Why is My Flight Late?!?

One day as a teenager, my dad and I were trying to fly home from Calgary after a ski trip. Our flight was delayed, and I didn’t understand why. Dad asked the gate agent, who said that our aircraft was delayed somewhere else and was arriving late into Calgary. As silly as it might seem, that was a big epiphany for me. Apparently, planes didn’t magically appear at our gate for us to board at the scheduled departure time. Hmm.

Now that I actually work for an airline and see the other side of delays, I thought some of you might have the same question: why is my flight late?

Think of an airline as a really big, really expensive bus system. A bus starts the day at one location and goes from place to place for the rest of the day, picking up and dropping off passengers at each stop. Any number of things can go wrong along the way to throw off the schedule – traffic, passenger issues, bad weather, mechanical problems, you name it. An airline works the same way except with many more rules, many more people involved, and many more details to work out. And if one flight is late, all the remaining flights that day for that aircraft are likely to be late as well.

If a flight is late, sometimes the cause is easy to pinpoint. Perhaps an engine won’t start. Perhaps there’s a huge thunderstorm over the airport. Perhaps some yahoo called in a fake bomb threat that the authorities must investigate. But sometimes the cause isn’t so easy to identify. Here are some examples of less obvious causes for a flight delay:

ATC – Air traffic controllers are high-tech traffic cops whose jobs is to ensure that the aircraft don’t fly too closely together. Due to FAA-mandated safety rules and operational considerations, each airport can safely handle only a certain number of airplanes per hour based on the runways in use, weather, and available navigational equipment. A smaller airport like Houston Hobby or Chicago Midway might be able to land 20-30 flights per hour. A huge, well-designed airport like Denver or DFW can handle over 100 arrivals per hour under normal conditions.

Problems occur when the airlines schedule more flights to land somewhere than the airport can accept during a given time. This happens daily for short periods in some airports such as Philadelphia and New York’s LaGuardia and Newark. If demand exceeds capacity for only a short time, air traffic controllers might force some inbound flights to fly in a circle once or twice, a maneuver known as holding. Each “donut” might take 5-10 minutes. If demand is expected to exceed capacity for a longer period of time, perhaps due to bad weather or a runway closure that reduces the amount of traffic an airport can handle, ATC starts delaying flights on the ground at their departure airports. This leads to late flights, frustrated and confused passengers, and lots of phone calls to Dispatch.

This graphic shows traffic in Philadelphia. The horizontal white line shows how many aircraft the airport can handle every 15 minutes. The green bars show the number of aircraft scheduled to arrive. From 1315-1345z, the airport is way overloaded, and many of those flights will spin a bit. That’s why we dispatchers plan extra fuel for PHL flights that arrive during that time.

Weather – Yes, weather in general might be clear as a cause of delays, but what specific aspects of weather actually cause problems? Thunderstorms are a highly visible example. Obviously, it’s not safe to take off or land in a severe thunderstorm. They can also cause problems if a line of storms lies between the two airports. Our planes can fly over a less severe storm, but some thunderstorms can reach 40,000, 50,000, or even 60,000 feet, too high for us to fly over them. In those cases, we must fly around them, which could add hundreds of miles and 30-60 minutes or more to the flight.

Fog is another problem. The FAA issues rules, known an minimums (or minima, if you want to be more grammatically correct), that tell us two important restrictions for when we can land:

  1. The minimum visibility needed to approach the runway
  2. How low the pilots can descend before seeing the runway just before they land

For a good runway, we might need 1/2 mile visibility and 200 foot “decision height”, meaning the bottom of the cloud layer must be 200 feet or higher to allow the pilots to see the runway and decide whether to land or not. If we don’t have enough visibility or high enough clouds, we can’t land, meaning we hold or divert. In order for a flight to depart, we must have reason to believe that the weather will be good enough to land when we get there. Part of the dispatcher’s job is to monitor the current and forecasted weather and ensure that they are good enough to launch the flight. If not, we have to throw the flag and tell the pilots not to leave yet. As you’ve probably guessed, that generates some phone calls to our office as well.

Wind can also play a role. When an airline builds a schedule, the planners estimate how long each flight will take based on the expected route, the airports involved, and historical data on flight time and winds. The upper-level winds where our airplanes fly generally blow from west to east and tend to be stronger in the winter. However, the winds are a little different each day. An unusually strong tailwind helps a flight arrive earlier than scheduled, sometimes 30-45 minutes early. Conversely, an unusually strong headwind can delay the flight by the same amount of time. Dispatchers try to plan our longer flights with more favorable winds, but on some days, there’s just not a viable way to make a flight reach its destination on-time.

Maintenance – If the engine won’t start, that’s a pretty obvious mechanical problem. But not every broken part must automatically delay the flight. Some parts, such as a broken tray table or busted lavatory, don’t inhibit the safe operation of the flight. Other parts are only necessary for certain flights, such as the onboard weather radar for a flight that could encounter thunderstorms or ice removal equipment for a flight into a snowstorm. If something breaks on an aircraft, the captain, dispatcher, and mechanic must decide whether the flight can proceed as planned or whether the part must be fixed prior to departure. Sometimes it can take a while to figure out the best course of action or to actually fix the busted item. We want to operate safely and legally above all, which sometimes means delaying or canceling the flight.

Passenger Issues – Connections generally tend to increase delays. A late inbound flight might have passengers connecting to several other flights. Customer-friendly airlines like Southwest try to hold those departures to ensure the connecting passengers make it on, especially if the delay isn’t too long or it’s the last flight of the day for that market. Sometimes, especially for an airline like Southwest that handles LOTS AND LOTS OF BAGS, it can take extra time to get all the bags on and off, particularly for connecting passengers whose bags need to be retrieved from one plane and transferred to another.

TSA – On rare occasions, the TSA detects a possible security breach and requires all passengers in a terminal to be rescreened. Other times the TSA security line is running extra slow, and many passengers are unable to reach their gates on-time as a result. The airline might delay flights for any of these reasons.

Delays can occur for other reasons such as human error or computer problems, but the causes above are responsible for the vast majority of flight delays. Delays tend to compound over time, so as a general rule, your flight is more likely to be delayed at the end of the day versus the morning. Our office has a special group of people who try to reduce delays by reassigning aircraft from one flight to another, a mixture of art and science known as “swapping”. Our pilots and airport employees also work to mitigate delays where they can by working faster and smarter, focusing on problem flights, asking for shortcuts in the air, and making up time on the ground.

I hope this helps clarify things a bit. If you have any questions, post them in the comments. I love talking about this stuff. =)

Disclaimer: Although I work for Southwest Airlines, all opinions expressed on this website are solely my own and do not necessarily represent those of my employer, its leaders, or its board of directors. Yes, they make me say this. 🙂