Skyfaring: A Journey With a Pilot (18 page)

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 signal 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.

The designers of radio altimeters must also consider the deceptively simple question of where, exactly, zero is. That is, where the airplane itself begins. Given the radio altimeter’s use near landing, it makes sense to define a height of zero not by the bottom of the fuselage, where the radio altimeter itself is likely mounted, but by the bottoms of the wheels when the landing gear is extended. But this isn’t straightforward either. When a 747 comes in to land, the landing-gear legs—shock absorbers, essentially—are longer because they are not compressed. Additionally, the plane itself is nose-high, tilting upward. (Many airliners point upward not just in the climb, but throughout the cruise and much of the final descent, too, an upcast geometry of flight that partly explains the brake pedals on meal trolleys, the subtleties of flat beds, and why it is almost always harder to walk toward the front than the back of a plane.)

Because in flight we want to know the height of the lowest point of the plane, the radio altimeter starts counting not from the altimeter itself, or from where the wheels end when they are on the ground, but from roughly where the wheels end when there is no weight on them, when they are flying freely through the air.

At landing, though, the nose lowers and the weight of the plane presses down onto the landing gear, compressing it. Now the 747’s ever-truthful radio altimeter finds itself below where it understands the ground to be in flight. And this is exactly what it reports to us when we first walk onto a parked 747, adjust our seats, turn up the brightness on the screens, and start our takeoff preparations: that this airplane is 8 or 10 feet below the earth’s surface. Even the most sophisticated measure of our height above the earth’s surface is inconstant; it depends on whether we are coming or going.

—

One October a friend and I went to Iceland. We drove clockwise from Reykjavik, and late one night, several days and autumn storms later, we rounded the island’s far southeast corner. In England the weather makes perfect sense to me if I imagine, even in the center of London, that I am on deck, at sea; if just past the newsstand or coffee shop on the far corner of the street I picture a pounding, Turner-caliber seascape. Driving in Iceland I repeatedly had the sense not that we were at sea—though the sea was almost always nearby—but that we were flying. I’ve never been more aware of the wind’s effect on a car. Merely staying on the road required a near-permanent force to be applied to the steering wheel, left or right depending on the direction of the road and the rain-laced crosswinds. Each gust knocked us halfway out of our lane.

Planes, too, must sometimes be briefly driven down a road in strong winds, on the runway at takeoff or landing. A plane on the ground can be steered with either the wheels or the flying controls—or with both. As the speed of a plane on the ground increases and more air flows over its
flight controls,
these grow in effectiveness, as a hand-wing extended from the window of an accelerating car might. This gives the unexpected and accumulating sensation that during takeoff you are simultaneously driving in the air and flying along the ground. In Iceland, after we stopped one night, I thought how much easier the driving would have been if Icelandic rental cars were equipped with some airplane-style controls, a rudder, perhaps—some recognition of and accommodation to their unintended life in the air.

Wind, to the earthbound observer, suggests a local event against a default background of stillness, a breeze passing over a fixed point on earth; over us. But higher up, the air itself, the reference frame of flight, is almost always in motion. Once I leave the ground I no longer think of wind as a flow of air that passes over us; rather, it carries us whole, as a river or an ocean current.

If you could produce a view of the earth showing only those things moving faster than 100 mph—a map of speed, a worldview made only of motion—you would see a few trains, and plenty of motorists in Germany, the lines of their velocity sketching out the network of autobahns. You would see many planes, materializing as they accelerated for takeoff and vanishing from the speed planet when they landed and slowed. Mostly, though, you would see the
jet streams,
the high winds that ring the earth. “Where are the jets tonight?” I might ask a colleague, or comment that: “All the way over we’ve been fighting a strong jet.” The
jet
in jet stream is said to derive from the streaming quality of the winds, not the aircraft we most associate with them. But these winds were only properly understood after we started to fly, and today the name is a pleasing convergence of the engine type and the wind patterns that both enable and shape our greatest journeys over the planet. The fastest jet stream I have flown in recently was 174 knots—a tailwind, thankfully. (Knots are nautical miles per hour; a nautical mile is equal to 1.15 regular, statute miles, so 174 knots is around 200 mph.)

There are many factors that determine which path a plane will follow between two cities. Sectors of airspace may be congested or temporarily closed, often due to military exercises, and the varying navigation charges that countries impose mean that routes longer in time or miles can nevertheless be more cost-effective. En-route weather conditions are another consideration. But in the absence of such factors, the primary task that flight planners and pilots face is to navigate the high winds; to harness them by hitching a lift on a sky river that is flowing the right way or to actively avoid them, fleeing the tempests that would enormously slow an airplane’s progress over the world.

Over the North Atlantic, which so many planes cross en route between North America and Europe, a new set of wind-optimized routes is charted each day, one set for the westbound flights and one for the eastbound. Each day the westbound planes may arc far to the north, swinging high onto the Labrador coast to avoid the west-to-east winds that blow further south. That night those same planes may return to Europe in a momentous arcing, a vast and more southerly swoop, seeking out the heart of the eastbound jet stream that only hours earlier they went so far out of their way to avoid. Often the winds lever the paths of opposite-directioned planes away from each other so effectively that the route of an airplane flying from London to Los Angeles, say, will never once cross the path taken the same day by a plane from Los Angeles to London.

A
great circle
is the so-called straight line between two places, as a string would connect them around the surface of a sphere. (Indeed, in my airline’s office, to one side of a bank of flight-planning computers stands a globe with such a string still attached to it, a relic of the age when great circles might still be plotted by hand.)

On flights from northern Europe to western North America, passengers are occasionally surprised to see from the moving map screen how far north we are—often over Greenland, where one of the planet’s most reliably spectacular panoramas of sea, ice, and mountains awaits those travelers lucky enough to have a window seat. The shape of these routes is often attributed to the marvel of great circles. But westbound planes often fly still further north than the great circle to escape the howling headwinds that would devour their time and fuel; the same winds for which that night’s returning pilots will be grateful, when they fly well south of the great circle, seeking the sky where the eastbound air tide is strongest. Occasionally I’ve started a flight from London to the west coast of North America on airways heading slightly northeast around a congested bit of airspace, rather than northwest. The overriding task on such a day is to escape the westerly wind. Only then do we take up our course.

While planes may move to or from these rivers of air, the rivers themselves transform and wander over the earth. They strengthen or weaken, twisting and drifting away from their typical haunts in long, languid sweeps across the cold miles of sky. The optimal route—the shortest and most fuel efficient—between two cities changes constantly, and so the route that an airliner flies between them can vary dramatically from one day to the next. Some days when I fly to New York, the last I see of Europe is Northern Ireland; the next week, flying to the same city, the farewell takes place over Land’s End. When the jet streams drift far from their traditional homes, skies that are normally quiet will fill with jets for a few hours or days, their pilots drawn like surfers to the most favorable sky-swells. The natural forces of the world shape even these, our most technologically advanced journeys; they guide our migrations with an all but biological simplicity.