Skyfaring: A Journey With a Pilot

by Mark Vanhoenacker

  Or you might think of air not as length or sphere but as depth. Here, again, there is truth and comfort in the natural analogy with water. Evangelista Torricelli, the inventor of the barometer, framed this in a 1644 letter: “Noi viviamo sommersi nel fondo d’un pelago d’aria.” We live submerged at the bottom of an ocean of air. Ralph Waldo Emerson, too, would speak of our enveloping air-sea, a few centuries later, in “this ocean of air above…this tent of dropping clouds.” There’s a particular kind of airport weather report known as a surface actual: the latest dispatch from the surface of the earth, from the bottom of the air-ocean.

  When you put your mouth over an empty plastic water bottle and inhale, the bottle collapses. Not, as we may think, because your inhalation pulls the sides of the bottle in, but because you remove the air that held the bottle’s shape against the crush of the atmosphere. The two Sumo wrestlers of the atmosphere press equally on the inside and the outside of the bottle, and the bottle holds its shape, immobilized; when the inner one is removed the bottle falls in on itself. The bottle of water I open in an airplane at high altitude crinkles and compresses when I descend, as it would if I plunged it deep into the ocean. Under our aerial sea the heft of the sky presses down on us, as water does on deep-sea creatures, or on us when we dive too deeply. We walk along the bottom of our air-ocean, as unaware of our water as David Foster Wallace’s young fish. Occasionally we board a plane and swim-fly upward.

  The depth of the air: I have flown over London on the fifth of November, and I have flown across the United States, from end to end, sea to sea, on the Fourth of July. The height of fireworks above our backyards and barbecues is nothing, really, in the towering scheme of the atmosphere. Each perfect burst has the scale of a coin at the bottom of a pool, or a tiny fire-flower, as fireworks are known in Japanese, underfoot.

  When we pass directly over a radio beacon at high altitude, the navigation computers show our distance from the beacon as zero. But we can also display the raw data from these receivers, and at the moment we consider ourselves to be at a beacon, the data may still show us 6 or 7 miles away from it. Away—as in above. Imagine sailing over the deepest ocean and passing above a watery light on the sea floor, its glimmer fading as it rises through the volume. The typical altitude of an airliner is about the depth of the Challenger Deep, the deepest known point of the oceans.

  I think of somewhere that is 7 miles from my home. It would take two hours’ brisk walking, or seven minutes’ driving at 60 mph, to cross the distance down to the surface of the planet. The air is as nothing, and yet there is so much of it.

  We may also gain a sense of the air by considering the force it exerts. I turn up the music and extend my arm from the window of a fast car on a warm summer day. When I change the angle of my hand my entire arm jumps or dips excitedly, almost beyond resistance. My hand pushes down on the air—like a wing, like the skis of a waterskier racing over a lake. I turn my hand so my palm faces fully forward, and my arm snaps back, landing against the window frame, as if a waterski or an oar had turned inadvertently, to catch the full force of the racing water.

  From the rear cabin, passengers, not pilots, have a clear view of the little panels—there may be several—on the back of the wing that swing up or down as the plane turns. These are ailerons—little wings. The aileron was the creation of the English classicist and inventor Matthew Boulton (his forward-looking 1868 patent was titled “Aërial Locomotion Etcetera”). Consider what happens when on one wing the aileron dips and on the other wing the aileron rises. It’s not quite the full story, especially on modern fly-by-wire aircraft, but in the simplest terms we can imagine that on the wing where the aileron dips down, it deflects the air downward even more forcefully, effectively increasing the lift on that side of the aircraft. The whole wing rises in response. The opposite wing, where instead the aileron rises up, creates less lift, and so that wing lowers. One wing rises, the other falls. This is banking or rolling, part of how a plane turns.

  Imagine a bike or car that you could easily steer with small alterations to the angle of your hand extended in the wind—by “warping the gale,” by “blading the wind’s flank,” as Hart Crane (“What marathons new-set between the stars!”) put it. This view from the passenger’s window seat—of the wing and the world in their proper arrangement, and the whole sky vessel rolling in response to the gentlest stirring of small panels—is not available to the pilots of many large planes, though there is no better picture of the invisible air.

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  It is an irony of flight—particularly of modern high-altitude, high-speed flight—that while an airliner inhabits the air even more fully than a boat inhabits the water, it quickly leaves most of that air behind. It’s true that the atmosphere is a desperately thin band of air around the earth—thinner, comparatively, than the skin of an apple, as warnings about air pollution often remind us. But the greater shock is how quickly the atmosphere attenuates, even within this thin skin. Air is not distributed evenly throughout the atmosphere. Unlike water, which is barely compressible, the air accumulates at the bottom of its ocean; it piles up under its own considerable weight.

  If I picture the earth from space again, I might remember that it is about 8,000 miles in diameter. To climb 3.5 miles—about halfway up to a typical airliner’s cruising altitude—is to have barely left the earth at all. Yet at this altitude about half the earth’s air is already below. You could still breathe here though, albeit with difficulty—most people climb Mount Kilimanjaro, which is nearly 4 miles high, without using oxygen tanks. Higher up still, at a typical cruising altitude of 7 miles, around four-fifths of the world’s air is already below the airplane. Airliners do not yet enter space. But in terms of escaping the atmosphere that sustains everything we know, they get us most of the way.

