Stress as defined by the American Heritage Dictionary[1] refers to “A mentally or emotionally disruptive or upsetting condition occurring in response to adverse external influences and capable of affecting physical health, usually characterized by increased heart rate, a rise in blood pressure, muscular tension, irritability, and depression.” or “A stimulus or circumstance causing such a condition or ’A state of extreme difficulty, pressure, or strain.’

In terms of Human Factors and human performances; Stress can be defined as ” a set of psychological and physiological responses to external and internal stressors. [Wickens & Holland (2000 12)].

Stress has also been defined as ” a disturbing physiological and psychological influence which produces a state of severe tension in an individual” [Stokes & Kite (1994 11)].

Theoretical frame

Stress is the human body’s response to demands placed upon it. These demands can be either physical or psychological (pleasant or unpleasant) in nature [FAA (2007 4)]. Stressors are the key factors in which stress will be created, but with its aptitude varying from one another. Stressors may include internal factors such as fatigue, frustration, anger or external forces such as heat, noise, vibration time factors. These stressors results in a number of different responses including physical responses such as reduce capacity in individual performances or physiological response such as increasing heart rate, sweating etc. Stress is inevitable and it can have several positive or negative effects which vary from each individual response to it.

Stress is part of our daily lives whereby the amount of stress each individual experience will somehow affect each individual performance ability [Green, Muir, James, Gradwell, & Green, (1996 5)]. Stress allows oneself to believe the demands placed upon them and how they cope with the demands. With perception differing from each individual, it is each person perception of his abilities rather than his actual abilities which contribute to one’s stress experience.

Stress often has three manifestations. Firstly, stress will create a phenomenological experience in terms of emotions as a consequence of stressors. Closely linked, a change in physiology is often associated with it. This my be in terms of a short-term change such as increasing heart rate during a first attempt to fly an aircraft[Hart & Hauser (1987 6)]. Thirdly, human information processing will be effect either improving its performances or degrading it [Wickens & Holland (2000 12)]. These three manifestations will have many effects being classified and categorised as Chronic or Acute differentiating from external or internal factors.

Two Classification for Stress

There are two main classifications for stress.

Chronic Stress

Chronic Stress refers to stress that has a long term background of demands caused by both internal and external factors which mainly consist of physical, environment and personal issues. Chronic stress is accumulated through time and are often mild in terms of stress level [Earl (2006 2)]. Yet chronic stress can be an add-on effect with acute stress which will intensify its disastrous effects.

Environmental Stressors (External Factors)

Environmental stressors are environmental conditions which will cause physiological stress to individuals being affected. Individuals with different tolerant levels will response differently to the effects accommodating themselves to the demands of its stressors [Earl (2006 2)]. When one feels stress by one condition, it will naturally reduce their tolerance over other conditions. Here is a list of environmental factors which will create chronic stress for individuals.

Most individuals obtain optimum performance in temperature of 20 to 23 degree Celsius. Therefore, when temperature rises to 30 degrees or more; or falling to 15 degrees or less, people tend to feel uncomfortable and performance decreases drastically [Earl (2006 2)]. Thus in aviation context, aircraft cockpits are maintained at about 22 degrees in order to allow pilots reduce the risk of temperature being a stress factor.

2.Noise and Vibration
Noise at low levels will be able to simulate individual to increase in their performances. Noise may be used to help to maintain performance levels during boredom periods or fatigue stages. Likewise, it can be use to masked over distracting sounds [Green, Muir, James, Gradwell, & Green, (1996 5)]. For example, Audio warnings on aircrafts during cruising flights will ensure pilots awareness of its situation or any problems which might occur. However, excessive noise can be annoyance and it will lead to cardiovascular or other physiological conditions.

Vibrations on the other hand will result in physical stress to be created due to its vibration frequency. 1 to 4 Hz will often result in interference with individual’s breathing, 4 to 10 Hz will often lead to chest and abdominal pains on the body, 8 to 12 Hz will cause backache while 10 to 20 Hz of vibration will affects individuals physical body with headaches, eyestrains, pain on organs and body tension [Green, Muir, James, Gradwell, & Green, (1996 5)]. Therefore, pilot’s seats in cockpits are often modified with thick cushions, and sheep’s fur coat in order to minimise any vibration which might be a stressor affecting piloting performances.

3.Low Humidity
Humans are mostly comfortable at humidity levels ranging from 40% to 60%. Low humidity will result in dryness on individual body, resulting in irritation of any sort. Such consistent effects will eventually evolve in to a stress factor if not properly dealt with [Stokes & Kite (1994 11)]. For example, on board an aircraft, humidity level can be as low as 5% due to the hot bled air from the engines under going a moisture removal system prior to pressurizing the cabin. Thus this might result in urinating problems or water retention problems which will lead affect one’s stress level. Such discomfort might therefore affect one’s performance [Wickens & Holland (2000 12)].

