The term „Human Factors“ describes the influences on human characteristics and needs (Lit. 19.1-20, Fig. "Human factors in maintenance and overhaul"). That can be work specific very different. Stand, e.g., organisatoric aspects in the foreground wich can be summarised under rather keywords like career aspects and organisatoric might position in the foreground (Lit. 19.1-1). To this fits rather the term „Human Factor” than the factor human/man. In the medical or psychological area we find other connections. In English is the „Human (Factors) Engineering“, meaning ergonomics a main request (Lit. 19.1-3). Thereby this term is rather limited at the technical area. Of special importance is the so called „Human Factors Design Guide”. This compilation preferential deals with the hardware of the work environment (Fig. "Human factors in maintenance and overhaul"). This aspect for example dedicates especially the US-Federal Aviation Administration (FAA, Lit 19.1-3 and Lit. 19.1-4). This should be of particular significance for maintenance and overhaul of aeroengines.
The operation experience, especially in connection with the „Aloha-accident“ (Fig. "Accident caused by maintenance deficits"), shows maintenance typical influences which particular frequently emerge in connection with problems and failures. Those, separate chapters will be dedicated:
Figure "Human factors models for understanding"(Lit. 19.1-24): The SHEL-model together with the Reason model („Swiss Cheese Model”, Ill.19.1-0.2) serves to recognise the importance of the „Human Factors“ for safety and effectivity in the aviationf (Fig. "Risk increasing by disregarding human factors 3"). It is merely to see as a help for the understanding of the term „Human Factors” and should enable an impression for the one another influence respectively the interaction of the areas Software (approach), Hardware (machine), Environment (surrounding) und Liveware (human). The influences are teethed displayed (fixed components, no gear!). For an optimum they have to fit exactly. Is this not the case tensionings develop up to the breakdown of the system. Does this happen at one position this can also influence other areas. That this represents the reality rather good show many examples of catastrophic failures and accidents (e.g., „Pensacola“, volume 4, Ill. 188.8.131.52-9.1 up to Ill. 184.108.40.206-9.4).
'Liveware' considers the human as center. It contains influences which are determined by him.
Bodily conditions: to this belong size/height and strength.
Needs: food, breathing air, sanitary conditions.
Uptake capacity for informations. Cognition/senses, psychology and physiology.
Handling of informations depends, e.g., external from display units. Individual influences are knowledge, memory, stress and motivation.
Acting and reaction: to the personnel influences count communication and teamwork. Acting/actions are influenced by biomechanics. e.g., movements.
Environment/surrounding affects the feeling by temperature, vibrations, humidity, sound/noise, daytime and acceleration forces/inertia. To these belongs also the arrangement of the workplace.
Interaction of the model areas::
L-H applies for the system human-machine. To this belongs the arrangement of the workplace. Because of the adaptability of humans shortcomings often are identificated not until a catastrope occurred.
L-S stands for the interaction of instructions, manuals, checklists computer programs etc.
L-E: to those belong problems with the biorythm, air conditions, toxic materials, oxygen shortage and g-loads.
L-L stands for interpersonal contact. It's especially important for teamwork. Further aspects are leadership and cooperation. Those must be seen in close connection with the company climate.
Figure "Model for iInteraction of accident causes" (Lit. 19.1-24): This Reason Model reveals the situation for a failure/accident (Fig. "Influence of human factors at accidents", Fig. "Risk increasing by disregarding human factors 1" and Fig. "Accident caused by maintenance deficits"). For this some simultaneous conditions are necessary. Those are illustrated as holes in obstacles which stand for the zones of influence. Do they form a line, the „beam of disaster” will pass. This correlates with the Murphy Principle: `everything that can happen will happen' - this is only a question of time. The model has the following `obstacles':
Requirements (decision makers): upper management. It provides the superior targets. Thereby we have the stress field of safety and costs.
Processes (line management): here on hand of the targets the decisions for the implementation are made.
Surrounding (preconditions) for the performers skill, knowledge and motivation. Important are the technical preconditions like tools and equipment/jigs as well as work conditions.
Practice (productive activities) stands for the execution of the work. Thereby deviations from the instructions (unsafe acts) make protections (defences) ineffective. So the chance for a failure respectively accident develops.