  Such a rapid fading of the atmosphere is related to one of aviation’s more curious revelations about the air: that altitude itself is a fluid concept.

  Even before a plane takes off, there is nothing simple about altitude. The earth is not a perfect sphere. Its true, misshapen character must be approximated for purposes of navigation, and whichever magisterially monikered model is used—World Geodetic System 1984, for example—must be carefully noted on our charts. Altitudes may be referenced to mean sea level, but this, too, is only an approximation; sea level depends on the tide, on the season, on which side of the Panama Canal you are on.

  The elevations of airports vary hugely. Mexico City’s airport, for example, is around 7,300 feet above sea level; Amsterdam’s Schiphol stands below sea level. Pilots may joke that a flight from Mexico City to London is downhill, and so it is. Even airports themselves, even individual runways, which we imagine to be necessarily flat places, are markedly three-dimensioned. In Dallas, the official airport elevation is 607 feet, but the elevation of one runway’s threshold is nearly 100 feet less. In Mongolia, at Ulan Bator’s airport, the elevation difference between the opposite ends of the same runway is more than 200 feet. That is nearly the height of a twenty-story building, or a 747 standing on its tail.

  As creatures of the air, it’s fitting that planes calculate their altitude by measuring air pressure. The air lies most heavily on places that are lowest, the places that have the most air piled above them. A barometric altimeter equates high air pressure—lots of air weighing down—with low altitude. The air, we’re told, is as weighty and real as books stacked on my outstretched hand. On the ground the altimeter feels the weight, we might say, of ten books, and converts this reading to an altitude. Then, as a plane climbs, there is less air above it, and fewer books. The altimeter senses less air weight, less air pressure, and reports a higher altitude.

  But there are problems with this simple equation. No device can be perfectly accurate, not least when it is mounted to the outside of a moving airplane. In addition to such instrument errors, another problem arises from how altimeters convert air pressure to altitude. They use a formula known as the standard atmosphere. It is an ave
raged model of the ideal sky, an air-Esperanto, a paradigm of how altitude, pressure, and temperature typically relate to each other on our home planet. But the actual conditions on any given day will never exactly match the standard atmosphere.

  These errors may be quite large. Imagine you are on top of a mountain in autumn. An altimeter registers the weight of air pressing down upon it and accurately equates this with an altitude of 10,000 feet. But on a winter’s day, in the same situation, the cold will cause the air to become denser and sink. More of the atmosphere’s air will pile up below the mountain, and less will be above the altimeter to weigh down on it. This decreases the pressure around the altimeter. All the altimeter knows is that there is less air-weight, and so it reports an altitude higher than its true altitude.

  So even in sophisticated airliners, pilots must manually apply cold-weather corrections to the heights of mountains. We might say that we’d normally wish to pass above a certain 10,000-foot mountain at 12,000 feet; but it is so cold out, we decide to treat the mountain as if it were 12,000 feet, and overfly it at 14,000 feet instead. This gives the all but inescapable impression that granite grows in winter—that when cold falls over the land, mountains rise further into the sky, until spring comes and they descend.

  A more fundamental problem for altimeters is that air pressure varies in each individual place as both time and weather pass. It also varies between different places on the earth. Imagine an airplane parked on the ground as a low-pressure system arrives over an airport (generally, high pressure is associated with fine weather, low pressure with poor). The altimeters sense less air weighing down from above. Who knows if a lower-pressure system has arrived, or if the plane has started to climb up in the sky? Not the altimeter.

  Similarly, a high-pressure system moves in, and there is more weight of air on the altimeter. Is it weather, or has the plane descended? The altimeters can’t tell. It’s a regular occurrence to board an airplane that has spent the night at an airport at sea level, say, and for all the altimeters to claim that I am well underground, or already, as far as the altimeter knows, flying slowly upward into the sky. If the pressure is changing quickly, then even though we are parked at the gate, utterly stationary both vertically and horizontally, our displayed altitude will rise or fall before our eyes.

  The anomalies caused by such variations are dealt with by adjusting the altimeter to the local weather conditions; we give it a new starting point so that it can calculate our altitude from the hour’s pressure more accurately. One of the first tasks to be carried out when we sit down in the cockpit is to obtain this pressure setting. As we dial it in, the altimeters spin contentedly in response, and the plane, for all it knows, rises from beneath the earth or descends toward it. In making these adjustments in the first minutes in the cockpit I feel a mild sense of mental exhalation, a relief that at last the air and ground of a morning are properly arranged.

  Controllers check that we have the correct, most current pressure setting. Not only is an accurate pressure setting crucial to each pilot’s awareness of their height above the ground, but the safe vertical separation of aircraft from each other depends on all the planes in a given area referencing their altitude to the same pressure setting—the same model of a given hour’s air. When the air pressure changes, air controllers may announce the change over the radio—to “all stations,” i.e., all aircraft. When such a pressure change is given all nearby pilots dutifully adjust their altimeters. The displayed altitudes in all the planes approaching Heathrow or Atlanta or Dubai then all change at once, harmoniously flickering upward or downward together. These pressure settings are so important that we have formal procedures to ensure the multiple altimeters on an aircraft are all set correctly. If they are not set to the same pressure the plane may flash a message to us: “BARO DISAGREE” (baros, meaning weight—of the air).

