Aviation Error Models and Theories (9.8.1) PDF

Error Models and Theories (9.8.1) Learning Objectives 9.8.1.1 Explain types of human error and models/theories used to classify and explain human error (Level 2). 9.8.1.2 Identify the three stages at which memory failure can occur (Level 2). 9.8.1.3 Explain types of human error and mo...

Error Models and Theories (9.8.1) Learning Objectives 9.8.1.1 Explain types of human error and models/theories used to classify and explain human error (Level 2). 9.8.1.2 Identify the three stages at which memory failure can occur (Level 2). 9.8.1.3 Explain types of human error and models/theories used to classify and explain human error (Level 2). Summary It has long been acknowledged that human performance is at times imperfect. Error may be de ned as when a planned sequence of mental or physical activities fails to achieve its intended outcome. Aircraft maintenance depends on the competence of many types of workers. Examples presented in this training highlight errors that aircraft maintenance workers have made which have contributed to aircraft incidents or accidents. One thing is clear: aviation is an industry where errors can have terrible consequences, but it is an industry where great improvements have been made because people have made errors. In the past, aircraft components and systems were relatively unreliable. Modern aircraft by comparison are designed and manufactured to be highly reliable. Therefore, it is more common nowadays to hear that an aviation incident or accident was caused by human error. This topic considers the likely types of error that occur during aircraft maintenance and the implications if these errors are not spotted and corrected. Strategies for managing human error in aircraft maintenance are discussed. 2023-01-12 B-09 Human Factors Page 260 of 340 CASA Part 66 - Training Materials Only  Error Models and Theories Types of Human Error To appreciate the types of errors that can be made, researchers have looked at human error in a number of ways and proposed various models and theories. These models attempt to capture the nature of the error and its characteristics. Some of these models and theories will be brie y discussed: Types of Human Error Design versus operator-induced errors Variable versus constant errors Reversible versus irreversible errors Slips, lapses and mistakes Skill-, rule- and knowledge-based behaviours and associated errors The ‘Swiss Cheese Model’ Failures 2023-01-12 B-09 Human Factors Page 261 of 340 CASA Part 66 - Training Materials Only  Design versus Operator - Induced Errors In aviation, emphasis is often placed on error(s) of the front-line operators, who may include ight crew, air traf c controllers and AMEs. However, errors may have been made before an aircraft ever leaves the ground by aircraft designers, people who manufacture the components, or those who assemble the aircraft. This may mean that, even if an aircraft is maintained and own as it is designed to be, a aw in its original design may compromise operational safety. Alternatively, awed procedures put in place by the airline, maintenance organisation or air traf c control management may also lead to operational problems. Image by Kelly Lacy from Pexels Flight deck environment Aircraft manufacturers are very careful to exclude the potential for design-induced errors. Human factors experts work for all major manufacturers, trying to ensure that the human/machine interface is as good as possible. But still, despite all this care, it is possible for unintended design faults to become apparent after the aircraft has been designed, approved and built. An example of a design de ciency can be found in the British Aerospace BAE 146 cockpit [CS]. 2023-01-12 B-09 Human Factors Page 262 of 340 CASA Part 66 - Training Materials Only  Source: Photographer Mario Nonaka, 2005 (from Airliners.net) A design de ciency can be found in the British Aerospace BAE 146 cockpit Shown inside the red circle are three T - handles: The two red handles are (from left to right) elevator disconnect and aileron disconnect. These levers allow the ight crew to physically split the ight control systems in the event of part of the system becoming jammed. The black handle is the park brake. But it can be seen that the aileron disconnects handle and the park brake handle: Are situated next to each other Are both about the same size Operate the same way (pull up) Are oriented the same way (unlike the elevator disconnect which is turned 90 degrees to the orientation of the other two handles. Although the designers have gone to the trouble of ensuring one handle is coloured red and the other black, this feature is ineffective if the pilot is not looking or is in a dark cockpit. Regularly in line service, when parking or when stopped for an extended period during taxiing, a pilot will reach down to the park brake handle to pull it and engage the park brake. If they reach down without looking, it is very easy for the pilot to mistake the aileron disconnect handle for the park brake handle. Pulling the aileron disconnect handle immediately splits the aileron system. The aircraft cannot take-off until the system is reset. 