4. Importance of Atmospheric Pressure: Kharkov Accident (1975)
Pilots while landing or departing are given value of Atmospheric pressure in terms of QNH or QFE by Air Traffic Controllers. Atmospheric pressure value allows pilots to accurately know aircraft height above sea level or terrain. If a wrong QNH/QFE is given, then altimeter will give pilots an incorrect altitude read out. For example it is possible that on-board altimeter is showing that aircraft is at 1000 feet above terrain but in reality it might be at an altitude of 500 feet. So atmospheric pressure plays important role especially while landing when the altitude error can cause aircraft to hit terrain and ending up in a disaster.
That day, it was an Aeroflot Airlines Antonov-24 aircraft flying from Rostov on Don to Kharkov. While at an altitude of 4800m, crew requested further descend. ATC gave it descend to 2400 metres. After reaching 2400m, crew again requested further descend for landing at Kharkov airport. At that time atmospheric pressure was 737 but ATC by mistake announced 757. Pilots also read-back 757 that ATC did not hear-back properly and did not rectify it to 737. So aircraft system was now showing wrong altitude because of wrong value of atmospheric pressure and it was flying at lower altitude than the altitude shown by equipment. ATC gave clearance to descend to 400m on crew’s discretion. When aircraft was 15 km from airport, ATC gave them further descend to altitude of 300m. After some time of this clearance aircraft hit the terrain and crashed, killing 19 of 50 people on-board.
So this crash was due to wrong value of atmospheric pressure passed by ATC to Pilots. But this could have been averted had the ATC heard back properly the value (757) of atmospheric pressure confirmed by Pilots. Also pilots did not react to the altimeter warning 50m above the first barrier.
5. ADC Airlines Flight 86 Crash (1996)
This Nigerian domestic flight was operating between Port Harcourt to Lagos on the day of collision. This Boeing 727 of ADC Airlines was at flight level 240, under the control of Lagos ATC at the time of collision. Pilots of ADC flight requested descend from which was delayed by ATC as there was another aircraft (ELF Petroleum Business Jet) at flight level 210, which was a traffic for this flight. Due to this ATC was unable to give them descend. At the same time, there was another Triax aircraft flying at flight level 160.
After some time when the controller finally cleared the ADC flight to descend, he thought he had cleared to aircraft to FL100 at an earlier stage, but the aircraft was still flying at FL240. So ADC aircraft started descend from Flight level 240 with a potential traffic at flight level 160. While passing through flight level 160, TCAS warning came as there was Triax Aircraft at the same level (flight level 160). The flight crew took evasive action but in doing this manoeuvre the aircraft rolled to an excessive bank angle and crew lost the control of aircraft. It was falling with a speed of sound towards the ground. After a few seconds, Aircraft got crashed near a place Imota, killing all 143 people on-board.
The primary reason for this fatal accident was wrong assumption and lack of situational awareness of the Air Traffic Controller.
6. Gol-Embraer Mid Air Collision 2006
On September 29, 2006, A business Jet (Aircraft Type Embraer Legacy 600) was flying northbound from Embraer factory in Brazil to Manaus for a delivery flight. Gol Airlines flight 1907 was flying southbound from Manaus Airport to Rio De Janeiro. Both Aircraft were flying at flight level 370 which went unnoticed by Brazilian ATC. At 1648 local time both aircraft collided over Brazilian state of Mato Grosso. Business jet winglet split the left wing of Gol’s Boeing aircraft which caused excessive roll of the aircraft which disintegrated and crashed killing all 154 people on-board. Embraer aircraft and its occupant were lucky enough to land safely at a Brazilian air-base.
In the investigation it was found that both aircraft were assigned same flight level at same point of crossing. Although as per flight plan of Embraer aircraft, after a certain point, it was supposed to fly at flight level 360 but Air traffic controller did not instructed it to descend to flight level 360 assuming that Aircraft was already at flight level 360. Second point was that Embraer aircraft pilots were unable to make contact with ATC for almost 57 minutes. Had they contacted ATC, it might had been possible that ATC could have rectified his own mistake of assuming the aircraft to be at flight level 360 instead of its actual level i.e. Flight Level 370.
Third point was that Transponder of Embraer aircraft was accidently in switch off mode which went unnoticed by ATC as well as the pilots of Embraer aircraft. Because of this none of the concerned aircraft got any TCAS warning regarding imminent danger and kept flying towards each other, eventually colliding over Amazon.
So Air Traffic Controllers errors can be fatal for aircraft and its occupants. They are required to be well-equipped as well as almost hundred percent correct all the time. Here, we need to understand that Air traffic controllers are also human being and they can contribute to creation of accident by various reasons as loss of situational awareness, unintentional wrong information passed to pilots, misinterpreting the crew’s requirements or due to adamancy as well. They need to be provided with more sophisticated systems with possibility of more control and prediction of individual decisions. Other important factor that can minimize ATC errors will be provision of adequate manpower at all the ATC stations which will help in reduction of workload of Controllers.