by Dan Johnson

We all know that dehydration makes good pilots into bad ones, and causes accidents. But along with good ideas, misconceptions about thirst and hydration keep getting spread around, like bad jelly on good toast. "Thirst can't be trusted" tops the list. Like many misconceptions, it's partly true: thirst isn't a "fuel gauge." This is an oversimplification; well intended, but like all oversimplifications it fails to educate, and sometimes misleads, causing bad decisions. This common advice, given to pilots and athletes, assumes that people are dumb animals who need to be fooled into doing what's best. Such people exist -- we've all met a few -- but they're not the type who would read this article.

Thirst Can Be Trusted

The truth is that thirst can be trusted -- but you must know how it works. If you're happy with the idea that thirst can't be trusted, that the only way to avoid dehydration is to keep the pee-bottle full or the nappie wet, simply stop reading here and do something more interesting, like reviewing consumer ratings of adult diaper capacity or skin adhesives for condom catheters. But if you do believe this, please don't run around teaching it. It's like teaching that the best way to avoid stall-spin accidents is not to teach spins. It might "work," but it doesn't make a more proficient pilot.

OK, now that I have you upset, let's talk. In truth, thirst will not keep you ideally hydrated. Thirst is not a fuel gauge; it's a warning light. Thirst begins after we're already mildly dehydrated, it doesn't increase in intensity proportionately with our degree of dehydration, and it's relieved before we've fully rehydrated. But thirst can be trusted because it's consistent and reliable; if you understand how it works (read on) and pay attention to your own thirst, it can teach you how and when to hydrate.

When does thirst begin?

This varies from person to person. Studies of dehydration variously estimate that we are 2% - 3% dehydrated (i.e. we've lost that much of our body weight in water) before thirst begins. But it's not correct to make the obvious connection and assume that the average 170-pound pilot must lose 3.4 pounds of water before feeling thirsty. Last summer, when some of us weighed in and out at the gliderport, it was pretty clear that pilots can learn to detect thirst at 1%. Why is this? Are the researchers wrong?

Here's the key: In the scientific articles there's a lot of talk about percent dehydration, but a lot of silence about just what zero dehydration might be. You might think that your normal, comfortable status is a non-dehydrated situation. Wrong. In fact, it's not all that easy to define what is a "fully hydrated" human.

Our bodies are fancy soup, with a specific gravity of 1.010-1.012 (somewhat more dense than water). If the specific gracity of our urine is less than 1.010, we're trying to get rid of extra water -- we're over-hydrated. If it's more, we're conserving water and becoming dehydrated. People operate most of the time in water-conservation mode because it's inconvenient to urinate often. On the average, we run happily around, about 1% short of optimal hydration. The worst that happens in this state is a hard stool, not usually something to write to the editor of the local newspaper about.

This means that we can normally tank up with water equal to about 1% of our body weight without causing excessive, inconvenient urination. Since a pint is a pound, one or two pints of water early in the day can give us a head start on avoiding thirst (and dehydration -- remember, thirst means we're already dehydrated).

This also means that since thirst begins at 2% dehydration, it begins after a loss of 1% of our usual body weight. We're not conscious of the first 1% that we're always short of, and once we're thirsty we can get back to our mildly parched normal state by drinking water equal to 1% of our body weight.

The delayed onset of thirst is pragmatically useful: If we became thirsty as soon as we needed an ounce of water, we would be continually looking for water, a waste of time. If thirst were exactly proportional to the need, we'd be in agonies of thirst, a distraction, when we don't need to be.

Thirst is a reliable signal that we need to drink a pint to a quart (depending on our size) right now and to increase our rate of fluid intake

Because thirst begins after we are mildly dehydrated, we shouldn't wait until we're thirsty to begin hydrating. The few studies on dehydration and mental performance show that mental acuity and coordination are already decreasing at 1% dehydration, and are consistently and significantly decreased at 2%.

Is this clear? You lose your edge before you get thirsty. You can trust this. Thirst is a reliable sign that you are actually impaired, not a casual hint to take a slurp of water to wet your whistle. It's a sign that you should not get into an aircraft right now.