Figure "Human factors in maintenance and overhaul" (Lit. 19.1-2): The „Human Factors“ influence the work at the hardware like during maintenence and overhaul (Fig. "Important influences in the maintenance"). With this it differs of such which are rather organisational tasks. With this more individual goals push into the foreground, for example the career. So the factors and/or their assessment can be very subjective (Ill. 19.1,.1-1 and Fig. "Effective communication").
In contrast the work on site may evaluate especially the technical/specialized qualification. So specialist work and with this safety requirements can be underestimated by managing hierarchies. Misunderstandings are then the consequence and lead to different priorities. This becomes especially noticeable in the personnel policy at areas like motovation (Fig. "Importance of motivation"), workload and experience. „Maintenance Resource Management” (MRM, Fig. "Importance of motivation") is dedicated to this problem.
Work performance is measured with efficiency (costs, expenditure of time), product safety and safety of work. Those are essential influenced by the factor motivation (Fig. "Importance of motivation"). This must again be seen in connection with the work environment, which composes of many single factors. More immaterial influences like motivation (Fig. "Importance of motivation"), knowledge (Fig. "Influence of human factors at accidents") and experience („softfacts“, grey labeled) can be distinguished from „hardfacts” like workshop installation or maintenence friedliness (Fig. "Maintenance specific trauning"). Those factors influence the human and backwards also by him respectively his claims and demands for an optimal work performance.
Figure "Important influences in the maintenance" (Lit. 19.1-20): With those „Human Factors“ important influences in the maitenance of airplanes are compiled. Related posters are available in the internet.
„1” Lack of communication: This may be about the most important problem factor (Fig. "Direct communication influences", Fig. "Effective communication" and Ill. 19.1.1-3). Communication can take place verbal and written. Single or in combination it can come to misunderstandings or informations can be lost. The modern means of communication like monitor/display or handy can also increase the risk of problems.They must not replace the safety of a documentation.
„2” Complacency (overestimation of one's own capabilities): This can develop in connection with frequent repeated work steps like checks/tests. For example, was never a fault/bug found the ability to judge can be affected (volume 4, Ill. 17.3.1-9).
Asking is rather a sign of expertise and for the safets necessary critical thinking.
„4“ Confusion, avocation: About 15 % of all maintenance faults are assigned to this influence. Typical example is a interrupted task, at which continuation the imagination exists to be further as really progressed (Fig. "Risk of shift changeover with not accomplished maintenance").
„5” Deficient team work: often in connection with communication problems. These aggravate with the team size.
„6“ Exhaustion/fatigue is very malicious because is not until an extreme case concious to the concerned person. This virtually avoids to be identificated in time. The, for the maintenance typical shift work, can favor the problem.
„7” Shortage/lack of parts, logistic problems (lack of resources): The typical employee is proud to bring an airplane back into the air. For him as practitioner („can-do“), a contrary decision is not easy.
„8” Workpressure/ timepressure: just in the airplane maintenence prevails a high pressure to finish the work. It's an art to realise if this is not realistic.
„9“ Lacking self confidence /certainty (lack of avertiveness): it can, even if seldom, be necessary that certainty is required. For example to ensure that the right is done and not leaped. Thereby also personal drawbacks can arise.
„10” Stress, a normal situation as long as it is tolerable. It depends on whether to identify the point of time.
„11“ Lack of awareness/insight: Especially experienced can loos the awareness for possible consequences of their action. Don't cover manuals the fault we can afterwards hear: „The common sense should have been sufficient”.
„12“ Behavoural norm/habits (norms): This influence must not be underestimated. Usually humans want to belong to a community and so they accept ther norms. It's difficult to withdraw from this `peer pressure'.
Figure "Affects at safety due to maintenance" (Lit. 19.1-21 and Lit. 19.1-23): The tendency to increased outsourcing of maintenance work has its problemaic aspects.
Resposibility: In case of the „Alaska Airlines“ accident (Fig. "Accident caused by maintenance deficits") there was a juristic decision: airlines can not be made responsible for the maintenance work of contracting partners (contractors), if they didn't know them and by sufficient examination could not find them out. This is contrary to the opinion of the FAA: „…carriers have unrestricted and unconditional responsibility for the independent maintenance contractors…” . This means the airline bears the fully responsibility for the maintenance work of the contractor.
Symptoms of the problematic: maintenence faults (Fig. "Influence of human factors at accidents") play a considerably greater role at accidents and incidents as long be thought.
