When flying over a thick blanket of clouds, you can occasionally spot long stripes, like lines drawn in the sand. This is usually caused by large rivers because the cold water makes the air directly above them descend instead of rise.
The good news is that these sorts of microbursts are relatively rare around airports. Also, detection has become easier with Doppler radar (on the ground) and wind shear detection devices (in cockpits), and pilots are better trained to avoid and handle wind shear than ever before. Although there were several wind shear crashes in the 1960s and 1970s, training and early detection have meant almost none since 1985.
Snow and Ice
Airplanes can lift hundreds of thousands of pounds into the air not just because they have wings but because the wings are a very specific shape. If the shape of the wing changes even a little bit, the airplane isn’t as efficient and requires more power to keep aloft. That’s why airliners are so sensitive to ice forming on the wings. In some cases, just a half-inch (12 mm) of ice along a wing’s leading edge can reduce its lifting power by 50 percent.
So before a plane takes off in a snowstorm, the airport ground crew must spray the wings with a deicing fluid to keep snow and ice off them until it’s airborn. If takeoff is delayed for some reason, the aircraft may have to be deiced again while it waits, just in case. Fortunately, once a jet airplane is in flight, heat from the engines is circulated through the wings to keep them ice-free. Smaller, propeller-driven planes often don’t have this heating system and so are more susceptible to the dangers of ice when airborne.
One of the most fascinating natural wonders in the air is called the glory: a rainbowlike light that sometimes surrounds the shadow of the airplane in or on the tops of clouds. Glories are caused by diffraction of light in the cloud’s water droplets (the larger the water droplets, the smaller the glory). Actually, the center of the glory isn’t the airplane; it’s where you’re sitting in the airplane.
In-flight deicing isn’t just used for flying through storms. At high altitudes, the air is so cold that water vapor instantly turns from gas to ice as soon as it has something to latch on to (like a wing or even tiny particles in the engine exhaust). Pilots may not be able to see the full length of the wings from the cockpit, but they generally turn on the heaters as soon as they see ice begin to build up on the windshield wipers.
By the way, snowstorms produce concerns on the ground as well as in the air. A half-inch snowfall doesn’t sound like much, but over a 10,000-foot runway it means clearing thousands of cubic feet of snow. Mounds of snow along the sides of a runway can’t be too high, or else they might catch a wingtip as the airplane taxis to the gate. Also, the glycol- and urea-based fluids that airports use to deice airplanes and runways become toxic to fish and plants if they’re carried into nearby lakes and rivers by snow runoff. Some airports have specially designed drainage systems or wastewater treatment plants just for this purpose.
Flying Forecasts
Pilots are keenly aware of weather conditions at all times, and they won’t even take off if it isn’t safe. Besides, there are strict rules regarding what pilots may and may not do in every sort of weather condition, so any inconsistency in the pilots’ personal judgment is not an issue.
During a flight, pilots constantly monitor the onboard weather radar so they can avoid areas of heavy precipitation or anything that even looks like a thunderstorm. Each airline also employs a team of meteorologists who forecast the weather around the clock in order to minimize flight delays and other problems. If the need arises, the airline can even radio special weather reports to the pilots. (Many pilots insist that these weather forecasters are among the great unsung heroes of the aviation industry.)
One of the trickiest conditions to forecast is fog, which is just very low cloud cover. Back in 1945—long before modern warning systems were the norm—a U.S. Air Force bomber pilot became disoriented in the fog above New York City and crashed into the seventy-ninth floor of the Empire State Building, blasting a twenty-foot hole in the side of the building and killing all three crew members plus eleven people in the building.
Today, flying through fog isn’t a problem, but when it comes to taking off and landing, each airport and each airplane are rated for a minimum allowable visibility (the distance you can see the runway and other objects clearly). Some airports also impose a minimum ceiling (the distance of clear air from ground to clouds), often about 200 feet. However, some experienced pilots with special training and some aircraft with special equipment are allowed to land in low-visibility and low-ceiling situations.
Believe it or not, some newer aircraft (like the Boeing 777) and pilots are even rated to land in virtually zero-visibility conditions based on electronic instruments that are installed at some airports and on the airplane. In these cases, the autopilot can do everything from land to taxi to park outside the gate.
Ultimately, as scary as harsh weather can seem when flying, it’s extremely rare for weather conditions to be the cause of an accident in this day and age.
Turbulence
“Did you have a good flight?”
You can hear this question uttered thousands of times each day by friends and relations greeting passengers at airport gates, but if you listen carefully you’ll find that what is really being asked is: “Was there much turbulence on the flight?”
For most people, a good flight means a smooth flight, and there’s nothing like bad turbulence to make passengers swear they’ll never fly again (though a screaming baby in the next row is a close second). Even relatively calm fliers tend to find themselves glancing nervously around when an airplane starts bouncing and shaking through the sky. After all, how much turbulence can an airplane take?