2023-01-12 B-09 Human Factors Page 263 of 340 CASA Part 66 - Training Materials Only  The park brake is used only when the aircraft is on the ground, so this design de ciency will never compromise aircraft safety in ight. But resetting the aileron system on the ground takes time and specialist skill. This design de ciency has proved to be quite expensive for operators. Human factors experts are involved in every aspect of cockpit design, from the comfort and ergonomics of the seating arrangement, ensuring that the pilots can physically reach all controls, to the eld of vision through the windscreens, the colour of warning and caution lights, the style and volume of warning and advisory chimes, and the methods of tactile warnings, such as a stall warning. Variable versus Constant Errors It can be seen in the gures below that variable errors in Target (A) are random in nature, whereas the constant errors in Target (B) follow a consistent, systematic (yet erroneous) pattern. Constant errors may be predicted and therefore controlled, whereas variable errors cannot be predicted and are much harder to deal with. If we know enough about the nature of the task, the environment it is performed in, the mechanisms governing performance, and the nature of the individual, we have a greater chance of predicting an error. In the example below: Ri eman A’s pattern exhibits no constant error, but large variable errors Ri eman B’s pattern exhibit’s a large constant error but small variable errors. The latter may be easier to predict and to correct (e.g., by adjusting the ri e sight if it were out of alignment). Reason (1990), originally from Chapanis (1951) Variable versus Constant Errors 2023-01-12 B-09 Human Factors Page 264 of 340 CASA Part 66 - Training Materials Only  Reversible versus Irreversible Errors Another way of categorising errors is to determine whether they are reversible or irreversible. The former can be recovered from, whereas the latter typically cannot. For example, if a ight crew miscalculates the fuel to be carried, that is a reversible error because pre- ight checklists may alert the crew to the problem, or they may have to make an in- ight diversion to take on more fuel. But if the ight crew accidentally dumps fuel in ight, this situation cannot be reversed. A well-designed system or procedure should ensure that errors made by AMEs are reversible. Thus, if an engineer installs a part incorrectly, it should be tested or inspected and corrected by procedures before the aircraft is released back to service. The concept of duplicate/dual/independent inspections is common because it gives a greater chance of errors to be captured and reversed before they can do harm. Duplicate/dual/independent inspections are carried out after critical aircraft systems have been worked on, allowing two independent people to inspect the same system. Image by Jeswin Thomas on Unsplash Prototyping bread board 2023-01-12 B-09 Human Factors Page 265 of 340 CASA Part 66 - Training Materials Only  Slips, Lapses and Mistakes It can sometimes be confusing when different error types are listed. Some people have never considered that there are different error categorisations. Common examples can help with clari cation. These classi cations are often important when analysing what went wrong and how to prevent it happening again. Sometimes, when someone has made an error, an immediate suggestion is that the person may need more training. But if the person just forgot something (see below – a 'lapse'), then more training will not necessarily help. They just plain forgot. Slips, on the other hand, are when someone tries to carry out a task but for some reason fails in their action. Like a goal kicker who is consistently missing the goal, they look like they are kicking the right way, but perhaps their technique is not quite right. These errors can bene t from more training. Type Action Misperceptions Things are not what they seem Slips Right idea, wrong action Lapses Just plain forgot Mistakes Wrong idea; action as intended Violations Not the prescribed procedure The most well-known error types are slips, lapses and mistakes: Slips can be thought of as actions not carried out as intended or planned, e.g., transposing digits when copying numbers, or mis-ordering steps in a procedure. Lapses are missed actions and omissions, i.e., when somebody has failed to do something due to lapses of memory and/or attention or because they have forgotten something, e.g., forgetting to replace an oil ller cap or latch an engine cowling. Mistakes are a speci c type of error brought about by a faulty plan/intention, i.e., somebody did something, believing it to be correct when it was in fact wrong, e.g., an error of judgment such as mis-selection of bolts when tting an aircraft windscreen. Slips typically occur at the task execution stage, lapses at the storage (memory) stage and mistakes at the planning stage. Alongside slips, lapses and mistakes are two other important error categories, misperceptions and violations. 