Delhi airport faces disruption after ATC system failure

More than 100 flights were delayed at Delhi’s Indira Gandhi International Airport (IGIA) on Friday morning following a technical issue with the Air Traffic Control (ATC) system. Airport authorities confirmed that efforts were under way to resolve the problem.

With over 1,500 aircraft movements each day, IGIA is the busiest airport in India. In a passenger advisory, Delhi Airport stated, “Flight operations at IGIA are experiencing delays due to a technical issue with the Air Traffic Control (ATC) system. The team is working hard to find a quick solution.”

Travellers were advised to remain in contact with their respective airlines for the latest updates on flight schedules.

How air traffic control works, and why the technical glitch caused delays

At major airports such as IGIA, air traffic management is a complex, multi-stage process involving several ATC units and specialised systems that ensure the safe and efficient movement of aircraft both on the ground and in the air.

Filing and distribution of flight plans

Before each flight, airlines or pilots submit a flight plan to the ATC system. This includes information such as the route, altitude, estimated timings, and destination.

The Automatic Message Switching System (AMSS) is responsible for automatically distributing these flight plans to all relevant ATC units and agencies,  including the Aerodrome Control Tower (which manages surface and take-off control), the Area Control Centre (AMCC), radar controllers, and approach and departure units.

Each flight plan includes address codes to ensure it reaches the correct controllers, airports, and regional centres. This automation enables hundreds of flight plans to be processed and shared instantaneously.

Ground operations – Surface Movement Control (SMC)

Once the flight plan is processed, Surface Movement Control (SMC) manages the aircraft’s movement on the ground, including taxiing from the gate to the runway.
SMC operates on a dedicated radio frequency (RT set) to communicate with pilots, ensuring the safe and efficient movement of aircraft and ground vehicles.

Take-off and departure control

After take-off clearance is granted, the pilot switches to a departure frequency, where the departure controller provides instructions on climb path, heading, and altitude until the aircraft reaches a designated height.

At this point, control is transferred from the tower to a radar controller, who monitors the aircraft’s position and ensures safe separation from other traffic.

Area control

Once an aircraft reaches cruising altitude, it comes under the jurisdiction of the Area Control Centre (AMCC).
The AMCC manages a large airspace region, monitoring aircraft travelling between airports and coordinating with other regional centres as flights move between different control zones, or jurisdictions

Controllers use fixed geographical reporting points (landmarks) to track aircraft positions and maintain orderly traffic flow.

 Approach and landing

As the aircraft approaches its destination, control is handed over to the Approach Control Unit, also known as Instrument Approach Procedures (IAP) control.
The approach controller guides the aircraft from cruising altitude to its final approach path before transferring control back to the tower controller for landing clearance.

The role of the AMSS, and why the glitch led to delays

The Automatic Message Switching System (AMSS) is a critical part of the ATC network. It automatically routes and shares essential information,  including flight plans, weather updates, and control messages,  between various ATC units, both within the airport and across regional centres.

When the AMSS at Delhi Airport became “temporarily unserviceable”, the automatic transmission of flight data stopped. Controllers were forced to process and distribute flight plans manually, a slower process that caused bottlenecks in coordination across ATC units.

Since every stage of a flight,  from taxiing and take-off to en route control and landing, depends on real-time data exchange, the slowdown affected:

• The approval and sequencing of departures and arrivals

• Coordination between controllers in different airspace sectors

• Updating of radar and tracking systems with flight details

This reduction in efficiency significantly slowed down the rate at which aircraft could be cleared for take-off or landing, resulting in more than 100 flight delays.

The impact

The ATC system functions as a tightly coordinated chain of units, SMC, Tower, Radar, AMCC, and IAP, all relying on the AMSS for seamless communication.
When this automation fails, manual intervention slows down every part of the process, causing widespread delays. While such failures do not usually pose immediate safety risks, they can have a major operational impact at high-traffic airports like IGIA.

Vipin Kumar, Chairman of Airport Authority of India did not respond to queries from Voice&Data till the filing of this report.