In the air, it's a message that large quantities of water should be drunk immediately, and if no water is available in the cockpit, it's time to land and solve the problem. (This is especially true if you're airsick -- water is not absorbed when we're nauseated.)

We can learn from thirst

Thirst is analogous to shivering: shivering is a sign that we are already hypothermic; to repeat: Thirst is a sign that we are already dehydrated. You can trust it: thirst reliably tells you that you have become dehydrated, that you've missed your goal of staying hydrated.

As we noted, thirst appears at about 2% dehydration. The actual level varies from person to person, and sensitivity to thirst decreases substantially in old age. We can train ourselves to suppress this and other body appetites, or to be more sensitive to them; we can discipline ourselves to respond to thirst.

We can also learn from urine color. Dilute urine made by the well-hydrated pilot is pale, white; concentrated urine made by the pilot flirting with dehydration is golden and sinks to the bottom of the toilet. As you empty your bladder before takeoff, think of this. If your urine is dark, drink now, a pint or two.

You can use thirst as a guide to learning how to hydrate. Thirst is a clear sign that your present hydration strategy isn't working and needs to be revised. Let yourself get dehydrated in safe conditions, such as during ground operations, and note the conditions under which your thirst appears: how hard were you working; how appropriate to the temperature was your clothing; what was the air temperature, the wind, the relative humidity; to what extent were you out in the sun? When you become conscious of thirst, ask yourself whether you might have been suppressing this appetite, whether there might have been some earlier inklings of thirst you might have picked up if you'd been paying better attention.

Lost Ability

Dehydration is a problem because physical work capacity and mental sharpness both decrease. At 2%, athletes have lost about 5% of their aerobic capacity -- not an issue if your only activity is to gently roll someone else's glider up to the flight line -- and coordination and thinking ability are substandard.

Interestingly, there are many studies of dehydration's effects on athletic performance, but few on mental performance. But enough have been done to know that our ability to do arithmetic, to recall words we hear, to trace a line quickly and accurately on a piece of paper, all are decreased at just 1% dehydration and steadily worsen as the water deficit grows (Figure 1). At 5% - 8%, you can probably keep walking, but you'll have trouble figuring out what direction you're going, and you surely won't be able to thread a needle or calculate a heading.

How Much to Drink?

Thirst should guide your rehydration strategy. If you weigh a hundred pounds, and you sense thirst at 2% dehydration, when you notice you're thirsty you're down a quart already. This should lead you to immediately begin rehydrating. Simple mental arithmetic should tell you how much to drink to be fully rehydrated, and how much to drink each hour to maintain hydration.

For example, if it took you two hours assembling gliders in the sun to get thirsty, and you plan to continue working as ground crew today, you need to drink a cup of water every thirty minutes to stay hydrated in these conditions in addition to the quart you need to restore yourself, to change yourself from a raisin to a grape. Double these numbers if you weigh 200 pounds, as I do. Yes! a pint every thirty minutes!

Or -- more important -- if the glider you were assembling and pushing 200 yards to the flight line was your own, you are now about to launch your glider in a dehydrated state. Instead of hooking up, delay your launch and rehydrate. If it's a busy day at the gliderport, the wait for a tow may be 45 minutes anyway; you can spend this time standing in the shade of the upraised wing, avidly drinking.

Estimate Water Needs

Does this give you some clue as to the water you should be carrying? On a hot day (or with vigorous activity even on a cool one) you will lose 300-600 ml (a cup to a quart) of water every hour. You might lose up to twice this much if you've donned heavy clothing for high-altitude flight. You may need to allocate one or two quarts of water just for the ground-operations phase in order to ensure that you don't scare the spectators with an interesting takeoff.

While we're aloft, we usually go up where it's cool or even cold; we don't exert ourselves much while flying, and the rate of sweating abates considerably, perhaps down to the minimum of about 1-2 ounces per hour. You can use this as an indication of the minimum amount of fluid you should consume during flight, about an ounce every half hour, with no thermal stress and no sensation of thirst.

We can be fooled

We need to be disciplined about rehydration, as thirst may be, under some circumstances, quickly slaked by small amounts of water. For example, when bicyclists were given water during a race and asked to estimate the volume of water consumed, they over-estimated their water intake tenfold -- when they drank a scant ounce, they said they had drunk more than a pint. Amazing. But in few studies have people been so dramatically wrong. The lesson is probably that when we're busy, distraction hinders accuracy.