  In my first year of commercial flying at least a dozen captains recommended the 1961 book Fate Is the Hunter and two gave me copies of it. Written by the aviator Ernest Gann, it contains many eye-watering tales from a former age of aviation. In one story, as their ice-shrouded plane descends toward mountains, Gann asks his captain, as if it is an ordinary question, whether it’s now the right time to purchase some altitude by throwing the passengers’ luggage overboard. Later Gann describes a harrowing descent in fog over water. He’s trying to come down sufficiently low to find Iceland, where he has no choice but to land, without going so low that he hits the ocean, which he is unable to see. His air pressure–based altimeters are functioning normally but, because he does not have a local pressure setting, he has no idea how high he actually is. In the end a colleague dangles a cable from the back of the plane and waits to feel it snag on the churning surface of the North Atlantic. “When you feel a tug,” Gann orders his colleague, “yell your head off.”

  Such stories make clear the importance of adjusting altimeters to local pressure when you are flying near the ground (or water). But even more surprising, perhaps, is that in the high, long hours of flight between airports, airline pilots abandon these local corrections entirely. We switch the altimeters to standard, a pressure setting derived from the standard atmosphere, that universal model of the earth’s air. In doing so, we shrug off the actual weather of the day—the hour-to-hour, place-to-place vagaries of the real-world atmosphere.

  To ignore local air pressure, of course, is to ignore our true altitude. And this is just the collective inaccuracy that high-flying airliners embrace. Whatever the altitude shown on the screens in front of you in the passenger cabin, whatever is displayed on the altimeters in the cockpit, the plane is almost certainly not at that altitude, because the pressure of air on the earth immediately below you is not known and, even if it were—from the weather report of a nearby airport, for example—our altimeters are not set to it.

  Even more curious is that airplanes following an altitude referenced to the standard atmosphere collectively and continuously adjust their measure of wrongness—gently climbing or descending as the true pressure around them changes with time, and as they move across the world into different realms of weather. It was a memorable moment of my training when I realized that a plane flying at 35,000 feet is unlikely to be at the same altitude as another plane, elsewhere in the world, whose altimeters also show it to be at 35,000 feet; or that if a plane could somehow hover in one place, precisely maintaining its standard idea of 35,000 feet, it would in fact slowly rise and descend with the weather.

  You might think of an ocean, of all the boats across its vast expanse rising and descending on their local swells; the simultaneous localness and global interconnectedness provided by the water. All the boats are on the surface, though their true elevation varies. An altitude referenced to the standard atmosphere is like such a surface: a membrane of air, pressed with indentations and textured with rises, shimmering invisibly over the aerial imperfections of the world and the air that lies on it. The world’s high-flying planes follow these both vertically and horizontally; from one moment and place, into other moments and other places.

  The high altitudes displayed in the cockpit are thus so detached from true altitude that they are termed flight levels, not altitudes—a distinction lost to both the moving map in front of passengers and every altimeter in the cockpit. Flight levels, then, though invalid to the extent that we colloquially equate them with true altitudes, are just what we might expect of an industry that works on a single time zone. They are both a leveling and a fiction; a globalization of the sky. Such a system—though its embedded inaccuracies may be surprising, and though some newer airliners allow pilots to call up a display of GPS-derived altitude—is both safe and purposeful. Many aspects of an aircraft’s performance are referenced to the standard atmosphere, and a shared, fixed altimeter setting ensures that nearby airplanes are properly separated from each other.

  There is a final idea of altitude that airline pilots must learn—radio altitude or radar altitude. Radio altimeters bounce a radio signa
l off the earth and calculate their height from the amount of time it takes for the signal to return. The radio altimeter only cares about how many feet of measurable space are directly below it—a figure that it often announces to us out loud in the cockpit. At low altitude their accuracy circumvents the vagaries of air pressure and the varying elevations of the hills surrounding airports, and in the vertical dimension, at least, radio altimeters partly replace the eyes of pilots during automatic landings. Radio altimeters are so precise that they must account for the time a signal takes to travel within the wiring on the aircraft itself. They are extremely reliable; though curiously their radio eyes can lose lock—lose traction, over certain kinds of ground cover such as blowing, long wet grass (a problem more for the pilots of helicopters than of 747s).

  The radio altimeter is the most precise measurement we have of our distance from whatever is immediately below us. But its very precision creates further conundrums. Sometimes at high altitude it will catch its reflection not on the ground but on another airplane. Though our air pressure–based altimeters show we are at 38,000 feet, the radio altimeter may faithfully announce “ONE THOUSAND” in the cockpit when we overfly a plane at 37,000 feet. That’s a number we expect the radio altimeter to register just before landing, when its interrogations are returned to it not by another airplane crossing under our radio shadow high over Mali or Missouri, but by the approaching earth.