Rating Turbulence
Pilots have four ratings for turbulence: light, moderate, severe, and extreme. If you have a glass of water in front of you during a flight and you can see surface ripples, you’re in light turbulence. If the water starts sloshing around but doesn’t spill out of the glass, you’re in moderate turbulence. It’s not until the glass is literally flying above your tray and the water is in your lap that you’re in severe turbulence. (By the way, if the bumps are steady and rhythmic rather than irregular, pilots call it chop instead of turbulence.)
Everyone knows someone who claims to have been in extreme turbulence, “so bad I thought for sure I was going to die.” However, the truth is that few people—even pilots who have been flying for thirty years—have ever experienced extreme turbulence. In fact, over 99 percent of the turbulence people feel on commercial airplanes is only light or moderate chop (even when it feels like it must be severe or extreme).
One reason for this is that strict rules prohibit U.S. airlines from flying into an airspace where there is known severe or extreme turbulence—like the center of a thunderstorm or a hurricane. Airplanes can fly into extreme turbulence and survive (the government frequently sends military or research pilots into hurricanes to measure wind speed), but since it’s almost impossible that you’ll find yourself in that situation, assume that the rest of this chapter deals only with light to severe turbulence.
What Is Turbulence?
What pilots know but most passengers don’t is that airplanes fly just as capably in the midst of turbulence as in smooth, calm air. Turbulence doesn’t make the pilots panic or clutch the controls in a desperate effort to control the plane. Turbulence doesn’t tear wings off commercial airplanes or shake the fuselage apart.
Flying through turbulent air is much the same as steering a powerboat across choppy water. In a boat, passengers expect the thud-thud-thud of the water hitting the bottom of the boat, the rising and falling seesaw of the boat’s hull, and the sometimes unexpected drops. Just as boats are strong enough to survive the impact of turbulent water, airplanes are built to withstand turbulent air.
In fact, some pilots who fly their own private airplanes on their days off actually look for turbulent air because it’s fun to fly through. However, pilots
on the job try to avoid turbulence simply because they know the passengers don’t like it.
The worse the turbulence, the more you tend to get air sick because the sensations you feel don’t match up with the movement you see around you. Those convenient disposable bags in the seat pocket in front of you have been around almost as long as passenger flight itself, and have been called everything from “burp cups” to “vomit bags” to the “SicSac.”
To understand what turbulence is, you have to understand that air is just like fluid. Remember: Just because you can’t see the air doesn’t mean it’s not there. Once you can visualize that airplanes are moving through a really light “liquid,” it’s easy to see what causes turbulence.
First, some parts of the air are warmer than others (like the air above parking lots on a hot day), and the hot parts rise because they’re less dense. As hot air rises, it pushes the cold air down. The lava lamp, that icon of the 1960s, is a pretty good visual representation of exactly this effect. Or imagine a fish swimming through an aquarium while you pour water into it: Some of the water is moving down, causing other parts of the water to rise up, and the fish gets a bumpy ride.
Now imagine the wind blowing over a mountain range: The air hits the mountain and gets pushed up, just as water in a stream is pushed over rocks. But the air above the mountains pushes the wind back down—the result is a wave of air moving up and down, also called a mountain wave. This can be surprisingly uncomfortable and is no fun to fly through. Fortunately, it is a well-understood phenomenon and easily forecast, so pilots can typically fly around it.
Another cause of turbulence is the “wake” from other airplanes. Just as a boat moving through water causes a wake, an airplane leaves a trail of turbulence in the air. However, the bigger the airplane and the slower it’s flying, the bigger the ripple. Small airplanes have to be very careful about flying behind large jets because they can literally be flipped over by the spiraling vortices trailing off the bigger aircraft’s wings. Fortunately, pilots and air traffic control keep this in mind and schedule plenty of space between airplanes.
High Winds and Turbulence
High winds don’t always cause turbulence; in fact, airplanes enjoy smooth flights in the middle of the jet stream, where winds can be blowing at over 200 mph. While small airplanes (which carry between two and ten passengers) can be blown around by strong gusts of winds—causing a sensation of rising, falling, and sliding from side to side—it’s rare for larger jet airliners to be affected by wind speed alone.
The turbulence you feel on these aircraft is typically due to changes in wind-speed or direction. That is, if the wind is blowing in one direction in one part of the sky and in a different direction elsewhere, then the boundary between the two areas will be filled with swirling eddies (just like water when two rivers converge). This is sure to cause a little turbulence because the airplane will experience slightly less lift over the wing, then more lift, then less lift, and so on until it passes through the boundary into the new, stable wind condition.
If the eddies are large and slow-moving, the turbulence may feel like driving over speed bumps slowly. If they’re tight and swirling quickly, it might feel like driving over those small, reflecting lane dividers on a road. Sometimes one wing gets slightly more lift than the other, and the airplane tips to one side and then the other quickly; again, this just means that the wings are hitting different parts of the airflow.
It’s also important to note that in most cases, turbulence doesn’t make the whole aircraft ascend or descend. Rather, when passengers in the front of the airplane feel a slight upward motion, the passengers in the rear feel the equivalent downward motion. Even a small change in pitch—where the nose moves up or down by three inches—can feel surprisingly strong when it occurs quickly. This is why seats near the wings (the fulcrum of the seesaw) often provide the smoothest ride.
