under which an aircraft technician is working. The authors are grateful to CWong for permitting to use the photograph of Boeing 737–800 series aircraft with registration number B-1791 available in https://www.jetphotos.com/photo/10528109.

Appendices and nomenclature

ATC

Air Traffic Control

CRM

Cockpit resource management / Crew resource management

FAA

Federal Aviation Administration

HFACS

Human Factors Analysis and Classification System

IATA

International Air Transport Association

ICAO

International Civil Aviation Organization

IVHM

Integrated Vehicle Health Monitoring

MEDA

Maintenance Error Decision Aid

MRO

Maintenance, Repair, and Overhaul

NTSB

National Transportation Safety Board

PEAR

People, Environment, Actions and Resources

PEAT

Procedural Event Analysis Tool

SHEL

Software, Hardware, Environment, and Liveware

SMS

Safety Management System