To the problematic belongs also the trend to an increating outsourcing. Because obviously the monitoring effort was not equivalet raised, it's to reckon with a rise of risks, like unsuitable tools and invalid manuals.
Warning signs and Symptoms:
Figure "Influence of human factors at accidents" (Lit. 19.1-22 ): This concerns an analysis of flight problems, that means, not oly of the engines. However the tendences may also be interresting for the engines. „Skill based errors “ are most frequently connected with aurcraft accidents (diagram left). Violations (definition Fig. "Technical terms in connection with overhaul problems") of the maintenance personnel, the operator and decision errors follow.
The most frequent decision errors (ca. 1/3) concerned departures from an instruction like a „Service Bulletin” or „Service Letter“. Next are problems during overhaul and the exchange of parts/components, each with about 10 %.
Skill based faults, that could be correlated to repair shops ascribed to assembly (ca. 30 %) and inspection errors, (> 15 %) . Violations of the mechanics have been similar frequent like skill based errors. Next are violations in connection with processes and instructions (>10%).
Violations at the operator who does the main tenance/overhaul himslf, stand frequently in connection with the compliance of the annual inspections (ca. 20%). Followed-up by problems at maintenance, overhaul and assembly (each ca. 10 %). With about 5 % each, not approved design changes, modifications and aberrations from the instructions were represented.
The right chart assigns accident causes in form of personal injuries. Especially catastrophic (death) seem to be influenced by violations of the maintenance personnel and the operator.
Figure "Importance of motivation" (Lit. 19.1-5, Fig. "Human factors in maintenance and overhaul"): Not only „hardfacts” are influential human factors. Also „softfacts“ must not be underestimated. As such, lack sense of achievement can dangerous influence the safety of test procedures (Lit. 19.1-5). For example, if over longer time periods no failure is found, this can lead to fatigue, decreasing concentration and frustration of the inspection personnel. However, if then a crack afflicted part exists, the possibility increases that the failure remains undetected (volume 4, chapter 17.3.1).
Figure "Maintenance specific trauning": The education anf further training of them maintenance personnel seems to be offered frequently world wide (Lit. 19.1-6 and 19.1-7).
Of especial importance is the „Human Factors Design Guide” (Lit. 19.1-3) which deals with an optimum workenvironment at about 1000 pages.
Of high significance are individual aspects like safety and health. To this a particular interrest is granted dangerous materials („Hazardous materials“, Lit. 19.1-3) and processes. Thereby not only the materials of parts and components itself are affected, but also auxilary substances and process media (chapter 19.2, Lit. 19.1-3).
The basic knowledge of task specific technical features was identified, but apparently definitely not always assessed in its importance. Technical basics ar not only important in the supervising level. Especially those, with the maintenance work employed practitioners (workers, inspectors, Ill. 9.1.2-1) need such knowledge that deposits also in description of the profession (chapter 19.1.2). Thereto belongs a certain term safety of the typical problems respectively failures, their images, mechanisms (Fig. "Problem symptoms registered by the user" up to 19-11 and Fig. "Job descriptions of repairers") as well as loads (see volume 3). So it gets possible to draw the right conclusions with sufficient likelihood and to initiate specific measures. The gathering of established experiences and to optimise continous the safety of the work needs also a sufficient understanding of the technical terms. That is a most demanding request which is often in its consequences not seen. At least temporary it shows in „orphaned” or unsuitable manned workplaces with high demands of experience and expertise (Fig. "Ill maintained lubrication leading to crash 1"). From the sight of literature and education offers (up to 2006) it seems just in this field to exist a backlog demand for practitioners.
Testing and checking/inspection (e.g., Borescope, Fig. "Borescope inspection" and chapter 220.127.116.11) stand often in connection with acute problems and serve the safeguarding. In many cases it's about test procedures adapted to the problem. Their handling and the result evaluation damands much expertise, experience and skill as well as assesssment ability. Suitable, also engine type specific courses/trainings, seem to be offered in the internet. In critical cases special briefings and practical exercises are required.
The documentation of the parts/components and the own work pattern increases. Helpful, often essential but not at all simpler for the individual is thereby the use of EDP. The introduction of an EDP-system demands considerably training with refreshments over a longer time. Especially the system has to show its ability in the practice and needs therefore, at least in the phase of introduction respectively testing an iterative approach.
