The near-miss at Mumbai airport: What happened and can it happen again?

medium111002649Two aircraft speeding one behind the other on the Mumbai airport runway early this week raised anxious questions as the clip went viral on social media.Has traffic density cut safety margins at the Mumbai airport?Is this kind of ‘bumper-to-bumper’ air traffic even normal?Had the aircraft taking off slowed down for some reason, would the landing aircraft have crashed into it?Who was responsible for this dangerous situation — the pilots, the Air Traffic Control (ATC) or a system that forces an infrastructure-constrained Mumbai airport to handle more and more flights?The incident was reminiscent of the runway collision at Tokyo’s Haneda airport on January 2.A Japan Airlines A350 aircraft landed and collided with a Japan Coast Guard turboprop aircraft that was on the same runway. Investigations revealed that Japan Airlines Flight 516 was given landing clearance, while the Coast Guard aircraft did not have permission to be on the same runway.In Mumbai’s case, airlines operating the two flights have both claimed they had an all-clear from the ATC — one to land and the other to take-off. Serious security concern at @CSMIA_Official Mumbai Airport yesterday putting 100s of life at risk

While @airindia ✈️ was in the process of take off, another 🛬 from @IndiGo6E was allowed to land on same runway @DGCAIndia takes action against #Mumbai ATC official responsible pic.twitter.com/nsJvHZrWTZ

