Skyfaring: A Journey With a Pilot (10 page)

Food, even, mirrors the pace of the turning world. Places in the sky take on the quality of a clearing at the bend of a hiking trail where I stop to eat lunch, marking a particular and regular point in the journey and my appetite. When I imagine Las Vegas, how it looks from the air, I think of it as a quiet spot in the sky that I associate with sandwiches and coffee, because it’s where—or when—I often have a snack before the busy descent into Los Angeles begins.

—

I’m a student pilot, near the beginning of my flight training in a small single-engine plane. I’m flying alone, somewhere northeast of Phoenix. I’m lost.

I am navigating, or trying to navigate, visually. I have a detailed chart that depicts mountains, roads, settlements, radio masts. I match what I see on the world below to the chart, and what the chart leads me to expect next I match onto the world, back and forth, back and forth, between the two. But the afternoon has grown much hazier than forecast, and for the first time I experience one of the effects that can suddenly make this kind of visual flying difficult. I can see straight down through the haze, but anything even slightly ahead or to the sides is obscured, an effect familiar to anyone who has ascended a skyscraper on a misty day.

A pit forms in my stomach when I realize I can no longer connect anything I see through the small window of visibility below the plane with anything on the map inside the plane. I work again from the last position I was sure of, and the minutes that have passed since I was sure of it, a rough calculation that suggests a time-dependent circle I must be somewhere on. But soon this ever-expanding circle will meet mountains, the jagged, sandstone-colored peaks hiding somewhere to the northeast of me in the haze. Hardly less worrying is the strictly controlled airspace around the international airport in Phoenix, from which my intended route would have kept me well clear.

I am about to call a controller for assistance, who would give me a code for the onboard transponder that would help the controller steer me home. Then I remember that there is a navigation beacon right at my home airport, in the eastern part of the Phoenix area. My instructor taught me to use such beacons weeks ago, almost casually; it was certainly not part of the lesson plan for this kind of flying. “Just in case,” he winked. I dial up the beacon. It flickers to life. I exhale, then follow the needle like a trail of electromagnetic breadcrumbs through the day’s unforecast murk. I turn sharply right and descend through the haze, and soon I see the runways ahead. I rest my hand on the top of the dashboard in relief and thanks, and then I land.

How do pilots and planes know where they are? It’s an excellent question, but one I am almost never asked now. The assumption is that GPS answers it, though small planes, like the one I flew in Arizona, may not be equipped with GPS receivers. Most airliners now make use of GPS. Often, it’s been added onto an airliner that was not originally designed for it. There are many such technologies—related in particular to communications, and to the avoidance of other aircraft, wind shear, and mountains—that accrete in aircraft systems as layers of progressively higher neurological functions evolve in living organisms, while older systems still twinkle in the lower-down layers.

One of these older systems is
inertial navigation.
It ensures that the airliners of the world could find their way home on the darkest and cloudiest night, even if all the GPS signals, air-traffic control centers, and ground beacons fell silent.

Imagine you have been blindfolded in a stationary car. Then you feel the car accelerate to roughly highway speed. After about an hour, you might guess you were 60 miles from where you started. Then, if you felt the car turn by 90 degrees or so, before driving for another half an hour, you could draw a triangle and have a further guess as to where you were. Similar to the vestibular system in our ears, an inertial navigation or reference system senses these two qualities: acceleration and rotation.

Acceleration is measured by accelerometers, which are relatively simple devices. To accurately measure rotation, inertial systems use gyroscopes, which are anything but straightforward. Originally mechanical (a spinning top, one of the world’s older toys, is a basic gyroscope), the gyroscopes in modern airliners most often use light rather than spinning discs or wheels.

The name given to such a light-based tool for measuring rotation is
ring laser gyroscope.
We think of laser beams as the very definition of straight. A ring laser gyro forces such a light beam into a closed path. Imagine a cube of glass with a tunnel drilled into it. The tunnel turns corners, forming a perhaps triangular light-course in the glass. At one point in the glass tunnel, light is fired in both directions. The beams travel around the loop with the help of reflectors, before meeting on the far side of the tunnel. If the device has not been rotated, then the beams arrive at the same time on the far side. But if the device has been rotated, then one beam will travel a slightly longer path through space, and arrive slightly later than the other.

A rough analogy is to imagine a round, frictionless billiards table (indeed, gyroscope means “circle watcher”). If you roll two balls away from you around the edge of this table, in opposite directions, toward a friend standing across from you, they’ll reach your friend at the same time. But if you start to rotate the entire table after you have rolled the balls, one will roll for a longer distance, which takes a longer time. It will arrive at your friend later.

Isak Dinesen wrote that language “is short of words for the experience of flying, and will have to invent new words with time.” The terminology of aviation is occasionally clumsy—we often speak of the brakes we use in the air as
speedbrakes,
for example, as if there could be any other kind. But the language of inertial systems is a high sort of technical verse, the engineering equivalent of Petrarch. The designers of these light-boxes speak of the
body frame,
the
local level frame
and the
earth frame.
They deal in
gravitational vectors,
the
transport rate,
the
earth rate,
and days referenced not to the sun but to
sidereal time,
the rotation of the planet against the background light of distant stars. The engineers responsible for inertial systems talk of
random walk
and
coasting, northing
and
easting,
and the
spherical harmonic expansions.

There is another dimension to the poetry of inertial systems on many commercial aircraft. They require a few minutes of perfectly motionless concentration and reflection on the ground before each flight. This moment of Zen, or a sort of preflight meditation that a nervous flyer might practice, is called
alignment.
Before a system can track the motion and orientation of the plane, it must know in which direction the center of the earth lies, which it senses from gravity, and which way the plane is pointing, which it senses from the turning of our planet. If the plane is moved during this alignment period, it will display a message that says in effect: “Please, be still until I am ready.”

