All pilots are limited by their training and by the nature and extent of their experience. But that is another subject. What we will talk about here is common to all pilots, regardless of experience. Pilots are human, and are all subject to the limitations of the human mind and body.
We have spoken about the almost linear relationship between altitude and air pressure below 10,000 feet (1 inch of mercury = 1000 feet). Here is the rest of the graph. The relationship sure isn’t linear above 10,000 feet.
So – in a nutshell – what is the effect of altitude on a human being?
The measure is TUC – Time of Useful Consciousness. It depends on the partial pressure of oxygen in the air the person is breathing. Here is a table:
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Numbers to remember:
- 10,000 feet
- 13,000 feet
- 30 minutes
Exam aside, what is important here? You will feel the effects of hypoxia above 10,000 feet. Your performance will be affected.
Another important point is that individuals vary widely in their symptoms. You could become giddy or sleepy. You could become intoxicated and all-powerful. Needless to say, all of these symptoms are detrimental to your performance as a pilot.
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The AIM is your reference to most exam questions about human factors, notwithstanding TC’s other publications on the subject:
- TP12863 – Human Factors for Aviation – Basic Handbook
- TP12864 – Human Factors for Aviation -Advanced Handbook
You see how the AIM presents euphoria as one of the symptoms. This slide also mentions that hypoxia is divided into four different types:
- Carbon Monoxide
- Alcohol Enhanced
What is going on here? As we have seen, the red blood cells distribute oxygen to where it is needed in the body. How much each red cell can carry depends on the partial pressure of oxygen in the air being breathed, so at higher altitudes (less pressure) less oxygen is absorbed.
But – for example- should there be an exhaust leak, or a leak in the heat exchanger (for cabin heat) – carbon monoxide (CO) can enter the cabin air. Haemoglobin (red blood cells) picks up CO 200 times more readily than it picks up oxygen. Carbon monoxide poisoning is insidious and deadly.
Another physiological mechanism that can keep oxygen from being absorbed is hyperventilation, which involves a close relative of CO – carbon dioxide. We know CO2 from bubbly drinks. CO2 is also our own human exhaust – what we breathe out and plants breathe in. It happens that our bodies regulate breathing by the necessity to offload CO2 – not by the oxygen we need, as you might expect.
Why is this important to pilots? Because if we get overloaded we tend to breathe faster than necessary – to hyperventilate. The remedy is to breathe oxygen if possible, but always to breathe slowly and deeply. That’s why, if we’re afraid or angry, we are told to take a deep breath.
Then there is alcohol-enhanced hypoxia, or hypoxia-enhanced drunkenness. This is another reason pilots should not fly after taking a drink.
You see here how the effects of alcohol and hypoxia are additive. If you drink a scotch in an airliner (8000 foot cabin) it will feel like two drinks. It’s a cheap drunk.
Drugs range from just as bad as booze to worse to much worse. Not a good idea.
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Here are a couple of easy exam questions, if you can remember which is which:
- Anaesthetics – 24 hours
- Blood donation – 48 hours
Notice the recommendation is that pilots should not give blood.
Looking for traffic is easy, right? Just look out the window.
Well, yes – but there is a bit more to it. We have to know the limitations of the human eye.
You have probably heard of the rods and cones. Rods are retinal receptors that are very sensitive, but cannot detect colour. Think of cat’s eyes. Cones sense colour, but need more light.
In the human eye, the distribution of rods and cones creates one kind of blind spot, and the point where the retinal nerves converge to form the optic nerve make another.
The Fovea has a high concentration of cones. It is at the center – the focus – of our visual field. The high concentration means we have sharp vision. That fact they are cones means we have a blind spot at night.
Where the optic nerve joins the retina, we have a blind spot all the time.