Have you ever wondered why, when talking about the shortest distance between two places, we say, “X km as the crow flies”? Why don’t we say as the plane flies because, after, all both the crow and the plane fly?

The answer is simple. Birds like crows fly straight while planes do not. Ask any pilot why this is so and pat come the answers: “Because birds are smarter” or “birds don’t have engines that can fail”.

But jokes apart, there are many reasons why planes don’t fly following a straight path, some of which are scientific, though all of them are serious for the safety of flights — and covering the longest distance in the shortest possible time for flyers’ convenience and airlines’ bottom lines.

According to Captain PP Singh, Senior Vice-President, JetLite, perhaps the most important reason why planes do not fly straight is wind patterns. “As we fly for the least possible time and not the shortest distance, just like google map does these days for road trips,” wind patterns make a difference, he explains.

Among the other reasons cited by Captain Singh are following a route structure which meets Air Traffic Control (ATC) requirements which is not necessarily a straight line, and airspace availability, as thousands of planes criss-cross international skies every hour.

In addition, aircraft have to stay within a specified time from a suitable airport. This varies from 60 to 240 minutes (called Extended Diversion Time Operations or EDTO) cleared for an aircraft. Earlier, aircraft had to be 60 minutes from a suitable airport but as engines became more reliable,this time was increased to 90 minutes of flying time to a suitable airport. “This approval is based on the aircraft’s reliability, an airline’s safety record and an airline’s record of completing successful EDTO flights for clearance of 180 minutes and beyond,” a pilot says, adding, “All aircraft have to ensure that at any point along the route that they take, a suitable airport is available where they can make a safe landing. Suitable in terms of runway availability, weather and available approaches.” This might not be possible if an aircraft is flying on a straight route.

Then there is also the issue of every country having Pre-Determined Routes or PDRs which help in controlling air traffic. “It is like a highway so that traffic moves in an orderly fashion. For example, any aircraft which is cleared from Delhi to London has to file a flight plan. This plan gives the flight path that the aircraft will follow in each country. These paths are pre-determined routes. A pilot knows these before taking off and feeds them into the navigation system,” says a former pilot. The same is true even for domestic flights.

Compass corrections

Captain Amitabh Singh of Air India points out that whether you fly straight or not depends on whether the aircraft is using satellite or ground-based navigational aids and how you depict this on the earth’s surface. Each map or depiction is based on different projections. “If you use ground-based navigation you can fly a straight line as you have to fly point to point following a ground-based navigation system like a VOR,” he says.

“When we are doing point-to-point then we do something called a Rhumb Line track,” says Captain Amitabh Singh. This involves taking the help of a compass but the compass has variations which keep changing at every intersection of a longitude and a latitude. Pilots have to keep making corrections for these changes in variations because, if they don’t, the aircraft will never reach its destination.

Adds Captain Amitabh Singh, “If you draw a straight line between London and Delhi and fly on a compass then you will never reach your destination as you will keep making corrections to the compass, thereby moving further away from your destination.”

Geopolitical factors

Geo-political reasons also determine the routes that planes fly. A senior pilot says that every country marks the routes for aircraft to fly in its airspace because of the overlying of its prohibited military areas. “So unfortunately it is not a straight line,” an Air India pilot points out, adding that to control crossing aircraft, a slightly left/right airway is also made.

Then there is also the issue of overflying rights over every country’s airspace and the costs it involves for airlines. Every airline needs permission to fly over another country’s airspace. For example, the Israeli national carrier EL AL is not allowed to fly over the Far East region and hence has to take a longer route, skipping the countries that do not allow it to overfly. Air India, which is the only other airline linking India and Israel with a direct non-stop flight, is allowed to fly over the Gulf countries making the flight shorter and faster than the EL AL connection.

Besides, every airline overflying another country has to pay fixed charges for using the ATC facilities in the country that it flies over, making flying aircraft a lot more complicated than birds finding their way from one point to another.