On a warm summer day, the greenhouse effect can easily raise the temperature from 90?F in the shade to more than 130?F in your cockpit. Unless you can cool yourself fairly quickly, you become so overheated in only a matter of minutes that your ability to think clearly, learn, and perform is severely degraded.
Because humans are warm-blooded, or, in technical terms, homeothermic, we maintain our body temperature at a core temperature of around 98.6?F. This temperature can vary up and down by an entire degree in the course of a day. If we exercise, our body temperature can go up, and careful studies of pilots have shown that if your core body temperature climbs by more than 2?F, your mental performance can drop by more than 20 percent.
To maintain peak performance on hot days our bodies work to maintain their optimum core temperature, and the body cools itself in a number of ways. First, we radiate heat from our skin. More than half of the heat we humans lose escapes from our heads. So, in the winter, we know to wear a warm, woolen cap, but in the summertime, we want to wear no more than a light sun hat to keep us in shade.
The second way humans lose heat is by evaporating sweat. We perspire, and as the water on our skin evaporates, it carries the heat away. This process also depletes the body's moisture, and it's not unusual for people to sweat more than a liter of body fluids an hour. If you're working in a hot, humid environment, two liters an hour of perspiration is possible. That is a lot of sweat! In fact, it is enough sweat to dehydrate you fairly quickly.
Dehydration means we do not have enough sweat to keep ourselves cool, and to keep our blood circulating. When we lack enough sweat to cool ourselves, heat exhaustion results. Go beyond this and you have heat stroke, a life-threatening emergency.
The body powers its cooling system by using salt to move sweat out. The pores in the skin use salt as a "pump" to draw water out of the blood. This is why sweat tastes salty, and why we lose salt and water when we perspire.
The body is actually very efficient at using salt, so we rarely need to take in more than we get by lightly salting our food. We do not need to take salt tablets. These tablets supply more salt than a human being needs for three or four days - even in the heat of summer. Salt tablets are fine for producing nausea and vomiting, but are good for little else.
More than anything, we need to constantly replenish the water our bodies use. If we perspire one to two liters per hour, we must drink the same volume of water for the body to keep up - and keep cool. During Desert Storm, troops carried two-liter bottles of water and were directed to empty them each hour. That meant that the average soldier could - and usually did - consume 24 liters of water per day. They perspired efficiently because the Arabian desert is not only hot, it's very dry.
In a humid environment, we don't perspire efficiently because the water can't evaporate quickly. It's harder for a body to cool itself in a hot, humid 90?F environment than in a dry environment at 100?F. The other trick is to keep air moving across your skin so the water evaporates. For those of us who fly sailplanes, no prop blast is available to cool us off on the ground until we finally get launched by a tow plane and can open a vent for ram air cooling.
Besides making sure that cool air blows across our face and neck so we can radiate and evaporate heat more efficiently, what else can we do to stay cool? We must replace the water our bodies use. We can drink water - plain, cool water.
What about sports drinks?
The original in this genre, Gatorade, is too concentrated. That is to say it contains too much salt and sugar. Many of the other sports drinks are not as concentrated, but in order to be palatable, they still have higher quantities of salt and sugar than are really necessary. This means they can dehydrate you more because you're adding more salt than water in proportion to what your body needs.
The idea is to replace the same amount of salt, sugar, and water the body loses to perspiration. The concentration of salts in the human body is only 0.9 percent. That's less than a teaspoon in a gallon of water. If you want to consume a sport drink, cut it in half with water. It doesn't taste nearly as good when it's diluted, but it cuts the amount of salt and sugar you're consuming.
Then we have caffeinated beverages such as soft drinks, tea (iced or otherwise) and coffee. You may want them in order to stay alert, but remember that caffeine is a diuretic. That means it forces your kidneys to produce urine and can actually dehydrate you while you are drinking liquid.
It's always a challenge for a pilot to fly and consume fluids at the same time because you frequently develop an urge to get rid of the excess fluids before you're ready to land. I'm aware that many pilots try to fly on the "dry" side so they can avoid having to use the relief bottle. But in the long run, a little discomfort from a filling bladder is tolerable if it means your body has sufficient liquid to keep your head clear. You can avoid the bladder problem by not drinking caffienated beverages before or during the flight.
When you carry a bottle of liquid with you in the airplane, fill it all the way to the top - or make sure it's vented. If you open that sealed container of hot liquid and there's air in it, you're going to find just how much the ambient pressure has decreased at altitude compared to the pressure inside the container. Most pilots make this mistake, but only once. Next time you'll carry an unsealed bottle.
Dress for the climate and your aircraft. In summer you'll perspire in the cockpit, especially before engine start and after shut down. So you want to wear clothing that will absorb perspiration and allow it to evaporate quickly. Cotton is good, and it also provides a degree of protection from fire that's much better than synthetic fabrics. The ultimate way to keep cool is a professional cooling vest that circulates chilled water through tubes in the vest.
All this reverses in the winter. Keep your head warm and you'll keep your body warm, too. Just one more thing. On your first cool-weather flight remember to make sure you have a carbon monoxide sensor - and a new one at that - stuck someplace where it's readily visible. Every fall we lose about a dozen pilots who turn on the heat for the first time only to find out the hard way that the airplane's heat exchanger cracked over the summer, allowing poisonous exhaust fumes to enter the cockpit.
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