and I mean precisely — North- South and East-West. It
makes the country look like a giant real-estate development; which it is.
One section has 640 acres. A quarter section, 160 acres, is the historical
homestead.
—Wolfgang Langewiesche, A FLIER’S
WORLD
The difference between 60 minutes and 120 minutes was dramatic: Before 1985, almost every trans-Atlantic flight was on a four-engine aircraft, like the Boeing 747, because a twin-engine would have to follow a circuitous route that was far from fuel-efficient.
In 1989, based on extensive studies of the integrity and durability of modern jet engines, the FAA extended the ETOPS limit to 180 minutes. This allowed airlines to fly twin-engine aircraft almost anywhere in the world, including Hawaii. In the year 2000, the FAA extended it even farther, letting some Boeing 777s fly 207 minutes from an airport, which permits even more direct routing between the United States and Asia. Today, the majority of flights across the North Atlantic are flown on the twin-engine Boeing 767.
Of course, the ETOPS limits still don’t allow for the most efficient routes on some international flights. For example, flights between the United States and Japan still have to hug the coast of Alaska a bit more than airlines would like.
In the near future, the FAA may raise the ETOPS limit for some aircraft to 240 minutes. One likely result of this decision would be the closure of some very remote airports—such as the one in the Midway Islands in the Pacific Ocean—which are kept open at great cost by the aviation industry for the sole purpose of extremely rare emergency landings.
Flights, flight maps, and flight speeds are always measured using nautical miles, which are different from ordinary miles or kilometers. One nautical mile is equal to one minute of arc along the length of a great circle (a circle that encompasses the Earth, like the equator). That means 1/60 of 1/360 of the length of a great circle. Unfortunately, the Earth isn’t a perfect sphere, so to avoid confusion over which great circle, the official nautical mile equals 6,076.1 feet (1,852 meters). That makes it about 1.15 times a mile, and 1.85 times a kilometer. A knot means “nautical miles per hour,” so 100 knots equals 115 mph.
Weather
Many people don’t realize that the same flight may use a different route each day. The reason? The weather.
Each airline has its own team of meteorologists who forecast the weather and help figure out the best routes to fly. Of course, commercial airplanes always avoid the centers of
thunderstorms or clouds of volcanic ash, which can sometimes make for indirect routes. But airplanes may also fly seemingly odd routes in order to avoid or get closer to jet streams, which are very powerful high-altitude winds that blow from west to east (in both the northern and southern hemispheres), especially during the winter months. Streams can be 100 miles wide and 2 miles from top to bottom, often blowing at more than 150 mph (130 knots) at the center.
A jet stream moving at this speed would either add or reduce 150 mph from a plane’s air speed, depending on whether the aircraft is heading east or west. That is why it often takes about an hour longer to fly to the United States from Europe than the other way around. So, each day, airlines and government officials adjust the long-distance flight routes, directing eastbound airplanes into the jet stream, and westward flights out of it.
Sky Highways
Navigation and air traffic control also keep airplanes from flying in straight lines from
departure to destination. In the early days of aviation, pilots had to watch for landmarks on the ground to know where they were; sometimes airmail companies would even light bonfires on hilltops as beacons that their pilots could follow. It’s not so different these days, except the beacons are radio transmitters sending out VOR (Very-high-frequency Omnidirectional Range) signals.
The Earth is spinning in space, and any given point on the equator is traveling eastward at over 1,000 mph (higher latitudes travel more slowly, as they have fewer miles to cover in the same amount of time). Fortunately, pilots never have to adjust for the rotation because the atmosphere spins at about the same rate. Wind speeds can make an even greater difference—
an airplane flying eastward in the jet stream would be traveling at more than 1,700 mph (as seen from outer space).
Before takeoff, the airline gives the pilots a flight plan detailing each point along their route. A 1,000-mile flight might include five VOR beacons, over which the airplane will fly as it makes its way in a relatively straight line from signal to signal. The paths between these beacons—
often called corridors or highways in the air—are about nine miles wide and are clearly marked on aeronautical charts.
For instance, a flight from San Francisco to Seattle would likely be “vectored” over four cities, each with a VOR station named with a three-letter code: Oakland (OAK), Red Bluff (RBL),
Medford (OED), and Portland (PDX). Some VOR stations are in towns with small airports, and others are transmitters in the middle of nowhere set up by the government.
If your flight is running late, the pilots may ask air traffic control for a more direct flight, perhaps picking different VOR stations or even bypassing one or more beacons. This is
increasingly possible due to Global Positioning System (GPS) technology that enables pilots to determine their exact location based on signals from satellites. In the future, GPS combined with longer-range airliners will make more direct paths possible, shortening flights and saving fuel and money.
From the air, the distinctions between residential, commercial, and industrial areas are easily understood while town, county, and state boundaries go unseen.
—Oliver Gillham, THE LIMITLESS CITY