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.
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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.
To the systems in modern airliners, though, magnetism is a fiction.
The distance between the magnetic and the geographic North Poles means that there are two kinds of headings a pilot may speak of: magnetic headings, referring to the magnetic North Pole; and true headings, referring to the geographic North Pole. The difference between the two, between magnetic and true, is called declination or more usually variation. Variation is not the same everywhere over the earth. In Glasgow, it is nearly negligible, at 3 degrees west; in Seattle the variation is about 17 degrees east; in Kangerlussuaq, Greenland, it is more than 30 degrees west. (Another complication is compass dip. The magnetic lines become vertical where the
y converge at the magnetic poles, as if you held a long blade of grass in your hand that was upright when it left your fist, but bent away to nearly horizontal further along its length. This means that standing at the magnetic North Pole, north is straight down, while south is directly above your head.)
Mariners were aware of variation, of course. Seafaring navigators once measured variation twice a day, at dawn and dusk, to track the local difference between true and magnetic headings. Cape Agulhas, the southernmost point of Africa that also marks the official boundary between the Atlantic and Indian Oceans, was named Cabo das Agulhas, the Cape of Needles, because five centuries ago Portuguese sailors noticed that magnetic and true north were nearly aligned here. Nowadays, pilots on a modern airliner can choose to display either type of heading. At the flick of a small switch the whole compass rose on our digital map will rotate left or right. It is a disconcerting moment when you first see a compass, which you imagine as a deep and incorruptible arbiter of direction, spin like a top.
Most of the time we fly on magnetic headings. The reason for this is largely historical. In the early days of aviation, pilots—like birds and mariners—only had magnetic directions to choose from, because they only had magnetic compasses. And so even today, when air-traffic controllers ask a pilot to take up a heading of 270 degrees, or due west, the controllers almost always mean not the 270 degrees that is actually west over the surface of the earth, but the 270 degrees that is displayed on a magnetic compass in that part of the world.
Yet the heading display on a 747, like that on most airliners, has no magnetic inputs. It is a surprise to new pilots, who have flown and studied and been tested on the vagaries and inherent errors of magnetic compasses, to realize that on a typical modern airliner there is nothing to sense the magnetism on the earth and feed it into the computers that generate our display of magnetic headings. There is only one magnetic compass onboard—a forlorn, technically isolated backup device that is never used in normal flight. On some aircraft it’s even hidden away, to be pulled out when needed, which is essentially never. It’s no small irony that the complicated electrical fields generated by the airliner’s systems themselves disturb magnetic compasses.
To display magnetic headings without using a magnetic compass, the plane consults its map of magnetic variation. The plane knows the pilots are not using a magnetic compass, but if they were, it knows that in this position over the earth, it would read this. And that’s what is displayed in the cockpit computers. In other words, the world’s airliners fly on magnetic headings derived from a preloaded map of magnetism, rather than actual compasses. If the earth’s north and south magnetic poles suddenly reversed themselves, or even if they stopped their eternal flickering one night, the pilots of commercial airliners would not see this on their screens—though birds, the pilots of small planes, and old-school hikers would notice immediately.
When I think of the long history of compasses and seafaring, or when late on autumn nights I see the northern lights, the solar wind catching in the harp-like lines of magnetism that congregate at the pole, it seems fitting that something as primeval and eerie as magnetism would have only such a ghostly sort of prominence in the shiniest machines of our age.
A further oddity of magnetism is that our elaborate fiction of it must be regularly updated. The magnetic North Pole, the star that our compasses orbit, is itself on the march—from northern Canada toward Russia, at a pace of several dozen miles per year, in a process known as geomagnetic secular variation. This motion means that the charts of magnetic variation must be routinely redrawn and the maps in the computers of airliners reloaded, even though nothing in the airliner’s computers can detect these changes. Runways numbered according to their magnetic direction (e.g. runway 27 points roughly 270 degrees on a compass) must occasionally be retitled too, their number-names spun, and all the airport signs remade or repainted, and all the charts onboard the world’s aircraft updated, to follow the latest twists in the planet’s old magnetic tale.
Machine
I’m at a small airfield in rural Massachusetts, aged sixteen perhaps. It’s a place that I occasionally came to with my parents when I was younger, to eat doughnuts and watch the small planes land and taxi in behind a low metal fence, the clear boundary of an airfield that many who love airplanes will have a memory of deeply wanting to cross. The planes park, the pilots and passengers get out, they walk into the lobby of the single-story building. They were in the sky; now they are here. They get into cars. They drive away, rescind a dimension, just like that.