Personal Stressor (Internal Factors)

•Domestic related stress

Such stress is produced by living conditions and individual factors or experiences. As stated above, chronic stress are accumulated, therefore stress at home or through individual’s perception can be added towards the stress people felt at work. Thus performance of individual will gradually decrease once the stress level exceeds one’s optimum level [Green, Muir, James, Gradwell, & Green, (1996 5)]. Factors such as home relationships, bereavement, financial issues or time management issues will likely to affect one’s concentration and further affect physical abilities [Stokes & Kite (1994 11)].

•Work related stress

Work related stress can be produced from the operating environment in our workplace or through the relationships with colleagues or the management. In many occupations such as a professional pilot, long term quantitative overload will result in stress symptoms. Without proper stress management, these stressors will degrade ones performance, reducing safety and health management of the company. High labour turnover, absenteeism, high accident rates can be accredited as symptoms of ‘organizational stress’ [Green, Muir, James, Gradwell, & Green, (1996 5)].

Acute Stress

Acute stress refers to stress that are caused by the task, situation or environment immediately rather then cumulative. This can be similar to being an Active Failure where situation must be dealt with on impact rather then latent failures (similar to chronic stress) which can be build-up and hidden for a period of time. For instance, pilots experiencing a sudden windshear or tailwind during landing; after then Decision Height (DH). Acute stress can be said to have high level of intensity as it requires a direct response and reaction in one’s performance.

Let’s us just recall one incident when a cat/dog or a pedestrian rushing across a busy road, most of the people will immediately avoid by either slowing down, braking hard, steering left or right. This response comes immediately though the incident just happens sudden. After avoiding the event, most of us will realise that our heart rate increases, taking deep breaths or having a ‘heart popping out’ effect. All these physiological effects can be associated with the acute stress we received during that short incident. But, this effect will vary from one individual to another due to our past driving experiences. A new driver will perhaps have a larger extent of this effect or even cause an accident as he/she will not be able to handle the situation. While for experience drivers whom had experience such incidents many times, he/she will be more confident in the ability to deal with the incident, therefore less likely to experience the high stress levels that prevented him/her from the ability to think or act clearly.

Effects of Stress on Performance

Adequate amount of stress can be desirable and can produce effects such as motivation. But, negatively it may degrade one’s performance. During low levels of stress, performance will improve as individuals are not expecting to perform any difficult task. This might create motivation which will allow more attention mechanisms to be more active [Green, Muir, James, Gradwell, & Green, (1996 5)]. As stressors or stress increases, the ability to perform tasks reliably and accurately will change. Depending on individual, optimum stress levels will allow one to perform optimally in more complex task due to his/her experiences and familiarity of the task. Once an individual is being over stress, performances will decrease and errors will be inevitable. Due to the high stress level, individuals tend to restrict and focus attention towards the primary demand which each of us perceive. Errors occur as information which might be important might be omitted due to the selective information process one will perform during stressful situations. Factors such as distraction from other stressors, or working memory loss during intense information processing will further degrade one’s performance level [Wickens & Holland (2000 12)].

After undergoing a high stressful situation (rejecting takeoff due to one engine failure), each individual will naturally changes his/her perception. Therefore, when an individual faces encounter another similar situation, the stress level subsequently decreases and its abilities to cope with such situation increases [Wickens & Holland (2000 12)].

Models of Stress

Stimulus Based Approaches [Earl (2006 2)]

This approach was derived from Sir Charles Symonds which he believed that by concentrating exclusively on external events or conditions (stressors) rather then on the individuals, this would be able to determine the amount of stress each individual experience. Stress is being viewed as a cause to performance. By manipulating the stressors which the researcher labelled as priori, stress level can be control. This research ignored the fact that individual differences and perception will have effects of stress levels and it has omitted the emotional aspects of human experiences which is the hallmark of human stress.

Response Based Approaches [Jones (1991 7)]

In contrast, response based approaches focuses on the reaction of stress rather then the external environment or stressors. Stress is viewed as a symptom rather then a cause. Yerkes Dodson Law for example has been historically the most extensive study for this approach. In this sense, such approach is curiously non-psychological as they acknowledge little or no role in the cognitive aspect such as perception.

Transactional Approaches [Endler (1975 3)]

This approach has been described as a radical redirection in stress research where it conceptualised stress as inhering neither person nor environment but the transaction between the two. It emphasizes the role of cognitive appraisal in human stress response in which a more psychological factor has been enforced as compared to the other two approaches. Such approaches acknowledge the subjective nature stress and emphasize the mental processes which mediate individual’s reactions providing a more substantive consideration.

Supporting evidence

Authors Green, Muir, James, Gradwell, & Green, (1996) [5] stated that among pilots the main sources of stress identified were:

•Lack of control or disruption for their personal lives due to poor rostering
•Anxiety and stress on flight checks
•Home to work interface
•Career stability and achievement
•Loss of career (medical)
•Insufficient flying resulting career back-lock and loss of additional income
•Lack of responsibility and decision making
•Interpersonal problems
•Management and organisational problems
•Domestic status
•Fatigue and flying patterns

It has been noted that the authors’ describe that theses areas stated will distort the pilots human body in five areas.