2023-01-12 B-09 Human Factors Page 266 of 340 CASA Part 66 - Training Materials Only  Misperceptions Misperceptions occur when a person fails to gather, or does not gather enough, information with their senses. Because of this lack of sensory information, the person can perceive situations incorrectly, for example not seeing a crack when a crack is present. Environmental considerations, for example, vibration or competing sensory information such as noise, can greatly affect the degree of misperception. © Aviation Australia Noise can impact perception if PPE is not worn Type 1 and Type 2 Errors There are also two particular types of error which can be associated with misperceptions and are referred to particularly in the context of visual inspection, sometimes called Type 1 and Type 2 errors: Type 1 error occurs when a good item is incorrectly identi ed as faulty. Type 2 error occurs when a faulty item is missed. Type 1 errors are not really a safety concern, except that it means resources are not being used most effectively because time is being wasted on investigation of items which are not genuine faults. Type 2 errors are of most concern since, if the fault (such as a crack) remains undetected, it can have serious consequences (as was the case in the Aloha Airlines accident). 2023-01-12 B-09 Human Factors Page 267 of 340 CASA Part 66 - Training Materials Only  Violations Violations sometimes appear to be human errors, but they differ from slips, lapses and mistakes because they are deliberate ‘illegal’ actions, i.e., somebody did something knowing it was against the rules (e.g., deliberately failing to follow proper procedures). AMEs may consider a violation well intentioned, i.e., cutting corners to get a job done on time. Sometimes, under certain circumstances, this may be justi ed. However, procedures are in place for a particular reason and must be followed appropriately to help ensure safety. If a person feels the procedures should be changed, then there are appropriate ways of initiating this change. An example of this is an Aircraft Maintenance Manual (AMM). Aircraft manufacturers will welcome feedback and suggestions to make the maintenance manual a more workable document. That is, the AMM can be changed. However, there is a distinct procedure for the change to be suggested, assessed and carried out. Until the change is made, the published procedure represents the best way and the approved way of doing the task. Intention is important when considering the nature of error. It is useful to ask the questions: Were the actions directed by some prior intention? Did the actions proceed as planned? Did they achieve their desired result? Image by Bruno /Germany from Pixabay Violation 2023-01-12 B-09 Human Factors Page 268 of 340 CASA Part 66 - Training Materials Only  Skill-, Rule- and Knowledge-Based Behaviours and Associated Errors The behaviour of AMEs can be broken down into three distinct categories: skill-based, rule-based and knowledge-based behaviour: Skill-based behaviours are those that rely on stored routines or motor programs that have been learned with practice and may be executed without much conscious thought. An example may be the skill of driving a rivet with a rivet gun; Rule-based behaviours are those for which a routine or procedure has been learned. The components of a rule-based behaviour may comprise a set of discrete skills. An example of this may be a pre- ight inspection, which is a procedure made up of a number of separate elements; Knowledge-based behaviours are those for which no procedure has been established. These require the maintenance worker to evaluate information, and then use knowledge and experience to formulate a plan for dealing with the situation. In aircraft maintenance, this is common in a line maintenance situation where aircraft defects are random and unpredictable and must be assessed and recti ed quickly. Each of these behaviour types has speci c errors associated with it. © Aviation Australia Types of errors 2023-01-12 B-09 Human Factors Page 269 of 340 CASA Part 66 - Training Materials Only  Skill-Based Error An example of a