The Federal Aviation Administration (FAA) defines en route delays as delays of 15 minutes or more during the length of the flight, after the aircraft has been pushed back from the gate. More specifically, a delay (of 15 minutes or more) incurred on a taxiway, on a runway, en route, or in the airspace holding pattern, is considered an en route delay.
En route delays must not be confused with airline-related delays, which are generally manifested before the beginning of the flight. Such delays can be caused due to mechanical issues, crew shortages, operational failures, and connecting passenger arrivals (for hub-spoke networks).
En route delays
Most commercial flights in the US follow planned schedules. Actual arrival times depend on the delays incurred during the flight. While airlines account for minor forecasted delays based on the regions they operate in, unforeseen circumstances can typically cause en-route delays.
Weather-related delays, as the name suggests, are a result of extreme weather conditions around the airport or along the flight path. Extreme winds, thunderstorms, rain, and snow are some examples of weather-related delays.
Severe weather around airports prevents incoming aircraft from landing safely, and outgoing aircraft from departing on time. Arriving aircraft are usually requested to enter a holding pattern until the weather is cleared out.
Depending on the duration of the delay and the reserve fuel onboard, flights either remain in a holding configuration or be diverted to alternate airports. Diverted flights wait out the weather on the ground before flying back to their original destination.
In the summer of 2014, a severe thunderstorm left 177 arriving flights in multiple holding patterns around New York area airports. More than half of the flights were diverted to other airports until the weather cleared up. In a span of just a few hours, the cumulative delay caused by the thunderstorm amounted to over 5,000 minutes (83 hours).
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In February, more than 3,000 flights into and out of the US were delayed due to a snowstorm that lasted two days. Flying through the path of the snowstorm, a majority of the flights experienced en route delays.
National Airspace System (NAS) delays
A variety of en route delays are attributed to the National Airspace System (NAS) delays. Such delays are incurred due to airport operations, heavy air-traffic volume, and air traffic control.
Scheduling more rotations than the maximum capacity of the airport can result in en route delays. Airlines in the US are known to increase operations during summer times and around major holidays. The overall increase in demand oftentimes exceeds operational capacities at airports, and causes delays.
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Runway closures also cause delays for departing and arriving flights. While planned runway closures can be accounted for during scheduling, emergency incursions on or around runways can trigger delays. On October 5th, an incident caused a brief closure of a runway at Dayton International Airport (DAY), which forced several arriving aircraft to divert to alternate airports.
Air traffic volume
Heavy air traffic volume, especially during peak departure and arrival times at large airports, can cause en route delays. Departing aircraft have to follow long queues on taxiways before receiving takeoff clearance. During winter, pre-flight de-icing is required in case of ice buildup on aircraft control surfaces. De-icing is usually performed after the aircraft has pushed back, and hence can cause en route delays.
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Congestion in the airspace is dynamic in time and space as it depends on ever-changing air traffic volume. Higher volume during peak times can send approaching aircraft into holding configurations for long periods of time.
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Airspace closures due to military excursions can also slow the moving traffic. If not planned properly, cross-network traffic passing through airspace intersections can impact approaching aircraft movements closer to the airports.
While airline operators generally account for a majority of anticipated en route delays in their schedules, delays due to severe weather cannot always be predicted.
Have you recently experienced an en route flight delay? How long was it? Tell us in the comments section.