In the lobby are a vending machine and a glass-topped counter through which I can see several shelves of maps and navigation tools for sale. On the wall behind is a bulletin board with all-capital letters stuck to it, of the type that you see in delis and diners. Here it’s a menu, too, a list of the aviation services provided at this airfield, and their prices. I know these prices by heart. I’ve been saving from my paper route and restaurant job, and now I’m here for my first flying lesson.
It’s early autumn, one of those clear, warm, bone-dry, and mosquito-free New England days, of the sort that draw people to Northern California when they realize they can enjoy them there the whole year. The leaves on the trees around the airfield perimeter are beginning to change; on the mountains nearby, as my mother would say, the color is “farther along.” I greet the instructor and purchase my first logbook, navy blue, from one of the shelves in the glass counter. We go outside and walk up to the white plane. I’m surprised to suddenly find myself on the other side of the fence.
Until today I’ve only seen planes from a distance or entered them through a jetway that masks nearly all of the experience. I’ve never touched the outside of an aircraft before. There is a surprising lightness to the plane. The doors feel flimsier than those on any car. There’s awkwardness, too, a sense that the plane is crafted for something other than motion on the ground or human comfort. There are plenty of opportunities to hit your head on something that looks expensive. The plane’s wheels are chocked, and the wings are tied to hooks in the tarmac. There must be experts whose job it is to design airfields and this must be one of the things they think of to install—hooks to tie our wings down to, when we are not in flight.
Naturally, the instructor is wearing aviator sunglasses. He is inspecting the aircraft with the seemingly contradictory mix of utter familiarity and deferential caution that I will later associate with pilots but even more so with the engineers who check and repair airliners. I follow him as he patiently explains what he is looking for at each point in his careful circumnavigation of the plane. He takes liquid from the bottom of the wing’s fuel tank, as if he is drawing blood, with a specialized tool that aspiring pilots may acquire along with their first logbook. He holds the clear column of what he has drawn up against the light, against the blue of the sky. He is checking for water. He pauses, stares straight at me. Water is bad news, he tells me. A new fact about water. Twenty years later I will read in my father’s notes that when he lived in Stanleyville, the city in the Belgian Congo now known as Kisangani, another missionary took him on a flight over the Tshopo River. Their small plane nearly crashed into the river’s reservoir because the fuel cap had been left off the night before, allowing rain into the tank.
The instructor and I have come full circle around the craft. The inspection is complete. He opens the door of the airplane and smiles, gestures, tells me again to watch my head. While I’m carefully climbing into the machine, he unties the wings.
Since I have become an airline pilot, I am occasionally asked: “What does it feel like to fly?” Perhaps the most honest answer is that I don’t know. What passengers see of the world is permanently framed by the iconic ovals, the windows cut from the hull of the vessel. Even pilots, with their wide and multidirectioned blessing of views, are surrounded by surfaces of intricate electronics, busy computer screens, buzzing radios—the plane and its permanent, metallic mediation of the experience of flight. Planes are n
oisy, particularly small ones. I have the sensation of truly flying, peacefully and silently as we do in our dreams, more when I’m swimming than in any airplane.
Aside from the lines that tie the wings of small planes to the ground, it’s seat belts that are the simplest reminder of the machine, of what, exactly, is flying. Whether as pilots or passengers, our experience of an airliner begins when we walk inside a contrivance the size of a building. To do what we call flying, we sit down in this. We tie ourselves to it.
Many pilots, of course, love the airplane precisely because it is a machine. An aircraft, a root that equals strength, skill. Nor is it clear that passengers, either, wish to pretend the ship away. We might consider why photographs taken from the window seat are almost always more evocative if they include something of the airplane’s structure—an engine, the curve of the window frame, or the lines of the wing. The airplane’s photogenic presence is something more than foreground technique. Perhaps the plane stands in the imaginative place of flight itself, of the experience we cannot have directly, as if looking from the window we say: “Yes, of course, we will never fly quite like we do in our dreams. Dreams are easy; this is real.”
Machines, indeed, hardly get more difficult. Pilots occasionally experience the unusual sight of an airplane indoors. A large plane inside a hangar looks much bigger still, in the same way that even a small car suddenly appears enormous and awkward in a garage. In a hangar you see the many steps, platforms, and hydraulic lifts that are needed to return an airliner’s structure to a human scale, much as the hulls of boats are brought close to human eyes and hands in dry dock. In some hangars the plane is all but disassembled for checks and maintenance, as if an actual airplane had enacted its own exploded view drawings, the engineering diagrams in which parts are pulled away from each other, for the purpose of understanding or duplication.
Skyfaring: A Journey With a Pilot Page 8
