For now, we're going to focus on the physiological category — Page 134, Lesson 204

I want to walk you through the categories of stress factors that we need to understand as professional pilots. We've already talked about what stress is and how it builds up, so now let's look at how we classify the different sources of stress — what we call stressors. The stressors we discussed earlier can be broadly subdivided into five categories: physiological, cognitive, non-professional, imaginary, and organizational. For now, we're going to focus on the physiological category. Physiological stress factors break down into two parts. First, there are external physiological factors — things like noise, temperature, and vibrations. These are sometimes called environmental stress. Second, there are internal physiological factors — things like hunger, fatigue, and lack of sleep. Let's start with the external physiological factors, and I'll limit our discussion to those stressors specifically associated with aviation — the ones you'll actually encounter on the flight deck. On the flight deck, there are a number of possible physical sources of stress: heat, vibration, noise, and so on. As we've already seen, the effects of stress are cumulative. That means the negative consequences from one source of stress are likely to lower an individual's resistance to other forms of stress. So if you're already dealing with heat, you'll be more vulnerable to noise, and vice versa. Let's talk about heat and cold first. A comfortable temperature for most people in normal clothing is around 20 degrees Celsius. Above 30 degrees Celsius, your heart rate increases, your blood pressure goes up, and you start sweating more. Below 15 degrees Celsius, the individual becomes uncomfortable and may lose feeling and some control in the hands — especially for fine muscle movement, which is critical for flying. The body's reaction to extreme heat is discussed later in this chapter, so we'll come back to that. Now let's look at noise. Interestingly, when you're bored or fatigued, some noise can actually raise your arousal levels and increase performance — it can wake you up a bit. But excessive noise, which means above about 90 decibels, will disrupt performance. Specifically, it causes three things: disrupted concentration, degradation of information being received in the working memory — which leads to an increase in workload — and an increased number of crew errors. So when designing aircraft warnings for system failures, care must be taken to ensure that the aural warnings should attract attention but not startle the crew. That's a deliberate design principle. Next, vibration. Vibration may affect the whole body, or specific parts of the body. Any vibration will cause fatigue and can affect both visual performance and motor performance — that is, your ability to see and to move precisely — leading to uncomfortable symptoms. The frequency of the vibration determines which parts of the body are affected, and the magnitude of the vibration determines the severity of the symptoms. Let me walk you through the specific frequency ranges and what they do. Between 1 and 4 Hertz, vibration interferes with breathing. Between 4 and 10 Hertz, you get chest and abdominal pains. Between 8 and 12 Hertz, you get backache. Between 10 and 20 Hertz, you get headache, eyestrain, pains in the throat, speech difficulties, and muscular tension, plus degradation of visual acuity — that's your sharpness of vision. Resonances of 30 to 40 Hertz applied to the whole body will interfere with human responses. If that same frequency range is applied to the head, although no physical damage is done to the eye, there is a possibility that visual acuity may be degraded. The resonance of the skull itself occurs at a frequency of approximately 1 to 4 Hertz, and this may also affect vision. Crew seats with anti-vibration mountings help to reduce these vibration levels. Helicopters can vibrate in all three axes — that's longitudinal, lateral, and vertical — at frequencies related mainly to rotor speeds, gearbox speeds, and engine speeds. Helicopter pilots will therefore be particularly susceptible to this stress factor.