Skyfaring: A Journey With a Pilot (19 page)

Planes also alter their vertical paths in response to the wind. At each point in a flight, a plane has an optimum altitude, based primarily on the aircraft’s weight. As fuel is burned the aircraft’s weight reduces and the optimum altitude rises. So in an ideal sky—one free of other airplanes and variations in the wind—a plane might climb continuously throughout its flight, until it was time to start the descent for arrival. But vertical differences in the strength of winds often overwhelm this ideal altitude, and so we may climb up early, to an otherwise inefficient altitude, because it is even more valuable to us to find the heart of a jet stream. Or we may descend to avoid one. On a recent flight from London to Miami, the Atlantic headwinds were so strong and wide that there was no way to avoid them horizontally. So, after climbing to 37,000 feet for the first few hours of flight, we descended to 29,000, surrendering nearly a quarter of our initial altitude, more than enough to make our ears pop. Later we climbed again, and then we descended; a kind of porpoising, a vertical wayfinding in the ocean of air.

Our flight paperwork and the onboard computers help us make such calculations. But we also have paper tables in the cockpit called the
wind-altitude trades.
The name recalls the trade winds that swept ships to the New World, ships that would then arc northward to catch different winds and currents home, in an echo of the aeolian geometry that pilots and flight planners deploy every day above the same waters. Another pilot at a different altitude might ask us what wind we are experiencing—our
spot wind,
it’s sometimes called—which will help them decide whether to climb, descend, or stay right where they are.

As with altitude, an airliner is also always calculating the most efficient speed for the moment-to-moment conditions of flight. You might think, as I did, that this most efficient speed would be the same regardless of whether the plane is experiencing a headwind, a tailwind, or no wind. But in the tangled calculus of the air, the damage wrought to an aircraft’s efficiency by a headwind is greater than the gift the same wind speed would offer, were it a tailwind. The stronger a tailwind, the less time you benefit from it, while the greater a headwind, the longer you are subjected to it. For this reason, the flight computers will suggest accelerating to what would otherwise be a less efficient speed to minimize the time we’re exposed to a headwind. While in a tailwind the computers will suggest slowing down; to linger awhile with the wind at our back.

Wind is so critical to flight calculations that some manuals refer to a new kind of distance: air miles, or
air distance.
Air distance adds the effect of the wind to the distance along the ground that the plane flies over. As a unit of length, it is as fluid as the air itself; the high-country mile. If there is any headwind or tailwind at all, and there almost always is, then a flight from London to Beijing, for example, will have a different air distance than a flight that follows precisely the same route in the opposite direction, from Beijing to London—and both these air distances will differ from the distance along the ground.

Occasionally, while still on the ground, after I load the route into the computers but before I enter the expected winds, the flight computers flash up a warning that the plane does not have enough fuel to complete the planned route without dipping into its sacrosanct final reserve of fuel. Then I tell the computers something about the winds over the world; the computers consider it and are satisfied. Though the miles on the ground are unchanged, though we have not even started to move, the air distance has been remade by the wind.

—

Richard Bach, the author of
Jonathan Livingston Seagull,
once titled an essay “I’ve never heard the wind.” Few pilots today ever do. Though they make up one of the natural world’s most dramatic physical presences and determine both the path and length of all our journeys, the winds exist largely as numbers on the cockpit’s screens, where a diminutive symbol (only an arrow, sadly, not a cockerel) turns like a weathervane. The wind’s dimensions are a regular topic of conversation among both pilots and flight attendants on a trip, as casually central to our daily life as delays on train lines or morning traffic jams. We do not mind a headwind on the outbound journey, because it often means a faster return home the next day. But we do not hear or feel the wind directly.

Jet streams occasionally produce turbulence, particularly at their boundaries. But often they are so smooth they suggest something like the opposite of their true dimensions. When it’s a little bumpy I remember that I am flying at 600 mph, in air that is moving at 200 mph, yet the plane is steadier than a car on a dirt road. Often the fastest winds are as smooth as glass. When the computerized wind readout shows even a routine wind for an airliner to experience, 50 knots for example, I think that anywhere but Iceland, perhaps, such a wind on the ground—58 mph—would make the news. We would struggle to stand against it, and yell to be heard.

