I asked my wife if she had any ideas for an article. She said, “You know, I really don’t know how you find your way from one city to another.” “Great,” I thought, “an article about how airliners navigate!” Or perhaps it was a sly insult; I can never really tell.
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Navigation is a really broad subject. Different aircraft have different types of equipment to help pilots find their way. I’ll stick with a generic airliner setup. And don’t worry… I’ll make this really simple and easy to understand!
Point-A to Point-B starts with a map (or an iPad)
Enroute charts are the road maps of the sky. They display airways that connect any two places you need to go. Airways are designed to keep air traffic organized and separated.
An airline dispatcher uses a computer to help analyze the weather and winds between the origin and destination. He or she then determines the most economic route using the airway system. For U.S. flights, this requested route is electronically sent to an FAA Air Route Traffic Control Center (ARTCC) that analyzes the proposed route and compares it to thousands of other requests as well as traffic currently in the air. The local Air Traffic Control (ATC) facility at the departure airport will tell the pilots just prior to takeoff if the requested route is okay (it usually is) or if any changes need to be made due to traffic congestion or weather.
Here’s a really simple route. We’re flying from Memphis, Tennessee (KMEM) to Knoxville (KTYS). The symbols MEM, BNA, and VXV are navigation radio aids (NavAids) at Memphis, Nashville and Knoxville. The lines connecting the NavAids are the airways we will be using: J42 and J46. The “J” in J42 stands for “Jet.” Jet airways are high altitude routes for, umm, jets. Our requested route in fancy ATC notation is MEM-J42-BNA-J46-VXV. Simple!
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Now that we have our route, we need to convince the airplane to follow it!
The Flight Management System or “The Box”
Onboard just about every airliner built since the early 1980’s is a box called the “Flight Management System” or FMS. Pilots call it simply “The Box.” The FMS is the ship’s navigation brain. When your captain comes over the PA in his captainly swagger and states “The computer shows us landing in Dallas in 43 minutes,” he’s not pulling that number out of his… head. He’s getting that time from “the box”.
The box uses several sensors to keep track of the aircraft’s position. Inertial reference system data, radio navigation signals, and on newer boxes, GPS position are all fed into the FMS to increase accuracy. The most important part of the box is it’s large database of the airports, navigation aids, and airways necessary for the route. Most everything that’s displayed on the enroute chart above is in the box’s database.
The Boeing 767 that I fly uses an FMS like the one pictured above. Take a look at the flight plan that’s been loaded into the box, it’s the same flight that I highlighted on the map; MEM-J42-BNA-J46-VXV. During our preflight prep, we type the route of flight into the box. In many cases, routes are preprogrammed (or “canned”) and we can type a short route code to retrieve them; this saves us a bunch of typing on long routes.
Enough preflight, it’s time to fly!
Now that our route to Knoxville is in the box, it will be displayed on our Horizontal Situation Indicator screen. This screen shows the route as a solid magenta line (J42 and J46). The big triangle at the bottom is our airplane. Sure enough, you can see the navigation aids we loaded earlier; MEM, BNA, and VXV just offscreen at the top.
After takeoff from Memphis, air traffic control might direct us to “intercept J42.” This means “hop on the magenta line and head toward BNA.” Our autopilot will follow the magenta line with pinpoint accuracy. The FMS can do lots of other cool stuff that isn’t within the scope of this article. If you want to learn more, check out the FMS Wikipedia page or the FMS SkyBrary Entry.
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Other ways we stumble around the skies
During most of a flight, it’s up to the pilots and our FMS to guide the aircraft along the assigned route. But what if another airplane comes cruising along in our direction? What if his magenta line happens to cross our magenta line at the same time and same altitude?
AeroSavvy’s First Law of Aviation: No two aircraft shall occupy the same space at the same time.
This is where Air Traffic Control (ATC) helps us out. The primary job of ATC is “aircraft separation”. Using radar displays and sophisticated computers, controllers have the big picture of all the airplanes in their assigned sector. They can spot a potential conflict while aircraft are still hundreds of miles apart. When they see a problem, they will contact one or both aircraft and instruct the pilots to change course or altitude to avoid the conflict. This usually happens a few times every flight; it’s very routine. As a passenger, there’s no need to worry – relax and enjoy your peanuts and coffee; you paid good money for them!
Another job of ATC is to help airplanes transition from the cruise phase of flight to landing. Pilots have everything they need on charts and the FMC to do this on their own; but around big cities, there are too many airplanes speeding toward the airport at the same time. To safely separate and space the airplanes to land on the runway, controllers will watch all the planes on their radar display and instruct pilots to change their speed and direction, guiding them with voice instructions onto final approach. This process is called “vectoring”. Once we land, a similar process happens on the ground while taxiing to the gate. A ground controller in the tower watches the busy taxiways and acts as a traffic cop to keep things moving in an orderly fashion.
Of course, there’s more involved in flying a jet from New York to Los Angeles than discussed here. However, you now know more about airplane navigation than most of the folks at your next cocktail party. (Unless you invite a bunch of pilots; which is a bad idea because they’ll yap on-and-on all night about airplanes!)
Coming up…
Have you ever heard aircraft radio communication? A few airlines let you listen to the pilots and controllers over the aircraft’s entertainment system. In Stuff Pilots Say! I decipher aircraft communication so you’ll know what to listen for. I’ve even include samples of authentic pilot radio gibberish!
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