Why do planes get put in holding patterns before being cleared to land? If there's bad congestion, wouldn't it be smarter to hold planes BEFORE they take off (or mid-route) instead of having a dozen planes flying circles near the congested airport?

That’s a great question and I suspect you may be asking on behalf of air travelers everywhere! For the answer and a little bit more, you might need a comfy chair and a bowl of popcorn.



Holding patterns are a necessary ‘court of last resort’ in the air traffic system. Like spare tires, we have to keep them handy but hope we never have to use ‘em! Normally, excess demand is managed in a more fuel efficient manner. But let me back up a bit.



Airports have ‘capacity’ numbers that tell those who work within the air traffic system just how many arrivals and departures can safely be handled there per hour, with little to no delay. Airport capacity is based on several factors such as the number and length of runways, weather conditions, available instrument approaches, etc. Capacity may be reduced by events such as a runway closure. Other capacity limiting factors include adverse weather conditions, aircraft accidents, inoperative landing aids, equipment failures within the controlling air traffic facility and such. When, for whatever reason, an airport’s capacity falls below the projected hourly demand – steps can be taken to limit airborne delays.



There are people in the air traffic system who’s job it is to continually monitor airport capacity and make necessary adjustments to the demand. One relatively simple way to control arrival demand on an airport is by implementing ‘mile-in-trail’ (MIT) restrictions. When MIT is used for a certain airport, air traffic facilities are advised to space their departures headed for that airport more widely than usual. Twenty miles in trail is a common restriction. MIT aims to provide controllers at the arrival end with a manageable flow that won’t exceed the airport’s limited capacity.



There are other tools in use to limit demand. If, for example, arrival capacity is reduced at Chicago O’Hare due to heavy snow and closed runways, a Ground delay Program (GDP) may be implemented. Basically, a GDP makes adjustments to the expected departure times of flights headed for O’Hare. These flights take their delay at the departure point. Without the GDP, they’d take off at their originally scheduled times and fly to the Chicago area, where many would end up in holding patterns because the airborne demand exceeded Chicago’s current arrival capacity.



If things get especially bad at the arrival airport, ATC has an even bigger hammer in their toolbox called a Ground Stop. When a Ground Stop is implemented for a particular airport – all flights destined for that airport must stay at their departure point until the Ground Stop is lifted.



These measures were devised to keep the air traffic system from becoming overwhelmed by airplanes with no place to go but the holding patterns. However; old school and inefficient as they are, holding patterns remain a necessity. Unforeseen, short term constraints at the destination airport may require some limited airborne holding. A good example would be when the airport needs to change landing direction. Some arrivals, already nearby, may have to be delayed somewhere until the change is complete. Holding patterns work well in such cases.



Keep all this in mind the next time, as Christopher Cross once wrote, “you get caught between the moon and New York city!”



Cheers,

Factor