Maritime cultures, such as those around the Mediterranean, still deploy many archaic names for winds—the Bora, the Sirocco, the Khamsin. Today, England has one named wind, the Helm Wind, known for occasionally shrieking down the western slopes of the Pennines in Cumbria. But it’s easy to imagine that the so-called Protestant wind that blew the Spanish Armada away from England might have become a general term for the east wind (“Popish” winds blew, too, a century later, to delay the arrival of William of Orange). America retains a few named winds, such as the Santa Anas of southern California and the Chinook, and even a fictitious wind, the Maria, from the Gold Rush musical
Paint Your Wagon
(from which the singer Mariah Carey gets her name and its pronunciation). Hawaii once had hundreds of named winds; whether you could list a place’s winds and rains there was a test of whether you were truly a local.

Imagine looking up one morning and seeing a Nile or an Amazon in the sky, in a slightly darker blue, a shimmering, partly reflective navy hue, twisting and curling in the north sky over your hometown, and migrating to the southern sky by the time you go out for lunch. Aside from the sun itself, such air rivers would be the most dramatic feature of the earth or the sky. We have so matter-of-factly fashioned the souls of cities, of literatures, of whole civilizations from rivers—the Danube, the Mississippi, the Yangtze. One manufacturer has even named its jet engines for British rivers—the
Spey,
the
Trent,
the
Tay
—to contrast their smooth flow with the unevenness of piston engines.

We might have made something more of the jet streams, have worshipped and built a complete mythology around their image, had our prescientific eyes been able to see them. Flying, though one day it will feel as old as sailing does today, is still new to us. It’s not nearly too late to style the sky’s high winds, to scatter up the seeds of an aerial heritage.

Though the jet streams run in sweeping, largely east–west bands around the earth, we might choose to give their different sections different personalities, as in England the names of some rivers and streets may change midstream. The high winds over the continental United States might be renamed the Wiley Posts, or the Post Winds, for Wiley Post, the one-eyed American aviator who was the first pilot to fly solo around the world, and who is partially credited with the discovery of the jet streams. Post died in Alaska in 1935, aged only thirty-six, in the crash that also killed the humorist Will Rogers. There’s an old-style aviation beacon named for Post and Rogers on top of the George Washington Bridge in New York, a memorial in light that once also served as a marker for aircraft taking up their westbound routes. “Our flight to Raleigh,” a pilot returning from the west might say, “is just under four hours tonight, thanks to the Post Winds.” Meanwhile, the winds over the North Atlantic, which blow so reliably and steadily from North America toward Europe, we could dub the Allied Winds, to help us remember they were best documented during the transatlantic resupply efforts of the Second World War.

It is a joy of my job that I’m occasionally tasked with drawing the winds. In the cockpit of the 747 many of our paper maps have been replaced by electronic versions. But there remains a set of disposable maps that cover the world, which are sometimes called
progress charts.
As a child I saw carbon copies of these mounted in the passenger cabin of the aircraft—the path of the plane, a charcoal-gray zigzag over blue sea and yellow land. I still have one or two that I asked the cabin crew for on those long-ago flights.

These charts have space for the date and flight number, and for the pilots to write their names and ranks. Completing a progress chart, I feel as if in another age I would be crouched over a table, in heaving seas, an oil lamp or heavy brass navigational instrument on the table to hold the paper in place. I like to draw the steady lines between distant waypoints in thick green or blue ink, to sweep over countries, mountains, oceans as only a pen, or an airliner, can do. Although we have other, computer-generated maps that show the predominant winds along our route, some pilots still draw the winds onto the chart, along with the forecast areas of turbulence.