Once an inertial system is aligned, it serves several important purposes. One is to navigate by summing up the accelerations and changes felt, as you might when blindfolded in a car. Another, less appreciated, function is to track which way is up. The attitude of a plane—the angle of its nose in the sky—is so critical to flight that it dominates the central screen, the
primary flight display,
in front of each pilot. This is the first thing I explain to guests in the flight simulator: the deceptively simple sky-blue and earth-brown horizontal division on cockpit screens shows not where we are, or in which direction we are moving, but rather, which way we are pointing (which is often markedly different from the direction in which we are moving). A plane that flies to the other side of the earth may, by the end of the flight, be close to upside down compared to where it started. Inertial reference systems keep track of what we might call
local down,
all around the world.

The intricacies these devices must grasp are subtle and numerous. When altitude increases, gravity decreases ever so slightly, and an inertial system must account for this. On a rotating fairground ride, the faster you spin, the more you are pressed against the wall; the plane follows a curved line around the earth and the inertial system must similarly account for the forces that keep it on its ever-bending path. They must account as well for the occasional gust of wind during alignment, the imperfections of the earth’s sphere, the temperature of the device itself. Consider, too, that on a chessboard, instructions to move left five spaces and forward four spaces, say, are commutative—the result doesn’t depend on the order in which you carry them out. But when it comes to the changing angle of the plane in space, it matters a great deal whether you rotated left, say, before or after you spun forward. An inertial system must unpick the details of the airplane’s rotations carefully indeed, in order never to lose sight of which way is down.

As navigation devices, inertial systems are not as accurate as GPS. In flight they degrade further as the hours and miles pass, and small errors accumulate and snowball through their dark calculus, eventually reaching the order of miles. The 747 has three separate inertial systems. We can display on our map screen where each of the three thinks we are. Each calculated position appears as a small white asterisk, informally called a
snowflake.
I have never seen all three snowflakes in the same position. Nor are the snowflakes steady. They quiver visibly on our map of the world.

Still, even with its tremulous inaccuracies and strict meditation regime, an inertial system retains one enormous advantage. In practice today, GPS data and the aircraft’s altitude are widely used to bound or limit the errors of inertial systems. But in theory, once set up, inertial systems do not need any outside source of information to know where you are, how fast you are going, and which way you are pointing. They just
know
—without looking at stars, maps, satellites, or scenery, without interrogating anyone or anything. Nor can they be interfered with externally—indeed, the development of inertial navigation was spurred by the need for accurate, jamming-proof guidance systems for missiles.

Flying over north London I can see a churchyard in which I sometimes sit with a coffee, where the tomb of John Harrison, “late of Red-Lion Square,” stands. Encouraged by the astronomer Edmund Halley, Harrison developed the “sea clocks” that helped solve the longitude problem, the difficulty with determining one’s east–west position at sea, an achievement so important that the officials who recognized it were known as Commissioners of Longitude. At such moments over London, as we come to the end of the planetwide countdown that every flight to this city effects, our longitude is nearly zero; it may ticktock from west and east and back to west as we cross the Greenwich Meridian in the next minutes of our approach pattern to Heathrow.

I reckon we could just about explain to an admiral or navigator from several hundred years ago how GPS works. We might say that we have essentially launched new stars into the sky, and that when we can see them, when we have a
line of sight
to them, their timed signals help us navigate. But imagine how much more impossible an inertial system would have seemed to our ancestors: a device that needs to see nothing, that you could cloak in heavy fabric, place in chains in a chest, cart across town, and roll down a hill, without its losing track of either its position or of which way is up. To our ancestors such a device—the stateliness of its sealed calculus, the wayfinding light that flickers deep within the darkened glass cube—might be more miraculous than GPS, or the airplane itself.

Before inertial systems and GPS were developed, aircraft navigators on flights over the sea, far from radio beacons, would use celestial navigation techniques to plot their position, cloud cover permitting. I have occasionally flown with senior pilots who still knew how to use a sextant. On modern 747s there is an overhead handle that we would pull in the event of cockpit smoke or fumes, to exhaust them directly to the atmosphere. (I once heard a perhaps apocryphal story of a long-retired pilot who would attach a hose to this vent in order to vacuum the cockpit.) This vent occupies what on previous 747s was a port designed for a sextant, a means of taking star sights; a hole in the airplane designed for clear nights and a bygone age in which celestial navigation was an unremarkable part of aerial wayfinding.

I have never crossed an ocean unguided by the constellations of GPS. But early in my career I occasionally flew an aircraft that had inertial navigation but no GPS, from London to Lisbon. On certain routes over the storm-mauled Bay of Biscay, the plane would sometimes veer out of range of the ground-based navigation aids on the French and Spanish mainlands. A small memo would then flash up on a cockpit screen, informing us that the plane had lost its last references to the outside world. It was now relying solely on its own internal sense of direction—it was thinking inside the box—to guide us to the far coast.

—

There is an arrow standing in a lake in a garden in Singapore that I walk past occasionally when I’m in the city, after lunch with a friend from childhood who now works nearby. The arrow, surrounded by water, points to England, to the observatory at Greenwich. It marks a spot chosen a century ago by surveyors of the earth’s magnetism.

The maritime, navigational use of compasses in the Mediterranean dates to the thirteenth century. It is pleasing to think of how long those who have moved in the blue between cities have been guided by the simple compass, of how long this energy from within the earth has been a light to us. Some birds use the earth’s magnetic field to navigate, too, and the analogy with airplane navigation appears sound—that independently, birds and humans stumbled across this unlikely gift from the earth, this unseen force that gives direction to lonely travelers and that it’s so easy to imagine we might never have known about.