environmental stress.
• Internal physiological factors (hunger, fatigue, lack of sleep etc.).
Figure 7.4 Some flight deck stress factors
External Physiological Factors
Discussion will be limited to those stressors associated with aviation.
Flight Deck External Physiological Factors
There are a number of possible physical sources of stress; heat, vibration, noise etc. As we have seen, the effects of stress are cumulative and the negative consequences of one source are likely to lower an individual’s resistance to other forms of stress.
Heat and Cold
7
Stress 7
Stress
Below 15°C the individual becomes uncomfortable and may lose feeling and some control in the hands, especially for fine muscle movement.
The body’s reaction to extreme heat is discussed later in this chapter.
Noise
When bored or fatigued, some noise can raise arousal levels and increase performance.
Excessive noise (above about 90 dB) will disrupt performance and cause:
• Disrupted concentration.
• Degradation of information being received in the working memory, leading to an increase in workload.
• An increased number of crew errors.
In designing aircraft warnings for system failures, care must be taken to ensure that the aural warnings should attract attention but not startle the crew.
Vibration
Vibration may affect the whole body, or specific parts thereof. Any vibration will cause fatigue and can affect both visual and motor performance, leading to uncomfortable symptoms.
The frequency of the vibration will determine which parts are affected and the magnitude of the vibration will determine the severity of the symptoms.
Frequency Effects/Symptoms
1 to 4 Hz Interferes with breathing.
4 to 10 Hz Chest and abdominal pains.
8 to 12 Hz Backache.
10 to 20 Hz Headache, eyestrain, pains in the throat, speech difficulties and muscular tension plus degradation of visual acuity.
Resonances of 30 - 40 Hz applied to the whole body will interfere with the human responses. If applied to the head, although no physical damage is done to the eye, there is a possibility that visual acuity may be degraded.
Resonance of the skull itself occurs at a frequency of approximately 1 - 4 Hz. This may also affect vision.
Crew seats with anti-vibration mountings help to reduce the levels.
Helicopters can vibrate in all three axes at frequencies related mainly to rotor, gearbox and engine speeds. Helicopter pilots will, therefore, be particularly susceptible to this stress factor.
Stress
7
7Stress
Low Humidity
The air conditioning system of a modern aircraft draws air from outside the aircraft via the engine’s compressor. At airline operating altitudes the temperature may be as low as -30°C to -55°C. At these temperatures the air is very dry, with a relative humidity as low as 5%.
For comfort man requires a relative humidity of 40% to 60%. At low humidity the individual becomes uncomfortable due to drying out of the mucous membranes of the nose and throat.
Eyes become sore as the tears evaporate rapidly and the tear ducts dry. Water vapour is added to the cabin environment during respiration, but as the air is continually replaced this will have a limited effect in increasing the cabin humidity. It would be possible to add water to the cabin air but the weight penalty of extra water to be carried is not considered commercially viable.
It is not advisable to take on a lot of extra fluids in these circumstances but drink only enough to maintain comfort.
Note: Humidity in the cockpit typically varies between 5% and 15%. Thus flight crew should drink sufficient fluids in flight to avoid dehydration. During flight, the relative humidity in the cabin is similar to a dry summer climate or to being indoors in the wintertime.
Caffeine and alcoholic beverages actually contribute to dehydration.
Dry air may cause irritation of the eyes especially if contact lenses are worn and these may have to be removed.
Dry air can also aggravate allergies or asthma.
The humidity control system within some aircraft may alleviate this problem.
In general, human performance is poor in an environment which is humid, regardless of the ambient temperature. Surroundings which are both dry and warm are most conducive to high performance.
Extreme Temperature Stress Factors
The human body is extremely sensitive to heat and cold and functions efficiently only over a remarkably small temperature range. The normal oral temperature is considered to be between 36.1°C (97°F) and 37.2° (99°F). Physical and mental performance starts to become significantly impaired at an internal body temperature of about 38°C. Apart from the skin and fat, which both act as insulators, the body has mechanisms, controlled by the ANS, which endeavour to cope with the change of body temperature and maintain equilibrium (sweating and shivering for example). When the body, however, is exposed to extreme temperatures with which these internal homeostatic mechanisms cannot cope, it reacts violently.
Extreme Heat Stress
Once the blood temperature rises to approximately 41°C (106°F), the self-regulatory systems of the body can no longer cope and the effects of extreme heat are:
• Excessive sweating leading to fast depletion of body fluids and electrolytes.
• This dehydration leads to a further rise in body temperature, thus exacerbating the situation.
Typical symptoms are: muscle cramps, giddiness, and fatigue.
7
Stress 7
Stress
• Cell damage - especially within the brain.
• Heat stroke.
• Coma.
• The body loses water through the skin, lungs and kidneys.
Should the blood temperature rise to approximately 43°C (110°F), death will result.
Note: Generally speaking the mind and body need an average of 2 weeks to acclimatise to a hot and humid environment.
Extreme Cold Stress
If the core temperature drops to approximately 35°C shivering declines and eventually ceases.
The effects of extreme cold are:
• Uncontrollable shivering and an associated need for more oxygen. Around 34.5°C, the shivering that will have started earlier will tend to cease.
• Cell damage - especially of the brain.
• Sleepiness associated with a feeling of contentment or apathy.
• Circulatory impairment and degradation of the sensory nerves.
• Severe damage to the skin and tissues (frostbite).
• Coma.
• Death.
It is important to stress that the effects of exposure to extreme temperatures are not restricted to the more dramatic conditions described above. Smaller temperature variations within these limits can have a detrimental effect on a person’s ability to perform a task.