Time and again I have criticised the airports in India for not having the runway, terminal and apron capacity in sync. While I can cite multiple examples, places like Patna or Raipur are prime examples of how the runway can support additional movements but the terminal is so small that it cannot handle load worth additional flights in case of Patna and there are limited bays available at Raipur – which boasts a state of the art terminal!
Airport capacity is a combination of Terminal Capacity, Apron and Runway and is unfortunately the lowest value of any of the three which dictates the capacity. For ease of use, the air side infrastructure like Runway and Apron is clubbed together and the Terminal Capacity is considered separately which includes the boarding gates, check-in counters and waiting area for departures and arrival gates and baggage belts for arrivals. A NAC (Notice on Airport Capacities) chart gives a picture of runway and terminal capacity in a single view and helps understand the status of the airport based on particular time bands.
A runway can handle a finite number of movements per hour. In aviation parlance this is known as R60 i.e. Runway movements in a 60 minute timeband. For airports which have heavy congestion like London Heathrow for example, the movements are also calculated at a 15 minute interval known as R15 and could go down up to 5 or 10 minute band as well. Airport infrastructure like Parallel taxi track, rapid exit taxiways (RET), navigational aids are some of the primary factors which dictate runway capacity. While there is a total cap on runway capacity, the number of departures and arrivals are also capped. As an example, for a runway which can handle 40 movements in an hour, all 40 cannot be departures or arrivals. Typically in the morning hours, there are more departures than arrivals and the trend reverses in the late evening hours. These are the times when base aircraft depart and arrive respectively.
An airport also has a finite space to park aircraft. No wonder then that globally LCCs are becoming favorite for airports since they are known to have a quicker turnaround which helps free up apron space and accommodate more traffic for the airports. The apron of the airport has markings to accommodate aircraft and is always carefully designed with traffic projections in view and type of aircraft which the airport is likely to handle. At larger airports, the bays are mixed mode – where a bay which can occupy a wide body aircraft is so designed that two narrow body aircraft could be parked at times when a wide body aircraft is not occupying the bay. In India, such examples can be found at Delhi, Mumbai or Bengaluru airports.
Runway capacity and apron capacity go hand in hand since an aircraft cannot be allowed to land and wait indefinitely for a bay to be parked and likewise the airport or airline does not benefit if there is a lot of apron space but runway limitations do not allow additional movements. While this sums up the air side infrastructure, the tricker part is the terminal infrastructure.
An airport terminal would comprise of check-in areas, security, security hold areas (SHA), immigration facilities, lounges, waiting areas, food and retail section on the departure side along with immigration facilities, customs, lounges, food and retail section, duty free and baggage belts on the arrival side. These are areas which are seen by the traveling passengers. There are many more sections which are away from the eyes of the passengers.
While runway and apron capacity is real time, a passenger arrives at the airport for his or her flight with some time at hand. This advance arrival is different for domestic and international and varies from passenger to passenger, thus taking away the predictability of passenger flow on a particular day. This is unlike how an aircraft arrives at an exact time which is known beforehand and any changes on the day of operations are also known in advance. To couple with this, terminals also have bottlenecks – at check-in, security, immigration, etc and it is important that passengers reach the boarding gates within time. Any delay leads to a cascading effect for the airline and also the airport.
Thus, terminal capacity is measured at an per hour basis and is different for arrivals and departures. Passengers tend to spend very little time at the airport when arriving as compared to when departing from the airport. If there are more flights departing between 0600 hours to 0630 hours, the terminal congestion will be typically between 0430 hours and 0530 hours. If the terminal capacity is exhausted, any more infrastructure on the air side would not help additional capacity since check-in, security and other terminal infrastructure hasn’t been augmented. Likewise, 30 minutes post arrival of a flight are the peakest for the arrivals section and availability of baggage belts dictates how satisfied the passengers are.
Understanding the NAC chart
The NAC chart depicts times when the capacity is full, denoted with “X”; when it is close to full, denoted with “C” and when the airport is not available for operations, denoted with “–”.
Lucknow – domestic for example shows up this NAC chart.
While the runway is not busy at any time during the day, there are certain hours when the arrival or departure movements are at peak without any scope for additional movements or close to that. Likewise, the terminal is busy. Even when the air side infrastructure can accommodate additional movements, the terminal cannot support those.
Calculating airport capacity is a challenging task and ever changing. In an era where airports earn a lot of revenue from non-aero activities like retail or food, passenger footfalls are very important but the space is always finite!