This time of year seems perfect for discussing the biggest issue in air travel. Delays. There may be a few different reasons for delays, but, for the most part, they are all caused by the need to maintain safety.
While it may seem that most delays are due to poor weather, and this is mostly true, the root of most delays stem from a lack of runways at your destination. I will stress that this lack of runways at our busiest airports is made most apparent during periods of bad weather, but is not exclusively due to weather. Also, some delays are due to bad weather in the en-route phase of flight, and have nothing to do with the weather at your origin OR destination. These are the delays that most puzzle the general public, in my opinion.
Let us start with some basics. Traffic Management has determined all the possible arrival and departure runway configurations at every major airport in the country. The use of each configuration depends on the visibility, cloud bases, and most importantly, the prevailing wind. Some configurations are capable of more operations per hour than others. The actual number of arrivals and departures per hour are publicly documented for each configuration at each major airport. They can be found here. I'll use this link to support a few examples.
Simply click on the plus signs next to East or West Directory, and then pick an ARTCC (Center) and then choose and airport. A chart should appear showing the relationship between each runway configuration and the arrival and departure rates in both optimal visual conditions and when instrument approach must be used. You'll notice the rates are different for visuals as opposed to when ILS approaches are in use, due to the increased separation required on final approach. I'll review these pages so you get the most out of them. Lets click on WEST DIRECTORY-ZDV-DEN (click on the plus symbols next to the words to open the menus). The first row in the DEN chart is what is called LAND SOUTH configuration. All operations are in a southern direction, as would be necessary when the winds are out of the south. Coincidentally, this configuration is also the preferred configuration when the winds are calm, as the residents of eastern Colorado tend to complain the least when these runways are used. Most importantly, landing south allows the highest arrival and departure rate due to the runway set up. The IFR column lists the arrival rates in the worst weather (during snow storms, fog, etc). The separation rules change slightly when the weather is better (VFR column) and the airport can accept a lot more airplanes per hour. During a nice sunny day, when pilots can fly visual approaches, they are simply told which plane to follow on final approach, and separate themselves. Pilots are very good at this, and can decrease the space on final down to as little as 2 miles. Taxiways at the airport are angled so aircraft can exit at high speeds, which allows them to spend less time on the runway, and thus, trailing aircraft can follow at an even closer distance. Pilots can adjust their speed to ensure they touchdown soon after the preceding aircraft has exited the runway.
During weather that requires use of ILS approaches, aircraft cannot maintain visual separation with other traffic. Controllers need to vector planes onto the final approach course and must maintain anywhere between 3 and 5 miles between planes. Lets do some quick math (not really my strength, but bear with me). During busy periods, as many as 10 planes can be lined up on final approach. If they were in visual conditions, that would require approximately 20 miles. When using ILS approaches, controllers end up with a 30-40 mile line. Thats almost DOUBLE the amount of space required for the same amount of aircraft. At airports with only one runway (or when a single runway is used for both takeoff and landing), spacing on final approach may need to be 6 miles to allow departures to depart between arrivals. I'll let you realize how much less of an arrival rate that is compared to a runway solely used for visual approaches. Now let us head east, where airports that have dual use runways are much more common.
Lets click on EAST DIRECTORY-ZNY-LGA. The numbers here are much smaller, yet the demand is higher. During ideal operations, LGA has an arrival rate of 44. If the winds are strong out of the northeast, the arrival rate can drop as low at 18. Where do the other 26 arrivals go? They hold at the departure airport waiting for their place in line. If you look at the Holding Capacities chart below, you'll see that only a few aircraft can hold at each of the arrival fixes. This is due to airspace constraints at the en-route level. Remember, these sectors are also sequencing (and holding) arrivals to EWR, JFK, and TEB. Allow me to point out that the total arrival rate during perfect weather for all three Ny metro airports COMBINED (LGA - 44, EWR - 58, JFK - 68 = 170 arrivals per hour) is only TEN airplanes per hour more than Denver's one airport (160). Perhaps we'll discuss that issue some other time.
Traffic Management is in control of the big picture. They use these arrival rates to determine when planes should depart. The goal is to induce delays at the departure airport before the airplane even leaves the gate, which tends to be more comfortable, and much safer than airborne holding en-route. This doesn't always work out, however. Storms may move faster, slower, or in a different direction than expected, causing a different arrival rate at different times. Emergencies disrupt traffic flows without notice. Runways close for maintenance, greatly reducing capacity. And sometimes, controllers just need to go to the bathroom (sorry, couldn't resist!).
In the next post(s), I'll go more in depth about how changing arrival rates affect the system, and how weather BETWEEN your departure and arrival airport can greatly affect your flight as well.