Skyfaring: A Journey With a Pilot (20 page)

The relationship of these speeds to one another is so peculiar that each is best regarded as a separate idea of motion. In the cockpit the four speeds are displayed in different places or at different times. The computers may automatically change the font size of one kind of displayed speed as it becomes less useful, or change what kind of speed a certain switch alters, or even seamlessly replace a display of one kind of speed with another that has become more relevant to a new stage of flight.

Mathematicians occasionally question whether the achievements of their field are created or discovered; I ask myself this about airspeed. I wish I could remember, before I learned to fly, what I might have guessed indicated airspeed actually meant. Why, I might have asked myself, do we need to specify
air
speed? Isn’t speed enough? And what about that
indicated,
which makes the term sound both fuzzy and very precisely qualified, as if the quantity in question isn’t quite real, or as if speed itself is the wrong word for what keeps us in the air?

The difference between
indicated
and
true
airspeed is something I might consider when I hold my hand out of a car window. Using straightforward numbers purely for illustration, at sea level on a windless day, driving at 50 mph, I might feel a certain force of the air on my hand. But now imagine I am on a road near the top of a high mountain, where the air is thinner. Though I am still driving at 50 mph, my hand would register less of a force from the slipstream because fewer molecules are hitting it. I am still moving at 50 mph, but it might feel like only 40 mph to my hand. We might say that my hand’s true airspeed is 50 mph, but its indicated airspeed is now only 40 mph.

The airspeed indicators—the speedometers—on a plane act something like an extended hand. They stick out of the sides of the plane, into the slipstream. They measure the pressure of the molecules hitting them. From this they subtract the pressure of the air that isn’t moving—that is, the background weight of the air, the weight that Galileo discovered air possesses (the same background measurement of pressure, by the way, that the altimeters use). Indicated airspeed can be thought of as nothing more than the pressure that speed adds to the air; the rough difference between what your hand experiences inside and outside the window of a moving car.

Indicated airspeed, then, isn’t speed in any normal, earth-referenced sense. Rather it is the feel of speed, the feel of motion through air.
Air force
or
air feel
would be better terms for indicated airspeed. Indeed, on some vintage aircraft, the airspeed indicator is outside the cockpit—a little panel that is deflected by the slipstream and read off against a scale of numbers beneath it, hardly more complicated than those light-hearted weather stations that feature a rock hanging from a board and instructions like: “If the rock is swinging, the wind is blowing.”

Airspeed, though, is not less useful because it is so inchoate and capricious. On the contrary, it is exactly what pilots need, because it is indicated airspeed, not true airspeeds that determines how much lift the wing creates. For this reason there are multiple, redundant systems to sense airspeed, and one of the most important checklists on the airplane relates to failures of airspeed indicators or, perhaps worse, a disagreement between them. It also explains why, when landing on a blustery day, you may hear the engines power up and down so often. If a sudden gust of headwind hits your car while driving, your hand out the window feels this sudden gust as an
increase
in indicated airspeed. In an airplane at just such gusty moments the telltale airspeed indicator spikes upward. In response the pilots may then reduce thrust to bring the amount of air feel back down to the target—until the wind drops, the speed slows, and power must be added again.

When we fly we leave behind many earthbound ideas, so it’s appropriate that the difference between indicated and true airspeed, small at low altitudes, grows substantially as a plane climbs. To conjure the same lift as at lower altitudes a high plane must fly faster in a true sense, to ensure that the same heft of air passes over the wing. At high altitude a jet’s true speed may be 500 knots—nearly twice the 270 knots reported by our airspeed indicators. A plane climbing at a constant indicated airspeed is in fact continuously accelerating, a sky-sorcery that will be undone only in the descent.

Ground
speed, meanwhile, adds the enormous effects of wind to our true speed through the air. It accounts not only for our motion through the air but the motion of the air itself over the earth. As its name suggests, it’s the closest to our traditional understanding of speed, which is why it is the speed typically shown on moving map displays. Imagine two boats traveling in opposite directions along a fast river. Their speeds over the surface of the water—their true speeds—are the same. But their speeds measured over the riverbed, or along the riverbank, are different. The boat heading upstream, fighting the current, is moving more slowly relative to the riverbank, while the downstream boat, carried by the current, is racing along the bank.

In the air, two planes traveling in opposite directions at the same altitude, one flying in the direction of a strong jet stream and the other flying against it, may have identical indicated airspeeds—the same feel of the air, the same number of molecules hitting their wings and speedometers. They may have identical true airspeeds, because their speed through the surrounding air—over the moving river—is the same. Yet their ground speeds may differ by 300 mph or more, because the surrounding air is carrying one quickly over the ground, while slowing the progress of the other. In a strong tailwind the ground speed of a jet can even exceed the still-air speed of sound, but the plane itself, in the frame of reference of the jet stream that carries it and surrounds it entirely, is not traveling supersonically.

Ground speed, though irrelevant to flight, is of great importance when getting into the air in the first place. At takeoff, though indicated airspeed determines when a plane can fly, ground speed determines when it will run out of runway. Air is thinner at higher elevations and temperatures, so a plane must use more runway length, or additional engine thrust for faster acceleration, or both, in order to gather the required feel of air over the wings. A plane that takes off at an indicated airspeed of 170 knots in lofty Denver or blazing-hot Riyadh is going much faster—is using up the runway much more quickly—than a plane taking off at the exact same indicated airspeed in sea-level Boston. That’s one reason long-haul flights from the Middle East traditionally depart late at night, when the air is cooler.