skill-based errors is an action slip. As the name implies, action slips are similar to slips previously discussed, i.e., an action not carried out as intended. An example of an action slip is a maintenance worker knowing they need a certain torque setting on a wrench to complete a job. But, because they are distracted by a colleague, they pick up another wrench which is set to the wrong torque. If nobody notices that the bolts have been tightened incorrectly, then this becomes an error. © Aviation Australia Example of an Action Slip Rule-Based Error Rule-based mistakes arise from: Misapplication of good rules Application of bad rules Non-application of good rules (violations). Rule-based behaviour is generally fairly robust and this is why the use of procedures and rules is emphasised in aircraft maintenance. Errors here are related to the use of the wrong rule or procedure. Take for example, an engineer conducting a pre- ight inspection: A misapplication of a good rule may be that the engineer forgets the correct sequence or misses a critical step when performing a pre- ight inspection. An application of a bad rule may mean the engineer never walks to the wing tips to check for leading edge damage during a pre- ight inspection because they have never seen leading edge damage except near the wing root. A non-application of a good rule (violation) may mean the engineer does not bother conducting the pre- ight inspection, even if procedures demand it. 2023-01-12 B-09 Human Factors Page 270 of 340 CASA Part 66 - Training Materials Only  Knowledge-Based Error Knowledge-based mistakes are more varied. They arise when people have to improvise in a new situation or use lateral thinking or a troubleshooting procedure. Errors at the knowledge-based performance level are related to incomplete or incorrect knowledge or incorrect interpretation. An example of this is when an engineer attempts an unfamiliar assembly or repair task and assumes they can ‘work it out’. Once he has set out in this way, he is likely to take more notice of things that suggest they are succeeding, while and ignore evidence that they are not succeeding. This is known as con rmation bias. © Aviation Australia Types of Errors (Rule-based and Knowledge-based) 2023-01-12 B-09 Human Factors Page 271 of 340 CASA Part 66 - Training Materials Only  Memory Failures A failure occurs when intentions to do something aren’t successful. Beyond the skill-based failures already discussed, humans are very prone to failure in a number of intellect areas, such as memory and attention, when attempting a task and not performing it correctly. Memory can fail at one or more of three stages of information-processing: Input - insuf cient attention is given to the material to be remembered. It is lost from short- term memory. Storage - the material degrades or suffers interference in long-term memory. Retrieval - the material is known but is not recalled at the appropriate time. Failures in each of these processes can cause forgetting. Image by Marcus Aurelius from Pexels Memory retrieval - the material is known but is not recalled 2023-01-12 B-09 Human Factors Page 272 of 340 CASA Part 66 - Training Materials Only  Memory Input Failures Why are we likely to forget someone's name once we’re been introduced to them? Because the name is part of a ood of new information about this person and often fails to get taken in unless we make a special effort to focus on it (then we often cannot remember what they looked like or what they did for a living). This tells us that giving just the right amount of attention to something is an important precondition for being able to remember it later. The second kind of input failure is the forgetting of previous actions. Again, this is due to a failure of attention. When we are doing very familiar and routine tasks, our minds are almost always on something other than the job in hand. That’s a necessary feature for the task to be done smoothly. The result is that we ‘forget’ where we put our tools down or nd ourselves walking around looking for something we are still carrying. Image by Juan Pablo Serrano Arenas from Pexels Focus and attention is important to memory retention Some other consequences of this kind of forgetting are: Losing our place in a series of actions - after a period of switching off, we become aware again of what we are doing but don’t know immediately where we are in the sequence. The time-gap experience - we can’t remember things about our journey walking or driving in the last few minutes, or what we’ve been doing exactly. For example, we may be in the shower and can't remember whether or not we’ve put shampoo on our hair. The evidence (if there was any) has been washed away, and we have been thinking about something else. In short, we’ve not been attending to the routine details. 