Air Traffic Controllers can be termed as the guardians of the sky. They ensure the safe, orderly, and expeditious flow of air traffic. But can we say that Air Traffic Controllers are always right?
With the increase in air traffic, the work of air traffic controllers has become very demanding and stressful which can produce errors. Such errors can break well-woven safety nets, ending in a fatal accident. In general, it is not possible that a single error can cause an accident as there are multiple safety nets or layers that when all infringed cause a disaster. It can be better understood with the Swiss Cheese Model as shown below. In this article, I have tried to describe some accidents which were caused by Air Traffic Controllers’ errors aided by other factors beyond the control of ATC.
1. Improper Co-ordination between ATC Units: Zagreb Mid-air Collision (1976)
In the mid-70s, Zagreb ATC Centre was one of the busiest area control centers in Europe as Zagreb was a common reporting point for traffic flying between northern Europe and South-eastern Europe, the Middle East, and Asia. One of the ill-fated flights, a Trident aircraft of British Airways Flight 476 was flying from London to Istanbul and another flight was Inex-Adria Aviopromet Flight 550 (a DC-9) from Split, Yugoslavia to Cologne, West Germany.
In those days, Zagreb ATC has three sectors named Lower, Middle, and Upper Sectors. The upper sector (US) was handling flights above 31000 feet, the middle sector (MS) responsible for flights between 25000 feet to 31000 feet, and the lower sector (LS) handled flights below 25000 feet.
On the day of the accident, Zagreb ATC was not having adequate manpower so all controllers were heavily work-loaded. British Airways flight was with upper sector maintaining flight level 330 (i.e. 33000 feet) and estimated over Zagreb at time 10:14 UTC. Adria flight 550 was in the middle sector at flight level 260 (i.e. 26000 feet) and requested higher. MS controller asked US controller for the climb of Adria aircraft to flight level 350, but US Controller was too busy to be interrupted. Then Middle Sector Coordinator came to US Controller, with the flight strip of Adria flight, and asked for its climb to level 350. US Controller saw the strip and said to the coordinator that yes, it can climb after the crossing of an aircraft (not British Airways) appearing on his screen. Upon crossing of concerning aircraft, MS Controller gave climb instructions to Adria to flight level 350. When Adria was at 310, MS Controller advised it to contact USC and release the squawk code assigned by MSC.
It must be known here that with help of Squawk Code it is easier for a Controller to identify an aircraft. In a normal situation, Adria would have contacted US Controller, giving its position and level and the Controller would have assigned it a new squawk code. Adria could not contact US Controller for a considerable amount of time due to heavy traffic or some other unknown reason. Also, US Controller was busy with other traffic so he did not contact Adria’s flight.
At 10:14 Adria contacted US Controller while climbing through level 325 for level 350. Realizing the imminent danger of collision, US Controller instructed Adria flight to stop climbing immediately, in doing so he reverted to his native language (not English). This meant that the British Airways crew was not able to understand the conversation involving their imminent collision with the Adria flight. So by the time Adria flight stopped its climb, it was at flight level 330 over Zagreb, at the same time and the same level as of British Airways flight. The controller’s last-minute effort to avert the collision turned a near miss into a collision.
All the 176 people aboard both flights were killed. So collision occurred due to improper coordination and heavy workload on the Upper Sector controller owing to manpower shortage.
2. To follow ATC or TCAS? Überlingen Mid-air Collision (2002)
Let’s first understand what TCAS is. Basically, TCAS is an onboard Traffic Collision Avoidance System that alerts pilots of Aircraft on a collision course to take necessary actions to avoid the collision.
That day, DHL Flight 611 was flying from Milano to Brussels while Bashkirian Airlines flight 2937 was flying from Moscow to Barcelona, Spain. At the time of the accident, both aircraft were flying in the airspace managed by Skyguide Company’s Air Traffic Controllers. It was a busy day for the concerned controller as he was managing two sectors simultaneously. Because of this, he was unable to recognize that two aircraft were on collision course.
When he realized the apparent collision, although too late, he instructed Russian Aircraft flying at Flight level 360 to descend to flight level 350 immediately as DLH was also on the same altitude (flight level 360). This was to be done to ensure to have a minimum specified vertical separation of 1000 feet between two aircraft. By the time the Russian aircraft started descending, TCAS came into the role and advised it to climb while TCAS advised DLH flight to descend. It was a confusing state for Russian pilots as there was a contradiction between ATC instruction and TCAS advisory. Russian aircraft again confirmed from ATC whether to descend or not. ATC once again instructed it to descend considering that the DHL aircraft was maintaining flight level 360. As ATC thought that he has sorted out the situation and was handling heavy traffic he did not check whether DHL was maintaining level 360 or not. Russian crew disregarded TCAS advisory and continued to descend but DLH aircraft that did not get any ATC instruction followed TCAS advisory and started descending to avoid the collision. Now both aircraft were descending and at an altitude of 34890, both aircraft collided.
After this accident, it was adopted by ICAO that in such situations when there is a contradiction between ATC and TCAS instructions; TCAS Advisory is to be followed by both aircraft irrespective of ATC instruction.
3. Sleeping while on Duty: Omsk Airport Crash (1984)
This accident was a chain of events that ended up in a disaster. On the day of the collision, Aeroflot Flight 3352 was flying from Krasnodar to Novosibirsk with an intermediate halt at Omsk Airport. It was morning at 5 o’clock (local time) when the aircraft was preparing to land in Omsk. The runway condition was slippery because of rain last night, so the Airport ground maintenance staff requested ATC Ground Controller to enter the runway and dry it. Ground Controller permitted them to enter the runway. But at that time, he was too sleepy to switch on ‘Runway occupied’ warning lights on and slept immediately. From here, the links of the safety net started breaking up.
Around 0525 AM (local time), the maintenance crew moved three vehicles, loaded with 7 tons of fuel, on the runway. All the vehicles were having flickering lights on their top but lights were kept off as maintenance workers found them too bright to concentrate on work. Now there were three vehicles on the runway with their beacon lights off, so for an aircraft on a landing path, it was impossible to see these vehicles on the runway. When the Aircraft was approaching for landing, the ground maintenance crew noticed it and contacted the ground controller whether any aircraft was about to land on the Runway. There was no reply from the ground controller so they ignored the aircraft thinking that the aircraft was not landing and they resumed their work again without paying any more attention to the approach path of the runway.
At time 0536, the aircraft requested permission to land from Approach Controller. The approach controller checked the runway status (which was not set to an ‘occupied’ state by the Ground controller) and found that it was available for landing. Pilots noticed some unusual contours on the runway and again confirmed whether to land from the Approach controller. The approach controller then contacted Ground Controller and received no response. Then he contacted the flight controller on internal radio and received an inaudible reply that sounded like ‘….ree’, he took this as ‘free’ and advised pilots to land on the runway.
While on a touchdown on the runway, the crew noticed that there were vehicles on the runway, they tried to turn the aircraft but it was too late. Aircraft collided with the ground maintenance vehicles one by one, igniting tons of fuel in vehicles and in the aircraft itself. 178 people lost their lives in this crash which was primarily due to ATC falling asleep on duty along with other reasons like switching off beacon lights by Ground vehicles, and landing clearance by the Approach unit without any confirmation from the Ground controller.