One the eve of 12th November 1996, Saudi Arabian Airlines Flight 763 and Kazakhstan Airlines Flight 1907 collided mid-air over Charkhi Dadri, a village in Haryana killing a total of 349 souls on board. The accident remains third deadliest aviation disaster till date in terms of total body count. The final investigation report listed various reasons that led to the collision ranging from language barriers and outdated radar equipment to restricted airspace policy.
The aviation industry is such a sensitive industry that safety is at the core of all its operations and activities. Every object that takes to the skies from the ground is subjected to countless regulations and stipulations making sure there is no room for even the slightest error that could endanger safety. Gravity could be a friend or foe depending on how things are handled 30,000 ft up in the air.
That said, mid-air collisions should be a thing of the past right? Fortunately, they are. Thanks to the ever evolving technology, we now have well defined intricate set of stipulations that govern the movement of these aircrafts while in air, thereby ensuring the safety of all concerned. Nevertheless, safety is never to be taken for granted and hence…
History has shown
Historical stats show that in a staggering 105 mid-air collisions that occurred between 1964 and 1968 , majority of them had convergence angles associated with one aircraft taking over the other . 35% were from 0-10 degrees- straight from behind while the rest 5% were from a head-on-angle. What is even more bewildering is the fact that most of these occurrences took place within 25 miles of an airport and below 8000 ft, primarily on final approach and in traffic pattern.
As is apparent, an increased traffic and higher closing speeds is what can be touted as the primary factors contributing to mid-air collisions.
To get a perspective let’s say, a jet and a twin have a closing speed of about 750 mph. The two planes converging at 750 mph would be less than 10 seconds apart when the pilots even first spot each other! – recipe for disaster. Hence, ICAO, which is the regulatory body for all civil aviation operations worldwide, came up with a well defined set of rules and regulations which would ultimately regulate all aircraft movements and oversee aviation safety globally.
Perhaps the most important separation when it comes to aircraft spacing, is the vertical separation. The ICAO States that all aircrafts below 29,000 ft be separated by 1000ft while those above 29,000 ft should be separated by 2000 feet-applicable to all IFR flights.
Reduced Vertical Separation Minima (RVSM)
RVSM was introduced to decrease the flight separation above 29,000 ft to 1000ft from 2000 ft. This was done in order to fly more optimum flight paths without compromising safety leading to fuel savings and also advocated a more efficient use of the available airspace. In 1988, the ICAO completed a study wherein it investigated the safe transition of separation from 2000ft to 1000ft between FL290 and FL410 and found it to be technically feasible. Only RVSM approved aircrafts may fly through this airspace and non-RVSM aircrafts “may” travel through RVSM route provided authorisation is obtained from the concerned ATC and guidance is given at all times.
In the event that the two aircrafts are separated LESS than the vertical separation minimum, horizontal separation must be applied:
Based on reports given by the pilots, this separation doesn’t require the active intervention of the ATC as the position of the aircrafts is verbally communicated between pilots . In some cases though, the ATC radar intervenes to provide the separation minima.
This separation minima is based on the position of the aircraft derived from dead-reckoning or from beacons. Aircrafts installed with beacons need to be at a certain distance from each other and their tracks to/from the beacon must diverge by a specified minimum angle. To be precise 5 NM horizontal separation up to 60 NM from radar head, 5 NM within 60 NM of ADS-B ground station when only ADS-B is used in non-radar environment. 10 NM horizontal separation beyond 60 NM from radar head, 10 NM beyond 60 NM of ADS-B ground station when only ADS-B is used in a non-radar environment. Under certain circumstances, a special authorisation is given for 3 NM separation from the radar head.
Aircrafts following tracks 45 degrees of each other and not laterally separated, are said to be flying the same route. Such scenarios call for longitudinal separation and it can be based upon time/distance measures by the DME(distance measuring equipment). What is to be noted though is that no two aircrafts flying the same route should come within 15 minutes of flying time of each other.
Speed Control Procedures
Also, there are certain speed control procedures that need to be followed subject to aircraft performance limitations; a surveillance controller may assign a specific speed to the aircraft in order to maintain/achieve required spacing. Speed control is also necessary to achieve the desired separation minimum or spacing between the successive arrivals. This in turn would improve the utilization of airspace and enhance the runway capacity to handle more aircraft.
All the speed restrictions shall be complied with as promptly as feasible and flown as accurately as possible within the limits of operational constraints. Aircraft unable to comply with the specified speeds must inform Air Traffic Controller (ATC) and report the minimum speed it is able to follow. In such cases controllers shall apply the alternative method to achieve the desired spacing between aircraft concerned.
Separation due to wake turbulence
Wake turbulence is one another factor to be considered when applying separation rules to aircrafts. An aircraft may fly into wake turbulence from a preceding aircraft which can cause it to enter a sudden unintentional roll leading to loss of control. Hence a minimum separation between departing and/or arriving aircrafts have to be maintained based on their MTOW ( classified as Jumbo, Heavy, Light, Medium) -as per ICAO wake turbulence category.
*MTOW- Maximum Take-off Weight, NM- Nautical miles
|PRECEDING AIRCRAFT||FOLLOWING AIRCRAFT||MINIMUM SEPARATION(NM)|
Please refer to the Enroute section of the Aeronautical Information Publication (AIP) document for further detailed reading about the Indian regulations in the event of collision hazard, Termination of surveillance service, ATS surveillance system failure and Communication failure procedure. The latest AIP is available on the Airport Authority of India’s (AAI) official website for public use.
With aviation making a remarkable comeback from the devastating pandemic, people have slowly started taking to the air. That means an increase in passenger loads translating to increased traffic in air. Though it will still take time to return to pre-pandemic levels the one thing that will always remain constant, is SAFETY.
Note: The data in this article is as per DGCA (India) rules. Please contact/refer to the relevant regulator’s website for laws pertaining to any other country or region.