2023-01-12 B-09 Human Factors Page 273 of 340 CASA Part 66 - Training Materials Only  Memory Storage Failures An intention to do something is rarely put into action immediately. Usually, it must be held in memory until the right time and place for its execution. Memory for intentions is called prospective memory, and it is particularly prone to forgetting or side-tracking, so that the action is not carried out as intended. It is, of course, possible to forget an intention completely, so that no trace of it remains. More usually, the forgetting occurs in degrees. Almost forgetting the plan entirely turns into the vague ‘I should be doing something’ feeling. Here, you have an uneasy sense that something needs to be done, but you can’t remember what, or where and when it should be done. Another fairly common experience is that you remember the intention and start to carry it through, but somewhere along the line (usually because you are preoccupied with something else) you forget what you came to some place to do. The place could be a shop, or you could nd yourself standing in front of an open drawer or cupboard. You simply can’t recall what you came to fetch. This is the ‘what- am-I-doing?’ or ‘what-am-I-doing here?’ feeling. The third possibility is that you set out to perform a plan of action, think you have completed it, but later discover that you left something out. A common experience is to return home to nd a letter you intended to post. Image by Andrea Piacquadio from Pexels This is the ‘what-am-I-doing?’ or ‘what-am-I-doing here?’ feeling 2023-01-12 B-09 Human Factors Page 274 of 340 CASA Part 66 - Training Materials Only  Memory Retrieval Failures Retrieval failures are among the most common ways that your memory can let you down, and increasingly so as you grow older. At their most acute, they show themselves as the ‘tip-of-the-tongue’ (TOT) state when you realise you can’t remember a name or a word you know. The word seems tantalisingly close on the tip of your tongue, in fact. The problem is usually made worse because another word or name comes into your mind, but you know it’s not the one you are trying to nd. However, you have a strong sense that somehow, it’s close to the target item. You may feel it sounds similar, has the same number of syllables, or is a name that belongs to someone who is related to or who works with the person whose name you are trying to nd. Image by Andrea Piacquadio from Pexels Retrieval failures occur more frequently with age 2023-01-12 B-09 Human Factors Page 275 of 340 CASA Part 66 - Training Materials Only  Failures Due to Attention Attention is a limited resource. If attention is directed to one thing, then it is withdrawn from another. Attention is closely bound up with conscious awareness. It selects a part of a much larger array of information for further processing. People have a limited capacity for attention and it is necessary for effective information processing. Types of attentional failures are: Strong habit intrusions e.g., you are a tea drinker and make 10 cups of tea per day. A guest asks for coffee, but you automatically make them tea instead of coffee. Omissions following interruptions e.g., forgetting something after being interrupted in the middle of a task. Premature exits e.g., departing from an action sequence before all the component actions are carried out, like leaving one screw undone. Common Errors in Aircraft Maintenance Errors arising from attention de cits, such as premature exits and omissions following interruption, are common in aircraft maintenance because much of the work is a multi-step process, susceptible to interruptions and missed steps. Rule-based errors are also common in aircraft maintenance. There are three main classes of rule- based errors: Misapply a normally good rule. That is, we can use it in a situation for which it is not appropriate because of some changed circumstance. Apply a bad rule that may get the job done but can have unwanted consequences. Fail to apply a good rule that was appropriate and should have been followed. These instances are violations, bending of the rules and non-compliances. 