In another study, people were dehydrated by depriving them of water for 24 hours; everyone rehydrated fully within 20 minutes after they were given access to water. In other studies, of athletes, typically about 2/3 of the water deficit was taken promptly after stopping, and the rest with the next meal.

Packing a lot of water is annoying; urinating aloft can be messy; condom catheters fall off, tubes kink, diapers leak, baggies spill, and our bladders can burst. We can suppress thirst consciously, and the symptoms of dehydration don't point to water. A friend of mine said after a week-long bike tour, "I get a little weak after a couple of hours; I don't know why..." His wife interjected briskly, "It's because he doesn't drink!" She was right. Physical weakness is one sign of dehydration. Unfortunately, a late sign of dehydration is impaired judgment -- including the idea that it's not necessary to drink yet.

When our body is working hard to conserve water, urine flow is scant. This may eliminate the need to collect urine during flight. But this is bad judgment. Your brain needs the water, and it is not hard to plan for urine collection.

Thirst Sensations Intensify

Thirst symptoms increase as we get more dehydrated, as illustrated in Figure 2. If you've suppressed thirst until it can't be ignored, you can use the intensity of your thirst as a reliable guide to just how important it is to not climb into that towplane, to get off the flight line, to devote earnest and full-time attention to sitting in the shade and rehydrating, or to consuming a large proportion of the water you brought with you on the flight. (You did bring water, didn't you?)

Let me suggest that if you are definitely thirsty during flight, and have less than a pint of water on board, you should seriously consider terminating the flight at the nearest safe landing field that has water. Thirst is a danger signal, an alarm, not a hint.

Weight Reveals Dehydration, not Moral Character

An excellent way to accurately estimate water loss is to weigh yourself. For about 25 bucks you can pick up a simple battery-operated digital electronic scale. (Don't get an analog scale -- they're too hard to read precisely.) Keep the scale on a hard level surface out of the sun, near the flight line (so pilots will actually use it).

Weigh yourself before you start the day's activities. Weigh yourself before you launch, to see if you're already dehydrated. Weigh when you begin to feel thirsty, so you know where your own thirst threshold is. And weigh at the end of the day, to gauge your success at keeping hydrated. Always weigh with the same gear, of course: holding that barograph in your hand only after a flight, and not before, will cause a delusion. Keep a log and a pen next to the scale, because we more or less forget exactly what that number was. (If your weight embarrasses you, lie consistently.)

Where Dehydration Occurs

Most of our water losses happen on the ground, before we fly. The folks most in danger from dehydration are the tow pilots, who work continually, in hot cockpits, at low and therefore hot altitudes. Next is the ground crew: the line boy, the SCUM, the pilots assembling.

The main reason pilots get in trouble with dehydration, I believe, is that we take off dehydrated, and then we don't correct the problem well enough. Once we get up in that cool air aloft, sweat losses diminish to a minimum, and we may lose as much moisture in our breath as from sweat.

Dehydration in the Cold

But it's important for pilots to know that in the cold, we will get dehydrated for another reason. When we get cold, the blood vessels in our hands and feet, arms and legs constrict to conserve heat. This makes the blood vessels over-full, and our kidneys, just doing their job, get rid of the extra. So when you get cold, even if you're already dehydrated, you'll lose water and some salt. Perhaps you've noticed that when you get cold, you soon have to pee -- this is why. There is no way, my friends, to avoid this cold-induced dehydration except by staying warm. And if it's really cold up there, you're not going to be able to wear enough to stay toasty warm.

This dehydration is not a risk while high aloft. In fact, it tends to compensate for the dehydration that you developed before you launched. If you hydrated aggressively on the ground and early in the flight, you lose much of it when you get cold. Nothing you can do about it -- this is how your body works. Set up the pee-collection device and use it.

