Why Airplanes Don’t Always Fly in Straight Lines to Their Destination
If you’ve ever been on a flight equipped with a screen that shows the flight path, you might notice some zigs and zags that make your direct flight look like a scenic air tour. There are a number of reasons for this, but most of the time it comes down to Air Traffic Control (ATC).
Some people think that air traffic controllers are the guys that stand on the ground, waving lighted wands to guide the plane up to the gate. Those guys are actually part of the ground crew and they only have control over your flight for the last couple hundred feet before you reach the gate. The rest of the flight is controlled by someone else and it’s not the pilot.
This system is a lot more complicated than it seems.
At the airport, the air traffic controllers sit up in the control tower. Those guys decide who gets to take off and land, which runways they use and when. They also direct planes that are moving around on the ground between gates and runways on the apron and taxiways. This aims to provide an organized flow of ground traffic and a safe flow of air traffic.
Once your plane has left the immediate area of the airport, the pilot must then communicate with a regional controller at an Area Control Center (ACC). If you’re on a long flight, you may get passed from one ACC to the next multiple times as you fly across the country.
The main purpose is safety. These controllers use radar to see all of the flights in their area and they are expected to provide proper separation between planes. Depending on the altitude, the minimum separation distance changes. That is because less sophisticated planes have pressure sensitive altimeters which are less accurate at higher altitudes — greater separation at higher altitudes compensates for risk.
At high altitudes (above 29,000 feet / 8870 meters), planes are required to have at least 2000 ft of vertical separation (it is usually 1000 ft when below 29,000 ft) although there are exceptions.
They even stagger the planes based on the direction they’re flying. Planes on the next level above and below you will becoming towards you. This makes it easier for the planes to see each other and ensure they’re at the proper height to avoid a collision.
Horizontal or lateral separation also varies. It’s not dependent on altitude but distance from the radar antenna. 3 nautical miles of separation are required within 40 nautical miles of the antenna, 5 nautical miles when more than 40 nautical miles from the radar antenna. This compensates for inaccuracy as the planes get further away from the radar antenna.
Many aircraft (especially large commercial planes) also have an on board system that is designed to alert the pilot when other planes are dangerously close or on a collision course.
When airplanes are over land, they actually have to fly where there is ground based radar that can track them. So your flight may be directed into “lanes” that keep the flights in an organized flow and within radar range. As I mentioned, having planes in radar view is important for preventing collisions but it’s also important for dispatching help if a plane stops responding or goes missing — they have an accurate last point of record.
Over the ocean, there isn’t any radar tracking. Many planes have on board GPS navigation systems, but there is no current standard to make that data available to flight control centers. This the why it was so difficult to find the Air France flight 447 that crashed in the ocean en route from Rio de Janeiro, Brazil to Paris, France in June 2009. Nobody noticed the flight was missing until it failed to make a radio call as it entered airspace controlled by a ground radar facility. When they tried to find the plane, the last location information they had was in a few short messages automatically relayed by the maintenance system on the plane as some systems began to fail before the crash.
The FAA (United States Federal Aviation Administration) is currently working on Nextgen, a new GPS based flight tracking system that will be much better than the current ground based radar network.
For now, some flights will let you listen in on the radio communication channels that pilots use to talk to controllers. If you ever have the chance to listen in, it’s very interesting. In fact, you can stream live feeds from LiveATC.net.
Related Post: Why Airlines Dim Interior Lights Before Night Landings
Written By: Chad Upton
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