— Nikhil Lakhwani (@nikhil_lakhwani) June 9, 2024 While the Directorate General of Civil Aviation (DGCA) has initiated an investigation into the incident, it will be a while before the final report is out. Meanwhile, the Airports Authority of India (AAI), the country’s air navigation provider, has already put in measures based on the preliminary findings.What are these steps? Is the Mumbai airport runway particularly intolerant of split-second human errors? TOI+ deconstructs the incident to explore its four significant facets.The incidentOn June 8, an IndiGo aircraft descended onto runway 27 and touched down behind an Air India aircraft which was taking-off.The Air India flight lifted off the runway merely a second before the IndiGo aircraft touched down. At that point of time, the distance between the two aircraft was about 2,100ft, said sources.The norm is to wait for the aircraft taking off to lift off and fly past the runway end before the aircraft landing can be allowed to cross the beginning of the runway.India’s busiest airstripThe 11,302ft-long asphalt strip that is runway 27 of the Mumbai airport is arguably the most expensive piece of real estate in the city. It is rented to airlines by the seconds, 24x7. Airlines book time slots for take-off and landing.An A320 or B737 aircraft takes an average of about 40-50 seconds of time on the runway for a take-off or a landing. Between each aircraft movement, a buffer or safety margin of about 20 seconds is maintained.The ‘Runway Occupancy Time’ or ROT is then a crucial parameter for flight operations at space-constrained Mumbai. It’s a tightrope act, especially during the morning and evening peak hours. medium111002509Unlike most airports where distance is used as the criteria to maintain a separation between two aircraft, at Mumbai time-based separation is used.Called ‘TTT’, which stands for ‘Time to Threshold’, the air traffic controller allows a take-off if the landing aircraft is a minimum of 70 seconds away from the runway threshold. “Time-based separation allows for more aircraft movements. In 24 hours, we save about 30 minutes,” said a senior air traffic controller.But then again, if an aircraft is slow in taking-off or if a landing aircraft comes in too fast, quick remedial steps need be taken by the air traffic controller.The two that came too closeTwo A320neo aircraft — one operating Air India flight AI-657 from Mumbai to Thiruvananthapuram and the other operating IndiGo flight 6E5053 from Indore to Mumbai, were involved in the incident.“The domino was triggered by the Air India aircraft,” said air safety expert Captain Amit Singh, based on data gathered from Flightradar24.The series of events that led to the incident occurred in the 90 seconds after an Iraqi Airways flight took off and the Air India aircraft entered the runway for take-off. “The Air India aircraft was slow to take-off after it received the line up clearance. Of course, one is assuming it received immediate take-off clearance,” he said.The Air India pilot should have informed the controller about the delay in taking off, who would have cancelled his take-off clearance.medium111043258 The Aeronautical Information Publication (AIP), which lists the norms for aircraft operations among other things, states: “When an aircraft is issued with a line-up and take-off clearance at runway holding point it shall be in a position to line-up and initiate an immediate take-off in one continuous movement. If unable the pilot should advice air traffic control.”But the Air India pilots didn’t do that. They continued with the slow take-off. Meanwhile, the InidGo pilots were on an approach to land.The Air India aircraft had not lifted off the runway when the IndiGo aircraft crossed the beginning of the runway (the runway threshold with piano key markings is further down the line).Did the IndiGo pilots err too? It appears so. Once they spotted an aircraft on the runway, they should have aborted landing and initiated a go-around. According to AIP, the ATC clearance is immaterial if a pilot spots a danger. “If pilots perceive at any point that it is not safe to continue approach, they must initiate a go-around,” it says.The incident occurred in clear weather. Pilots can hear each other and communicate with the controller on the radio communications channel. Senior pilots pointed out that IndiGo pilots should not have continued with the landing after sighting the Air India aircraft on the runway.The air traffic controllerThe DGCA has derostered the controller for investigations. Two days after the incident, the AAI issued a circular stating that the preliminary investigation revealed violation of the standard operating procedure (SOP) for ‘Time-Based Spacing’ between arrivals.The SOP states: “If the controller observes a delay in aircraft commencing take-off run after issuance of take-off clearance, the take-off clearance will be cancelled and the aircraft shall be instructed to vacate the runway immediately.”What that meant was, since the Air India aircraft took longer to begin the take-off roll, the controller should have cancelled its take-off permission, while also asking the IndiGo aircraft to carry out a missed approach or go-around.Captain Singh said: “The question is the degree of responsibility on each of them — the controller, the two pilots, and, most importantly, the runway infrastructure constraints at the Mumbai airport which puts the controllers and pilots in such situations.”The runway constraints at MumbaiThe incident occurred around 6.15am. None of the airlines involved in the incident were scheduled to be on the runway at that time.The incident, as mentioned earlier in this report. played out within 90 seconds after 6.14am when a delayed Iraqi Airways flight to Najaf took-off.This flight was scheduled for a 5am departure. The Air India flight to Thiruvananthapuram was scheduled to depart at 5.45am, but was running more than 20 minutes late and lined up to depart behind the Iraqi aircraft.At the same time, the IndiGo flight from Indore was in the queue to descend and land in Mumbai. This flight has a scheduled arrival time of 6.40am, but it came in to land early by 20 minutes.In the congested Mumbai runway, the effects of “time slot indiscipline” are massive and can create potentially dangerous situations. !function(){"use strict";window.addEventListener("message",(function(a){if(void 0!==a.data["datawrapper-height"]){var e=document.querySelectorAll("iframe");for(var t in a.data["datawrapper-height"])for(var r=0;r !function(){"use strict";window.addEventListener("message",(function(a){if(void 0!==a.data["datawrapper-height"]){var e=document.querySelectorAll("iframe");for(var t in a.data["datawrapper-height"])for(var r=0;r The peak hour declared capacity of main runway 27 is 46 aircraft movements (arrivals and departures). But the Mumbai air traffic control often ends up handling close to 55 flight movements in 60 minutes, as flights that are delayed or come early encroach into other flight’s take-off/landing time slots.Bunching up of flights during peak hours adds to the workload of both controllers and pilots. The major airports in the world and those in India, such as New Delhi and Bengaluru, are served by parallel runways, which allows for two runways to be used at a time.But Mumbai airport has a set of cross runways and so only one runway is in use at a time. More often than not, it is runway 27, the workhorse that handles air traffic that supports the Mumbai Metropolitan Region’s (MMR) 23mn population.MMR, in fact, is the only urban agglomeration in the top 10 most populated cities in the world that is served by only one airport that functions out of a single runway.What hope does new Navi Mumbai airport hold?Due to land unavailability, the possibility of having parallel runways at Mumbai airport has been ruled out. So, the only hope of respite comes from a second airport to serve MMR.In January, former civil aviation minister Jyotiraditya Scindia announced that the Navi Mumbai airport will be ready to handle flights by March 2025.“When Navi Mumbai airport becomes functional, Mumbai airport will get decongested to a considerable extent as airlines will be asked to move a chunk of their flights to the new airport. When the volume of flights goes down, the instances wherein the air traffic control is forced to handle more flights than the declared capacity of the runway will go down,” said a senior air traffic controller.A lot of hangar space and general aviation infrastructure will also be moved to the Navi Mumbai airport which will free up space at Mumbai airport. This will allow the construction of more airside infrastructure which could be in the form of extra parking bays or taxiways and apron, all allowing for ease in aircraft movement.While Navi Mumbai airport, in phase 1, will operate out of a single runway, the airport eventually will have a pair of parallel runways.