A further aspect of the documentation is the work according to documents like manuals, process specification, instructions and technical drawings. Precondition is, that the content is exact understood. Only then work according to the sense of the introcuction is possible. It must show if the electronic image presentation on site with the help of a labtop, represents in fact an improvement, compared to paper documents.
Figure "Activities connected with human factors" (Lit. 19.1-12, 19.1-19 and 19.1-20): The relisation of the „Human Factors-idea“ requires the cognition of the influence factors and its specific problems. Therefore from firms, authorities and schools available, respectively offered, approaches have been designed (Lit. 19.1-12, and in the internet: examples Lit. 19.1-6 and 19.1-7). The instructions/trainings are adapted to the requirements of the operators (airlines) and the maintenance shops. The multitude of the programs and activities to ensure the safety of maintenance and overhaul demonstrates the high significance of this undertaking.
The practical apprenticeship as a fundament of an education of a skilled worker/technician in the area of aviation technique should include the theme „Human Factors” and sensibilise for it. This is especially true for the military application.
For the extra occupational further training/education, seem to be two themes in the foreground:
The „Maintenance Resource Mangement“ (MRM, Fig. "Maintenance resource management MRM") and in line with this the „Maintenance Error Decision Guide” (MEDA, Fig. "MEDA Maintenance error decision aid" and Fig. "Models of MEDA").
Beyond that there seem to be a series of further programs of which the following will be briefly introduced.
„Assessment of Aviation Safety Programs“ (ASAP, Lit. 19.1-22) in maintenance organisations is a further program, based on the evaluation of aircraft accidents and incidents and serves the safety. An especially request of these programs to improve the confidence between maintenence mechanics and the supervising level.
The „ERgoNomic Audit Programs” (ERNAP) is a tool that was developed originally in and for the USA. It shall combine the different audit-approaches (tools). Thereby it servs the assessment of the ergonomics (human engineering) of maintenance, overhaul and testing. Also the designers can use this method for ergonomc improved processes and systems. Forms and practices can be found in Lit. 19.1-20.
Figure "Maintenance resource management MRM" (Lit. 19.1-6, 19.1-16 and Lit. 19.1-20): Maintenance Resource Management (MRM) is engaged in technical influences (see also Fig. "Affects at safety due to maintenance"). It concerns rather so called „soft skills“, which affect the suitable behaviour under particular situations (see also Fig. "Human factors in maintenance and overhaul").
Importanf subject matter:
Communication (see chapter 19.1.1): the experience has shown that problems and failures during maintenance and overhaul stood in connection with deficits in communication. A typical situation occurs for example during a shift changeover (chapter 19.1.1). In this situation the unmistakable handover of the work/activities of high importance for the technical safety. Also the instructions and the feedback between the skilled labour/technicians and the supervision is determined by an optimal communication.
MRM passed historical several steps (Lit. 19.1-8). From training about communication in the cockpit (Cockpit/Crew Resource Management Training =CRM) to the communication between performing an supervising maintenance personnel to training courses for mechanics. Thereby at safety relevant activities, the attention/concentration should be increased and the individual cooperation behaviour improved.
To estimate situations is a precondition for right actions. For example this requires the identification and assess of a critical situation (Fig. "Direct communication influences"). At technical problems an adequate expertise is essential (chapter 19.1.3).
Teamwork and synergy affect particular the safety/quality and efficiency of labour. The ability to work in a team must be learned and must be appraisable. It describes as well the behaviour inside the team, as also against outer influences like the hierarchy.
Leadership based on trust and competence affect the communication positive. Arrogance and unfair reactions may have a negative affect.
Conflict behaviour: conflicts develop mostly in the line with particular situations, at least subjective estimated as critical. In such cases overeactions or unsuitable reactions are preprogrammed. This provokes misunderstandings with high problem potential. So it is of high importance, to identify and evaluate such situations and to handle them suitable.
Decision making: seldom has a decision in line with a problem elimination only a positive aspect. This is expecially true for the typical complex technical problems of the aero engines. Those are from experience influenced by several contributory causes. Mostly this deals with a compromise between the action in line with the remedie and thereby connected evils (e.g., costs, loss of prestige). With an approach according to the MEDA principle a systematic, comprehensible decision making can be expected (chapter 19.2).