A pilot who does not wish to waste paper might save the chart for the flight home, plotting the return route and winds in different colors from the outbound ones. But if we ever have visitors to the cockpit after landing, especially kids, we will gladly give them the chart to take home. Someday even these last of our paper charts will be removed from cockpits; they will become relics of the early jet age. For now they remain, these maps of transience and air in every sense—hand-drawn charts of one day’s unique and wind-sculpted journey and of the great unnamed rivers of the sky that hindered us or blessed us and carried us on our way.

—

If altitude and distance are not straightforward concepts for planes flying high, neither are temperature and speed. Generally speaking, the temperature of the air drops as you climb, in the same way that mountains are usually colder than lowlands. An airliner climbs to high lands indeed, to a bitter world where temperatures routinely drop to minus 70 degrees Fahrenheit.

Temperature affects many things on an airplane—the efficiency of the engines, and the formation of ice that can disrupt both the engines and the airflow over wings. Measuring temperature, however, isn’t straightforward. In cold climates meteorologists warn about wind chill, about how the wind can make an icy day feel colder still. Airliners go so fast that they experience not only wind chill, but also what might be called wind heat. Fast air hits a thermometer and is brought to a sudden halt, compressing and warming greatly—an effect my brother is familiar with from pumping up the bike tires I’m always asking him to help me properly repair. This effect, part of what’s called
ram rise,
means thermometers in the slipstream generally report a temperature that’s much higher than the air’s ambient temperature.

On the Concorde, ram rise could heat the nose and the leading edges of the wings to over 212 degrees Fahrenheit—hot enough to boil water at sea level, and to cause the plane to stretch by around 10 inches in flight. On a 747 moving at less than half the speed of a Concorde, the effect is more modest. But a thermometer in the slipstream often still reads substantially higher than the actual temperature of the air—often at least 50 degrees Fahrenheit higher. The temperature that airliners experience at speed is therefore called
total air temperature,
or
TAT
(rhymes with “hat”). It is distinguished from s
tatic air temperature,
or
SAT,
which is the temperature the air around the aircraft would be had it not been compressed.

It’s reasonable to think of SAT as the real or actual temperature, and to find in the difference between SAT and TAT an inexact if pleasing comparison to a tenet of quantum physics, in which the act of measurement may distort or alter just what you are hoping to measure. The higher sensed temperature, though, is not a question of measurement. The forward-facing parts of the plane—such as the leading edges of the wings and the nose—are known, appropriately, as
stagnation points.
These entire surfaces experience the same heating effect as the thermometers do.

This heat, though problematic for the designers of supersonic aircraft, can be useful. Consider the fuel in the wings of an airliner. Fuel cools dramatically during a long flight in the high cold, but it cannot be allowed to cool too much. Typical freezing points of fuel are around minus 40 (a temperature that requires no
C
or
F
to follow it; it is the intersection of the Celsius and Fahrenheit scales) or colder. The static temperature of the air outside—the ambient temperature, shown perhaps on the moving map screen—is often colder than this. But the TAT, the experienced, wind-warmed temperature, is much higher. Indeed, nothing suggests the speed of airliners and the physicality of air quite like the fact that if the fuel starts to get too cold, the simplest way to warm it up again is to fly a little faster.

If flying overturns our everyday definitions of distance, altitude, and temperature, it scrambles our intuitive sense of speed most of all. In daily life we have only one idea of speed: how fast we move over the ground. If you ask pilots how fast their plane is going, they might pause before replying. They might say it depends.

In the sky there are four important concepts of speed. First is
indicated airspeed.
This is best imagined as the speed at which you’d guess you were traveling if you stuck your hand out the window and felt the air against it. In all but the most limited circumstances, indicated airspeed bears little resemblance to
true airspeed
—your actual speed relative to the air mass around the plane. Third is
ground speed,
your speed over the earth, which is perhaps nearest to our terrestrial understanding of motion, though it is irrelevant to everything about an airplane that has to do with the air, and often differs from indicated and true speed by hundreds of miles per hour. Finally there is
Mach,
the true airspeed of the plane relative to the local speed of sound.