Physiological effects with short-term changes in responses to a sudden or unexpected event, will lead to nervous system breaking down with symptoms such as dryness of mouth, sweaty palms, breathing problems or increasing heart rates.
Health effects are the most frequently observed due to stress for the pilots. Sleeping disorder, sexual disorders, allergies, colds, flu etc, are signs whereby pilots are overloading their capability to perform at their optimum level.
Behavioural effects which includes impulsive behaviour, nervous laughter, restlessness; excessive drinking and smoking etc are syndromes of stress.
Cognitive effects have been observed as pilots are lacking of concentration, forgetfulness and inability to determine priority.
Subjective effects such as anxiety, anger, aggressiveness, depression are recorded during this study.

Although all pilots being studied behave differently, it is the personality, the desirable impact and the nature of the stressors or event which determine the level of stress each individual are experiencing.

1. Stress levels in pilots at different phases of flight

A study conducted by Lee and Liu (2003 [8]) suggests that pilots' stress levels are highest during take off and landing. The study involved ten male pilots undergoing the analysis in a Boeing 747-400 flight simulator. Heart rate of the pilots were observed and was assumed to be correlated to physiological workload. Mean heart rate was observed to be at 88.6 bpm during landings and 83.2 bpm during take offs. Recommendations were made on individual stress management techniques and trainings.

2. Effect of stress on pilot's skills and abilities

A research carried out by Rebok, Li, Baker, Grabowski and Willoughby (2002 [10]) suggests that stress have a great impact on pilots' skills, abilities and health. The study invovled 1310 commercial pilots of age between 28-59 (mean age 45.5) and mean flight time 111,992 hours undergoing a 13-item survey namely Aviation Experience Survey. Using 5 point Likert scales and yes/no responses the data was analysed. Pilots who reported more anxiety or stress during flights states that their performance has been affected by stress and consiquently there is a great impact on their flying skills and abilities. However, the study suggests there is no evidence of correlation between stress and age.

3. Stressed controller behind Brazilia Crash

The CENIPA (2008 [9]) report on Brazilia Crash over the Amazon on September 29, 2006, concluded that the key factor behind the crash was the stressed controller. When Gol Transportes Aereos flight 1907 collided with Embraer Legacy 600 flight N600XL midair both the aircraft were at flight level 37000 feet. The controller was unable to take corrective actions to which NTSB report refers as "loss of effective air-traffic control". The controller did not take corrective actions to re-activate the transponder of N600XL nor he took appropriate actions to regain the communication with N600XL. NTSB concluded that the controller at Sector 7 was with inadequate skills and was unable to handle the stressing situation.

4. Stress behind John F. Kennedy Jr's Airplane Crash

The National Transportation Safety Board (NTSB) declared the crash was caused by the pilot's failure to maintain control of his airplane during a descent over water at night, which was a result of spatial disorientation. The NTSB also suggested that John F. Kennedy Jr.’s marriage may have contributed to a source of stress (Psychological stress) by the time of the crash. Kennedy had spent the final three nights of his life apart from his wife at a New York City hotel before the crash. Additionally, Kennedy's magazine, George, was in serious financial trouble.

Refuting evidence

See also

Stress Coping Strategies , Stress Management

Way forward (to do list)

1. American Heritage Dictionary. (2005).Definition of Stress. Information Retrieved on 16th September 2008 at Yahoo Official Website:
2. Earl, L. (2006).190.216 Aviation Human Factors Study Guide NZ: Massey University
3. Endler, N. S. (1975).// A Person-Situation Interaction Model for Anxiety,// in Spielberger, C. D. & Sarason, I. G. (Eds).Stress and Anxiety. Vol:1 John Wiley & Sons, USA: New York.
4. Federal Aviation Administration (FAA). (2007).Instrument Flying Handbook. USA: Jeppesen, Boeing Company.
5. Green, R. G. Muir, H. James, M. Gradwell, D. & Green, R. L. (1996).// Human Factors for Pilots (2nd Ed).// U.K: Ashgate.
6. Hart, S. G. & Hauser, J. R. (1987).In-flight Application of Three Pilots Workload Measurement Techniques. Aviation, Space and Environmental Medicine, 58, 402-410.
7. Jones, D. M. (1991).Stress and Workload: Models Methodologies and Remedies in Farmer, E (Ed.), Stress and Error in Aviation. Avebury Technical, Aldershot, Hants.
8. Lee, Y. H., & Liu, B. S. (2003). Inflight workload assessment: Comparison of subjective and physiological measurements. Aviation Space and Environmental Medicine, 74, 10, 1078-1084.
9. Brazilian Air Force (CENIPA). (2008). Final report: A-00X/CENIPA/2008. Aeronautical Accident Investigation and Prevention Center: Brazil.
10. Rebok, G. W., Li, G. H., Baker, S. P., Grabowski, J. G., & Willoughby, S. (2002). Slef-related changes in cognition and piloting skill: A comparisom of younger and older airline pilots. Aviation Space and Environment Medicine, 73, 5, 466-471.
11. Stokes, A & Kites, K. (1994).// Stress. Fatigue and Performance in Aviation.// Avebury, Aviation.
12. Wickens, C. D. & Hollands, J. G. (2000).Engineering Psychology and Human Performance. Upper Saddle River, USA: Prentice-Hall Inc.

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