Fig. "Risk increasing by disregarding human factors 1" and Fig. "Risk increasing by disregarding human factors 3" (Lit. 19.1-14 and Lit. 19.1-9 15): it came to this accident because of a faulty assembled safety clamp for the rear connection bolt of the engine nascelle towards the wing (sketches right , „1”). So the bolt could slip out of the bore of the connection. That lead zu the overload and the fracture of the still remained bolt of the front mount („2“) and to the detach of the nascelle. Five preceding cases (!) with a move of the connection bolts and a former parallel case became known from other airliners. All cases stood in connection with a faulty assembly of the nascelle connection.
In line of the investigation of this accident 18 questionings of the mechanics, inspectors and the management have been carried out. In addition came interviews of the inspectors of the supervisory authority for maitenance work at the operator. Deviations have been found, which were of interrest for the whole aviation industry. Additional informations have been gathered at other airlines, where similar problems had occurred. Important findigs of this case were (see Fig. "Risk increasing by disregarding human factors 3"):
Figure "Risk increasing by disregarding human factors 1": The maintenance personnel of the aircraft was not suitable trained and competent to achieve the necessary maintenance work and inspections of the heavy maintenance.This favoured faults.
It emerged that no regulary instruction/training in a class room took place.
The position of the director, responsible for education/training has not been assigned, his duty had been transferred to an other function. The training was carried out merely informal on site (on-the-job-training = OJT). Instead the lead mechanics have been responsible. This approach did also apply for the „General Engineering and Maintenance Manual” (GEMM) of the operator. Here inspectors and mechanics have been made specific familiar. Mechanics have been instructed when a training was required and the work gave time. New hired mechanics shared at a one-day introduction program that was engaged with basics.
Figure "Risk increasing by disregarding human factors 3": During the training the mechanics and inspectors had not the capability to identify an unnormal (nonroutine) status/situation of the task. The location could be labeled with a special Red Flag, to which a describing card was assigned. Questionings of the mechanics revealed understanding problems of approach and procedures.
A further problem was the operator specific EDP supported operations scheduling , documentation and monitoring system. A review in line with the accident investigation revealed plenty of difficulties in connection with this system. For example appeared cards two times and could so lead to a double execution of the task. The most frequent problems developed from deviations from the airplane maintenance manual as well as too few graphics and charts. This lead to an extensive adjustment effort with the airplane maintenance manual.
Besides the mentioned general problems of the education, organisation and documentation the investigations draw up else special problems in connection with the accident. All problems are compiled in Fig. "Risk increasing by disregarding human factors 3".
The lower sketch in Fig. "Risk increasing by disregarding human factors 1" shows deficits in the `working environment' during work at the mount of the nacelle. Neither the assembly platform nor the illumination or the deposition respectively storage of the extracted parts were according to the demands of such safety relevant work.
Figure "Accident caused by maintenance deficits" (Lit. 19.1-17 and Lit. 19.1-18): this accident has indeed nothing causal to do with the engines, hower is a impressive example for problems at maintenance and overhaul. It was the initiation of increased activities about the influence of the „human factors“. Hence it will be here covered more in detail.
The failure sequence began in the area of a rivet connection. Along the row of holes lengthwise the cabin wall at several positions fatigue cracks arose, after the additonal adhesive bonding of the riveted tension boom failed. This was intended for a fail-safe -behaviour of the connection, but showed production deficits. After the failing of the connection the shown failure occurred.
From the conclusions of the investigation report (Lit. 19.1-17) it can be resumed learned:
For the representative of an administration it's difficult to trigger improvements in maintenance programs of the operator outside the directive authorisation, alone by conviction.
The conclusion of the investigation about the main failure cause can be outlined as following:
The flight accident is explained by the failing of the maintenance program of the operator. Therefore dangerous detachments of the adhesive bonds and fatigue failures, which triggered the breakdown of the cabin had not been identified.
19.1-1 „Human Factor, Definition im Projektmanagement”, Internetzeitschrift www.Projektmagazin.de, 17.09.2006, page 1.
19.1-2 „Human Factors“, „Wikipedia” Lexikon im Internet, 17.09.2006, page 1-6.
19.1-3 Dan Wagner, „Human Factors Design Guide“, Final Report and Guide, January 15, 1996, zu beziehen durch: National Technical Information Service, Springfield, Virginia (USA) 22161.