Wind adds another complication to takeoff or landing. Once we are detached from the ground, the flowing wind forms its own frame of reference, which is why the air around a hot-air balloon is wonderfully quiet and still even when the balloon is traveling on a very steady breeze. Horizontally, such a balloon has an indicated airspeed of zero, and a true speed of zero, but a ground speed equal to that of the wind. Similarly, a plane in flight moves in the reference frame of the moving wind. But on the ground the wind passes over an airplane as it might over the branches of a tree, or a balloon that is tied to the ground.

Indeed, the airspeed indicators on a plane cannot distinguish between airspeed and wind, because to the probes, as to the wings, there is no difference. Taxiing around an airport on a breezy day, the airspeed indicators will flicker to life when you turn directly into the wind and drop off when you turn a corner and face away from it. They may register airspeed even when the plane’s wheels are stationary, just as you might by extending your hand from a stopped car on a windy day. Such a stopped plane, in an aerial sense, is already moving.

This reverses the everyday idea that tailwinds are advantageous. A tailwind is indeed a gift to a plane—but only once it is well away from the runway; when the wind carries it like a balloon and the plane’s speed through the air is added to the speed of the air over the ground. Before we are airborne the last thing we want is the wind at our back. If a plane is rolling down the runway at 10 knots in a tailwind of 10 knots, its airspeed is zero; and yet it is already devouring the available runway. A headwind, meanwhile, is a blessing. A plane parked in a headwind of 10 knots is already part of the way toward getting airborne, though it has not yet moved.

Our love of headwinds at takeoff applies equally to landing, when a tailwind is an unwanted addition to ground speed, to the rate at which we consume the runway. The desirability of headwinds explains why aircraft carriers will turn into the wind or accelerate; for both takeoffs and landings they are seeking a headwind or making their own. Back on shore, of course, a typical runway can be used in both directions, and many airports have multiple runways to cater for the vagaries of the wind’s direction. When the wind changes markedly, both arrivals and departures may be briefly delayed while air-traffic controllers reverse the flows of traffic coming to the airport and leaving it. This is how the unseen motions of air determine how you will arrive in a place; how the wind gods choose what to show you of a city, when you first come to it from the sky.

There is one further idea of speed that pilots must consider.
Mach,
a word that still sounds as futuristic to me as it did the day I first heard it, is the aircraft’s speed—its true speed through the air mass, not the indicated airspeed that the racing of the jet through the thin air sums to—expressed as a portion of the speed of sound in the local air. Mach is a peculiar kind of speed—a ratio, and so without dimensions or units.

When I was first taught about the speed of sound, in school I suppose, I assumed that it was only of interest to humans because we use sound as a means of communication, or because it explained why the light from a distant bolt of lightning arrives long before the noise. I’ve since realized, however, that there’s a good reason planes pace themselves against the same phenomenon that brings to our ears everything from Beethoven to thunder. There’s nothing arbitrary about our focus on the speed of sound. There is a speed of sound in iron, rubber, and wood, for example (all of which are faster than its speed in air). What we call
sound
is a kind of wave propagating to us, whether the voice of an opera singer, the pitter-patter of rain, or the noise from jet engines that Joni Mitchell (“I dreamed of 747s…”) called “a song so wild and blue.”

The bow wave that develops when a boat moves faster than the water can move away from it is analogous to the shock wave produced by a supersonic aircraft. A bird floating on the water would not sense the waves of such an approaching boat, just as a bird in flight would not hear the approach of a supersonic aircraft. The resulting pressure buildup, this aerial bottleneck, is what we hear on the ground as a sonic boom. We fly on Mach in order to pace ourselves properly against the limits imposed by the speed of sound, an elemental quality of the air we live in.

Airliners—since the retirement of the Concorde—fly below Mach 1, the speed of sound. But at high subsonic speeds the air over the top of the wings can reach or exceed Mach 1. This can result in the formation of shock waves that destabilize the pressure distribution around the aircraft, before the aircraft itself has exceeded the speed of sound. In order to stay within the aerodynamic design limitations that this imposes, airliners are typically engineered to cruise between Mach .78 and Mach .86. A normal cruising speed for the 747 is Mach .85, 85 percent of the speed of sound, read out as “decimal eight-five,” or “point eight-five,” or “Mach eight-five,” or just “eight-five.” If we are catching up to the plane ahead, a controller might tell us we are
in trail
of a slower jet; they might ask us to “reduce to eight-four.”

At low speeds on the 747, Mach is not even displayed. But at higher speeds—in what pilots may loosely term the
Mach regime
or the
Mach realm
—it is by far our most important measure of velocity. Fittingly, as we accelerate in the climb, our Mach number automatically appears in the same place on our screen where, when we were lower and slower, our ground speed was displayed. As we slow in the descent we switch again, from Mach back to airspeed. An air-traffic controller trying to space out aircraft descending through this boundary must therefore assign us two different kinds of speed to cover both the lower and higher speed dominions of the sky. “Start the descent at Mach eight-two,” a controller might say. “Then on
conversion
[or on
transition
] fly 275 knots.”

A further curiosity is that Mach, described to us by the science of fluid dynamics, is itself fluid. The speed of sound varies with temperature. So just as with altitude or indicated airspeed, the same Mach number relates to different speeds at different times and places. Mach is valuable not despite this variability but because of it. A plane traveling at a fixed Mach number will move slower when the air around it is cold and faster when the air is warmer. But at a constant Mach number, the aerodynamic conditions will remain the same. Sound, in other words, is as fluid as our most tender notions of music would suggest, and all our journeys in the higher sky are tuned to it.

Water