The risk is what happens when you come back down. You warm up. Your blood vessels dilate again. They dilate tremendously if you come back down to a hot airport, especially if it's humid. But if you've been cold, the water you need to fill these blood vessels is in your nappie, in your bladder, or out the pee tube. It's gone, my friend, and the warmer you get, the worse off you are. The consequence is low blood pressure, low G tolerance, fuzzy thinking or uncoordinated flying, and excitement for the spectators.

The only solution is to rehydrate vigorously as you warm up, a cup at a time, not a sip at a time, with the goal of drinking at least a quart and possibly two during the descent.

If you ride the escalator up and down, getting cold, then warm, then cold again, and again warm, it's important to realize that the body's fluid volume contracts anew each time you go up, and you need to rehydrate each time you descend. (Maybe you should connect a mouth tube to those tanks in the wings...)

When to Stop Rehydrating

Last, you can trust thirst to vanish before you're fully hydrated. When people are given free access to water after dehydrating though physical activity, they reliably drink about two-thirds of what they need, and finish making up the deficit at their next meal. So to rehydrate well, drink until you're satisfied, and then, when your stomach feels comfortable again, drink half again that much.

Enjoy your soaring, and remember that thirst is a reliable idiot light. Use it wisely.


Good pilots have crashes.

No one intends to crash.

Most crashes are from an unexpected malfunction of the pilot, not the aircraft.

Learning how the body works while in an aircraft can help anticipate this, and

may prevent your next crash.

I've noticed that pilots enjoy learning how their machines work and how to maintain them; they learn about weather, memorize regulations and airspace rules, and love stories about flying. But they don't spend much time learning how their bodies function and how to maintain themselves.

There are sound reasons for this. First, a healthy body doesn't need much maintenance -- it works well without our having to think about it. Flying an airplane is not a natural action like flirting or picking your nose, so doing it well requires study and training. Second, aircraft are exotic objects whose operation and maintenance demand education. Each comes with a manual. Our body's manufacturer provides no manual. Experts in medicine and physiology have written many "manuals," but rarely for pilots without medical training.

Most accidents -- about 80% -- are due to pilot malfunctions, not to aircraft malfunctions or to environmental conditions. The soft squishy thing that operates the controls is a lot more complex than the machine it operates. An aircraft is just a prosthetic bird with a removable brain. What the soft squishy part understands about itself and how it maintains itself have a great deal to do with whether the life insurance policy gets paid off.

When a pilot does something truly hazardous, talk amongst the spectators tends to imply that the pilot was dumb, inadequate, untalented, or poorly trained -- different from the rest of us, who have never done anything dumb. Well, maybe we did, once, but it was an accident, an aberration; we've learned better -- we're beyond that now. I've never, ever, heard a spectator say, "Wow, that was scary! That's something I might do."

I'm going to write a series of pieces in which I hope to persuade you that you -- yes, you, with all your intelligence, experience, good training, sound judgment, and knowledge -- might do something that looks really, really, dumb some day. Not only that, there is a very good chance that if you fly enough, you will, without necessarily knowing that it's happening, get into circumstances in which everything will seem perfectly proper and in control but that are very dangerous. You may crash, confidently.

It's going to take more than one column, as there are many different ways in which you can do this to yourself. I think that this stuff is really interesting, and I promise to do my best to show you why. But my goal is to give you clues that you can use to know that your body is trying to tell you that the thin edge of trouble is closer than it seems.

Meanwhile, who am I? I'm a bald guy, over 50, a physician in western Wisconsin, a specialist in internal medicine, an FAA-designated senior aviation medical examiner, and a commercially-rated glider and instrument airplane pilot with a lifelong interest in aviation and weather. I've been gliding since 1985 and soaring since about 1992. Work and other responsibilities held me on the ground until in 1995 I was able to buy a Blanik L-13, and formed a little club around it. We did hundreds of autotow ground launches until 2000, but the club withered and I sold the Blanik to buy a Ventus CM, a big change of emphasis.

I'm fascinated by the construction and function of the human body. This has led me to study pilot physiology (fortunately very closely related to human physiology), and to present talks at the last two SSA Conventions on subtle pilot incapacitation, "How to Crash Confidently," which I hope will be a continuing series on how we often become incompetent while being confident nothing is wrong.

Daniel L. Johnson, MD, FACP, Sr. AME
Copyright (C) 2002, All rights reserved.