19.1-4 „Aviation Maintenance” human factors research. FAA, „Flight Deck/Aviation Maintenance“, www.hf.faa.gov/flight.htm. 17.09.2006.
19.1-5 Aircraft Accident Report NTSB/AAR-96-03 (PB96-910403), „Uncontained Engine Failure Valujet Airlines flight 597 Douglas DC-9-32, N908VJ, Atlanta, Georgia, June 8, 1995”. (3680)
19.1-6 Weiterbildungsangebot „Magister Ludi“, Via Natale Battaglia, 8-20127 Milano Italy., www.magisterludi.com/aviation.
19.1-7 Weiterbildungsangebot „Maintenance Resource Management (MRM)”,Anbieter „Global Jet Stervices Inc“, www.globaljetservices.com.
19.1-8 J.C.Taylor, M.S.Patankar, „The Role of Communication in the Reduction of Human Error”, Seite 1-27.
19.1-9 G.J.Fogarty, R.Saunders, R.Collyer, „Developing a Model to Predict Aircraft maintenance Performance“, Proceedings des „10th International Symposium of Aviation Psychology”, May 3-6,1999, Columbus Ohio.
19.1-10 M.S.Patankar, J.C.Taylor, “Posterior probabilities of causal factors leading to unairworthy dispatch after maintenance”, „Journal of Quality in Maintenance Engineering“, Vol.9, Issue 1, May 2003, pages 38-47.
19.1-11 V.Mancuso, “Moving from Theory to Practice: Integrating Human Factors into an Organization”.
19.1-12 J.C.Taylor, “Evaluating the Effects of Maintenance Resource Management (MRM) in Air Safety”, Report of Research, SUC Project # NAR003, Vol.9, July 31, 2000, page 1-96.
19.1-13 J.C.Taylor, M.S.Patankar, “Four Generations of Maintenance Resource Management Programs in the United States: An Analysis of the Past, Present, and Future”, „Journal of Air Transportation World Wide”, Vol.6, No.2 - 2001, pages 1-32.
19.1-14 NTSB Special Investigation Report,, „Maintenance Anomaly Resulting in Dragged Engine during Landing Rollout, Northwest Airlies Flight 18“, Narita Japan, March 1, 1994, PB94-917006, NTSB/SIR-94/02, page 1-61.
19.1-15 Jim Hall, NTSB Safety Recommendation,, „Reply to A-04-218 through -223”, January 11, 1995, page 1-5.
19.1-16 E.G. Tripp, „Human Factors in Maintenance“, Zeitschrift „Business & Commercial Aviation” April 1999, page 82-87.
19.1-17 NTSB, Aircraft Accident Report NTSB/AAR-89/03, „Aloha Airlines, Flight 243, Boeing 737-200, N73711, Near Maui, Hawaii, April 28, 1988“, 1089, page 1-214.
19.1-18 Maintenance and Ground Operations Systems - Customer Support, The BOEING Commercial Airplane Company. 1987, „Aloha Airlines Maintenance Evaluation” Appendix I, Attachement to M-7360-87-3169, 1989, pages 215-244.
19.1-19 E.G.Tripp, „Human Factors in Maintenance“, Zeitschrift „Business&Commercial Airlines”, April 1999, page 82-87.
19.1-20 Civil Aviation Authority (CAA), CAP 716, „Aviation Maintenance Human Factors (JAA JAR 145)“,First Edition 7.January 2002 , ISBN 0 86039 832 3, Appendix A „Definition of Human Factors ”.
19.1-21 B.Alexander, „Serious Flawed Security - An Object for Maintenance Malpractice?“, Zeitschrift „Air Safety Week”, 13. Dec. 2004, page 1-3. (3974)
19.1-22 W.K.Krebs, Information zum „Aviation Maintenance Human Factors Execution Plan“ , Oct. 15th, 2004, FAA ATO-P R&D Human Factors, , page 1-9.
19.1-23 G.Drury, J.Ma „Do Language Barriers Result in Aviation Maintenance Errors”, Proceedings of the „Human Factors and Ergonomics Society 47th Annual Meeting - 2003“.
19.1-24 Civil Aviation Authority (CAA), Safety Regulation Group, CAP 718, „Human Factors in Aircraft Maintenance and Inspection” (previously IVAO Digest No. 12), First Edition 24.January 2002 , ISBN 0 86039 836 6.