The incident of two planes coming within a whisker of each other on the Mumbai airport runway last week has raised serious questions: 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, 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 Haneda airport in Tokyo on Jan 2 this year.A Japan Airlines A350 aircraft collided upon landing with a Japan coast guard turboprop aircraft that was on the same runway. It was found that the Japan Airlines flight 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 said they had an all-clear from ATC — one to land and the other to take-off.
Directorate General of Civil Aviation (DGCA) is probing the matter but it will be a while before the final report is out. Meanwhile, Airports Authority of India (AAI), the country’s air navigation provider, has already put in measures based on the preliminary findings.
India’s Busiest Airstrip
On June 8, an IndiGo aircraft touched down on runway 27 merely a second after an Air India aircraft took off. At that moment, the distance between the two aircraft was about 2,100 feet, sources said.
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.
The 11,302ft long asphalt strip that is Mumbai airport’s runway 27 is arguably the priciest piece of real estate in the city, rented by the second to airlines that book time slots for takeoff and landing.
An A320 or B737 aircraft takes an average of 40 to 50 seconds on the runway for a take-off or landing. Between each aircraft movement, a buffer or safety margin of about 20 seconds is maintained in normal conditions.
‘Runway Occupancy Time’ or ROT, is thus a crucial parameter for flight operations at space-constrained Mumbai. Unlike most airports where distance is the criterion for maintaining a gap between two aircraft, Mumbai uses a time-based separation.
Called ‘TTT’, or ‘Time to Threshold’, the ATC 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 movement. In 24 hours, we save about 30 minutes,” said a senior air traffic controller. But if an aircraft is slow to take-off or if a landing aircraft comes in too fast, quick remedial steps need be taken by ATC.
Too Close For Comfort
Two A320neo aircraft — AI flight AI-657 from Mumbai to Thiruvananthapuram and IndiGo flight 6E5053 from Indore to Mumbai — were involved in the inci dent. “The domino was triggered by the AI aircraft,” air safety expert Capt. Amit Singh said on the basis of Flightradar24 data.
The incident unfolded in the 90 seconds after an Iraqi Airways flight took off and the AI aircraft entered the runway for take-off. “The AI aircraft was slow to take-off after getting line-up clearance. Of course, one is assuming it received immediate take-off clearance,” added Singh.
Per the norm, the AI pilot should have informed ATC about the delay in taking-off, which would have triggered a cancellation of his take-off clearance. But the AI pilots didn’t do that. They continued with the slow take-off. Meanwhile, the InidGo pilots were on an approach to land.
The AI aircraft had not lifted off 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 goaround. The norms state that 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 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 AI aircraft on the runway.
The ATC Question
DGCA has derostered the controller for investigations. Two days after the incident, AAI issued a circular stating that the preliminary investigation revealed violation of the standard operating procedure for ‘Time-Based Spacing’ between arrivals.
“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,” the SOP states. Since the AI 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.
“The question is the degree of responsibility on each — the controller, the pilots and, most importantly, the runway infrastructure constraints at Mumbai airport that puts controllers and pilots in such situations,” said Singh.
Intersecting Runways
When the incident occurred, around 6.15 am, none of the airlines involved in the incident were scheduled to be on the runway. The delayed Iraqi Airways flight to Najaf took-off at 6.14 am though it was scheduled for a 5 am departure. The AI flight was scheduled to depart at 5.45 am, but was running over 20 minutes late. The IndiGo flight had a scheduled arrival at 6.40 am, but came in 20 minutes early.
The peak-hour declared capacity of main runway 27 is 46 aircraft movements (arrivals/ departures). But Mumbai ATC often ends up handling close to 55 flight movements in 60 minutes as flights that are delayed or arrive early encroach into other flights’ slots. But on the congested Mumbai runway, the effects of “time slot indiscipline” can create potentially dangerous situations.
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 Delhi and Bengaluru are served by parallel runways, which allows for two runways to be used simultaneously. 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 the workhorse runway 27.