Midair collisions: A rare nightmare scenario
(CNN) -- Midair collisions between large aircraft are a relatively rare phenomenon, especially at the high cruising altitude where the crash between a Russian Tupolev passenger jet and a DHL Boeing cargo plane occurred.
However, as the world's skies become increasingly crowded aviation experts say the chances of such potentially massive disasters occurring will inevitably increase.
"In the future we'll be seeing collisions at the rate of one or two per year," says aviation consultant Jim Eckes.
He points to a case last year involving two Japan Airlines planes that came within a hair's breadth of being "the worst accident in aviation history."
The two aircraft -- one a DC-10 and the other a Boeing 747 -- came within 10 meters (30 feet) of each other high above central Japan.
Between them the aircraft were carrying about 700 passengers and crew.
Eckes says the only reason a major disaster was avoided was because of the quick reaction and skills of the pilots involved.
The worst actual midair collision to date occurred in 1996 when a Saudi Arabian Airlines 747 collided with a Kazakh-owned cargo jet near New Delhi, India, killing 349.
Tuesday morning's crash over the Swiss-German border took place at what Eckes describes as "a crossroads of the sky."
The area near Lake Konstanz marks the intersection of some of the world's busiest air corridors as well as being at the frontier of German, Swiss and Austrian air traffic control.
Standard aviation rules dictate that aircraft traveling across the same territory in different directions fly at different altitudes.
Aviation experts say in the case of the two aircraft involved in Tuesday's collision the distance separating them should not have been less than 600 meters (2,000 feet).
What caused them to break that limit will be the focus of the investigation.
One possibility is that instructions from ground controllers were misinterpreted or were simply incorrect.
Eckes says that initial reports on the crash indicate that this human factor was the most likely cause of the crash, as has been the case with previous collisions and near misses.
Another possible factor may have been the handover from one country's air-traffic controllers to another -- a consideration that Eckes says could focus attention on Europe's extremely fragmented air space.
Aside from the human error possibility another consideration will be on the technical capabilities and their functioning on the two aircraft.
Many modern aircraft are fitted with a collision avoidance system known as TCAS, which can automatically pull the plane off collision-course or sound an alarm alerting pilots and telling them which way to turn.
However, although mandatory in many Western countries, TCAS is only slowly being retrofitted onto older aircraft and is not required on Russian-owned aircraft.
Eckes says the only way to ensure mid-air collisions do not occur is for all aircraft to be fitted with TCAS -- and even then the possibility remains that technical problems or poor maintenance will cause the equipment to fail.
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