The Air-Pocket Fallacy
The faster an airplane flies through turbulence, the more stress it can put on the aircraft, so airlines have “rough air” speed rules that instruct the pilots to slow down in turbulent conditions.
Sometimes when you’re flying, you experience the sensation that the airplane is dropping. Many people—even professionals—call this hitting an air pocket. It’s a terrible feeling—as if suddenly there is nothing holding the airplane up—and you could swear it dropped hundreds or thousands of feet in a split second. However, the reality makes for a much less interesting story.
There is no such thing as an air pocket—that is, airplanes can’t fly into a bubble where there’s no air any more than a boat can hit a “pocket of no water” in the middle of a lake. When you drive over a big pothole in the road, your car’s wheel may actually lose contact with the road for a moment as it drops, but there is no way for this to happen in an airplane.
Yes, the airplane may descend very quickly for a short time, but it is still surrounded by air. Often the “drop” happens when the aircraft flies into a column of rapidly descending cold air (a downdraft); other times the aircraft flies into an area where the wind direction is radically different. In either case, the airplane always moves through the offending air quickly, often following the short descent with a rapid ascent. Again, this is analogous to a motorboat cresting over a wave and dropping suddenly, only to be raised up on the next wave. It might feel and even sound as if the airplane is bouncing hard against something, but airplanes aren’t damaged any more than boats are.
Remember that our bodies are easily fooled in airplanes, and although we might feel like we’re dropping hundreds of feet, the reality is that airplanes just don’t do this. Pilots can watch the airplane’s altitude change on the cockpit’s altimeter, and they know that usually these sudden motions are only a matter of a few feet—maybe as much as twenty or fifty feet, but that’s not a big deal when you’re cruising six miles (9.6 km) up.
You Can’t Escape Turbulence
No passenger wants to fly through choppy air, but insisting on flying without turbulence is like expecting the water to be perfectly calm whenever you’re on a boat—it just isn’t going to happen. The important thing to remember is that as long as you’ve got a seatbelt on, air turbulence is not a safety issue; it’s just a comfort issue.
The people who are hurt in those very rare severe-turbulence situations are the folks who are walking around, like flight attendants. That’s why if there’s any chance of moderate or severe turbulence, the pilots ask the flight attendants (and everyone else) to be seated.
Flying in good weather is no guarantee of a smooth flight (clear air turbulence—or CAT—can happen even on a beautiful sunny day), but there are a few things you can do to minimize your discomfort. First, flights in the morning are often smoother than those later in the day, when the air has had a chance to warm up. Second, when turbulence tilts the front of the airplane up a little, the back usually goes down, and vice versa. Therefore, you can sometimes reduce the feeling of rising and falling by sitting near the middle of the airplane.
Remember that pilots and air traffic controllers are constantly working to avoid turbulence. When an airplane encounters some, it slows down and air traffic controllers try to assign it an altitude where the air might be more stable. They also redirect any aircraft that might be heading toward the rough air.
Air Traffic Control
Most airline passengers are aware that pilots are in communication with air traffic control (ATC) throughout each flight. United Airlines even allows passengers to listen in on this communications channel on some flights, though it can be difficult to understand the staccato bursts of coded language from pilot to controller and back again. In many ways air traffic controllers are the backbone of the airline industry—ensuring the safety of passengers and in-flight aircraft at all times—but who, and where, are they? Probably the best way to understand the air traffic control system is to follow a single typical flight.
From Gate to Gate
Before the pilots push back f
rom the gate, they must get permission from ground control, which is stationed in the airport’s control tower. Ground controllers keep watch over the gates and taxiways around an airport, using their eyesight (often with binoculars) and, at larger airports, ground radar to tell them exactly where every truck, cart, and aircraft is around the airport. As the airplane heads toward the runway, ground control hands the pilots over to tower control, also located in the airport tower, which oversees the runways, managing the precise timing of takeoffs and landings.
This kind of “hand off” from one air traffic controller to another is key to the whole system. A controller gives the pilots a radio frequency, like 121.83 (in megahertz), and one of the pilots dials it into one of several radios on the instrument panel. Then the pilot flicks a switch from the old frequency to the new one, so the pilots are never stuck “between channels.” Once on the new frequency, the pilots listen for a lull in the new channel’s communication in order to introduce themselves to the new controller by stating their airline, flight number, and location—for example, “Delta 486, approaching runway 11R.” The controller confirms recognition, in this case by repeating “Delta 486…,” and then provides further instructions.
If you have a window seat on an airplane, you’ll notice that the wings are typically bright silver while on the ground and a darker blue-gray when at cruising altitude. That’s because the metal wings are reflecting diffuse, scattered light while on the ground. In the air, the light isn’t as scattered by dust and water vapor, so we see a better reflection of the dark blue sky above.
The Flying Book Page 5
