screen or have noticed seemingly illogical flight paths between cities while browsing flight tracking websites. There are many reasons why flights don’t travel directly between airports, and this article will identify and explain some of the reasons why flight paths zig and zag enroute.
Winds
because of the polar jet streams’ presence over continental areas in both hemispheres.
, which are planned around the predicted wind for the forecast period.
The effect of wind on flight paths is equally apparent when you look at flights longer than 1,500 miles. The images below are examples of two airline flight plans between New York and Los Angeles on consecutive days. The more southerly route takes the flight over the Midwest (Kansas City and Albuquerque Centers, for example). In contrast, the more northern flight plan has the flight over the Great Lakes region with Minneapolis, Denver, and Salt Lake Centers. While the paths diverge significantly, the ground distance is nearly identical: only ten nautical miles. However, the winds shifted enough in 24 hours to necessitate a pretty drastic augmentation to the flight path.
Here’s an interesting tidbit that provides perspective on how important it is to plan around headwinds: Suppose you have two scenarios for a round-trip journey. One way is 3,000 nautical miles, so the round trip is 6,000 nautical miles in total. Both round trips are flown by the same plane at identical indicated airspeeds. However, one of the round trips is flown without any wind at all, and the other round trip is flown with a 100-knot wind at altitude. It, therefore, has a 100-knot tailwind on one of the 3,000 nautical miles flights and a 100-knot headwind on the 3,000-nautical mile return journey. Will both round trips take the same amount of time, or will one trip take longer?
wind. Hopefully this helps to illustrate the concept of avoiding the most substantial headwinds, if possible.
Airspace Restrictions
Of course, there are plenty of instances when headwinds just can’t be planned around. One of the driving factors that leads to flight planning around optimum wind conditions (and also causes non-linear flight paths) is airspace restrictions. Things like space launches, firefighting efforts, drone operations, satellite or GPS outages, and VIP movement all influence airspace closure. More than anything else (in the US, at least), the military dictates airspace that cannot be used for civilian flying.
In the US, military airspace is called an MOA (Military Operations Airspace) or “Restricted.” It is used to conduct high-speed military flight training and testing, firing exercises, and other classified activities. Due to the scale of the military’s work, tens of thousands of square miles of airspace across the continental US are reserved for military use unless otherwise authorized.
If you’ve ever flown between cities in the US West, there’s a very good chance your route of flight was influenced by military or restricted airspace. For instance, the straight-line route between San Jose, CA, and Phoenix would send flights directly through multiple restricted areas. Therefore, flights are always planned to skirt the edges of the airspace.
Likewise, flights between Dallas and Phoenix tend to go over northern or southern New Mexico, but hardly ever directly through the middle of the state despite it being the most direct route. As you have probably figured, there is a lot of military and restricted airspace within New Mexico, and it is therefore another prime example of an area where flight planning has to account for unusable airspace.
Weather systems
cloud tops can reach heights well above an airliner’s ceiling. Flight plans are built to circumnavigate the convection when bad weather is forecast.
A massive line of thunderstorms a thousand or more miles long builds over the Midwestern US a few times yearly in the summer months. Occasionally, this kind of convective line can stretch from Houston to Chicago with only a few gaps where flights can make it through in between. Rather than plan a route through the line of storms, dispatchers file routes over the Gulf of Mexico or north of the Great Lakes to avoid severe turbulence. When this happens, it usually means that multiple airlines are flying hundreds of flights along a common route, which leads to delays and speed reductions as more planes converge on the limited, viable flight paths. This is also why a flight from New York to Los Angeles might end up over the Gulf of Mexico rather than the heartland, though this only happens once in a blue moon.
Controller availability
A lack of personnel working in the ATC system has been a bugaboo in the US since the end of COVID-19. Nearly all en route centers, terminal control centers, and tower facilities are understaffed compared to where the FAA would like them to be, leading to an inability to handle as much traffic as the physical infrastructure would allow.
One of the busiest and most popular air traffic corridors in the US is up and down the state of Florida, much of which is controlled by Jacksonville Center. Jax Center, as it’s colloquially called, is one of the facilities most obviously suffering from the understaffing problem. A single controller is only allowed to work so much airspace, and the lack of controllers during some work shifts leads to particular routes and altitudes being closed or metered to keep the ratio of flights per controller at acceptable levels. Airlines who fly to lots of destinations throughout Florida (every major airline in the US, in this case) work closely with the FAA to figure out “slots” and “flows” for when their planes will be allowed into the airspace.
In extreme examples, some airlines that would otherwise overfly Florida to Caribbean destinations file flight plans over the New Orleans area. This results in the flights being handled by Houston Center rather than Jax Center, meaning that the flight won’t get a ground delay before departure if the airline cannot secure a slot within the airspace that corresponds to when the flight is scheduled to be in the airspace. This results in more time in the air, but the flight will arrive closer to on-time than if the dispatcher had planned a more straight-line route through metered airspace.
Terminal area departure and arrivals
The last point about the inability to fly in a straight line concerns procedures built into a terminal area. Standard instrument departures (SIDs) and standard terminal arrivals (STARs) serve as gateways in and out of large airports around the world. They provide traffic separation so that arriving and departing planes aren’t in conflict, and they also control speed so that planes flying on common routes don’t get too close to each other.
SIDs and STARs are highly efficient and lead to common transition waypoints rather than directly toward where a flight is heading. Look at the DEEZZ 5 SID from New York JFK (below). This departure is meant to flow traffic out of the New York area while avoiding arrivals and departures from LaGuardia, Newark, and Teterboro airports.
The DEEZZ 5 loops traffic far to the north over White Plains after receiving headings to fly after departure. It might be confusing to take off to the south en route to a city west of New York and feel yourself turning left rather than right. Though the pilots would prefer to make a quick right turn and head west, SIDs and STARs prevent this because they serve the needs of every plane in the terminal area rather than one specific flight.
Conclusion
Out of the five topics discussed, the largest and most obvious show-stopper for straight-line flying is winds aloft, followed by restricted and off-limits airspace. Most weather is avoided with a 10-20 mile deviation, while rarer lines of storms induce hundred-mile offsets.
If little is known about the circumstances surrounding flight planning and air traffic control, it’s undoubtedly confusing why planes can’t just take off and set a course straight for their destination. After all, the sky is a vast and open place compared to the streets on which we drive our cars. True though this may be, the rules of skies dictate specific routes must be followed, and wind requires that straight lines be sacrificed for overall efficiency.