2023-01-12 B-09 Human Factors Page 276 of 340 CASA Part 66 - Training Materials Only  Misapplying a Good Rule A good rule or principle is one that has been generally useful in the past, but sometimes can be wrongly applied. This usually occurs in a situation that shares common features with the one for which the rule was intended, but where the differences are overlooked. Applying problem-solving rules is often complicated by the fact that different problems can share common features. For example an AME inspects an aircraft where the pilots have complained about nose-wheel ‘shimmy’ and vibration. The engineer replaces the two nose wheels without searching for any other causes of the reported symptoms. The AME is using their past experience, when nose-wheel shimmy has been corrected by changing both nose wheels, especially if one tyre is worn more than the other. Thus, the engineer is following a rule that has a history of positive outcomes. But consider if the real cause of the nose wheel shimmy is worn linkages in the nose landing gear. Making assumptions about the cause of the shimmy and replacing the wheels without investigating further can lead to negative consequences further damage of the nose landing gear and the attaching hardware and structure. If left unchecked, this will result in failure of the worn components and a highly unsafe condition. Applying a Bad Rule Most people pick up bad rules (incorrect procedure or bad habits) from other people when learning a job. The bad rules might serve their immediate purpose on many occasions. But they are considered bad because they can lead to something going wrong at a later time, or they affect some other person or process that is not immediately obvious. An example may be a person who, under time pressure, skips a step in an inspection procedure because he has never found any problems when he has followed that step in the past. When time pressure is not so critical, the inspection is carried out as per the procedure. Bad rules can become established as part of a person's normal behaviour for several reasons: No one corrects the bad rule at the time. Applying the bad rule seems to get the job done, sometimes quicker and easier than other rules. Most of the time, there are no bad consequences. 2023-01-12 B-09 Human Factors Page 277 of 340 CASA Part 66 - Training Materials Only  Failing to Apply a Rule (Violation) Sometimes people deliberately violate a procedure by not applying the correct rule to their work. This could be a good rule that is appropriate and should be used, like a speed limit, or it could be a bad rule that makes a job unworkable, like a confusing maintenance manual procedure. Violations in aircraft maintenance are generally made because people have a genuine desire to do a good job. In some circumstances, there may be justi cation for a violation. A just culture will examine the details surrounding the violation before any punishment is imposed. There is more discussion of violations in the next section of this course. 2023-01-12 B-09 Human Factors Page 278 of 340 CASA Part 66 - Training Materials Only  The ‘Swiss Cheese Model’ It is common to nd when investigating an incident or accident that more than one error has been made and often by more than one person. The ‘Swiss Cheese’ Model is a pictorial representation of how sometimes a very particular set of circumstances, like multiple errors, must occur for safety to be compromised suf ciently to cause an accident or incident. Professor James Reason highlighted the concept of layered ‘defences’ against human error within an organisation. These layers are portrayed diagrammatically as several slices of Swiss cheese in Reason's model. Each layer has the ability to stop an error from having an undesirable outcome, e.g., an accident or incident. Examples of defences are procedures like duplicate inspections, pilot pre- ight functional checks, etc. Other defences include aspects of the organisation itself and, characteristics of its employees, including their risk personality and, their inherent desire to adhere to guidelines and rules. All of these things help to prevent or to ‘trap’ human errors, reducing the likelihood of negative consequences. Weakened defences are represented by the holes in the layers of cheese. Some of the holes exist because of latent conditions, and some because of active conditions. Latent failures have existed or have been generated at some point in the past and then lie dormant, such as a poor procedure. Other latent failures may have been introduced at the time an aircraft was designed or may be associated with a management decision. Errors made by front line personnel, such as AMEs, are active failures. When all the layers are breached, the errors aren't captured and can result in losses, such as incidents or accidents. Source: Bakeli (2017) Reason’s Swiss Cheese Model 2023-01-12 B-09 Human Factors Page 279 of 340 CASA Part 66 - Training Materials Only  The more holes in a system’s defences, the more likely it is that errors will result in incidents or accidents. But it is only in certain rare circumstances, when all the holes in all the layers of defence line up, that incidents or accidents will occur. To prevent incidents or accidents, it is best to have fewer holes, more layers or both. 2023-01-12 B-09 Human Factors Page 280 of 340 CASA Part 66 - Training Materials Only