Two airliners come within seconds of collision over Indiana, US

November 17, 2007

A midair collision has been narrowly averted between two commercial airliners over Indiana, United States. A Midwest Airlines plane carrying 24 people and a United Express aircraft carrying 31 people came within 600 vertical feet and 1.3 horizontal miles apart.

The eastbound Midwest Airlines jet was accidentally directed to descend into the direct path of the westbound United Express flight. The mistake was made after an air traffic controller erroneously removed the latter aircraft's identification aircraft tag in preparation to handing off the aircraft to the air traffic control centre for a neighbouring sector of airspace.

The Traffic Collision Avoidance System (TCAS) installed in the cockpits of both aircraft sounded emergency warnings advising the pilots to take evasive action. The Midwest Airlines plane promptly pulled up out of danger. Had TCAS not activated, it is believed the jets would have struck each other. "If they didn't suddenly climb, there would have been a convergence," said Midwest spokeswoman Carol Skornicka.

Had the aircraft struck each other, they would have been traveling at a combined speed of about 700 m.p.h. Airspeed would normally be higher, but congestion around O'Hare International Airport had slowed things down. The incident occurred at the end of a rush period, during a change in shift at the control centre.

Jeffrey Richards, president of the National Air Traffic Controllers Association, told reporters that pilots had mentioned three times in the course of the flight how close aircraft were coming, quoting them as saying to air traffic control on one occasion "Center, you really lined us up on that last clearance."

Near air misses are classified on a scale of D to A by the Federal Aviation Authority, A being the most serious. The incident was classed as a B, the third incident of such severity in just six weeks caused by mistakes at Chicago Centre. The minimum permitted distance for aircraft traveling at an altitude of 25,000 feet, as these two were, is five horizontal miles and 1,000 vertical feet.

Richards said that on the night of the incident controllers at Chicago Centre had been working close to the two hour working limit between rest breaks. "These controllers are fatigued from working such long stints and very few breaks compared to just three years ago," said Richards, who added that this was particularly true of the 26-year-old veteran who made the mistake. "Each of his sessions were right up to the two-hour limit," Richards said, adding that he was nearing the end of his shift and had returned from a break just minutes before the near-collision.

Richards said that at the time the building contained a staff of 11 controllers, five trainees and one supervisor, which is one supervisor and one controller lower than usual. He went on to say that before a retirement wave, 17 controllers monitored the airspace. He says this is a part of a larger issue whereby retirements are exhausting the supply of skilled operators, in what he describes as "a systemic problem,".

This latest incident raises important safety issues at Chicago Centre, which is currently undergoing a dispute between the controllers union and management over how many controllers are adequate to run the centre. The site is soon to have to handle large quantities of traffic as an annual holiday season starts. However, the FAA says that it currently has no issues with staffing at the facility, although Richards contends that low levels of staff and increasing workloads have long been a worrying factor at the facility.

Skornicka said of the incident that it was "part of a bigger debate taking place," and that she felt "this is just one situation that highlights the need for modernization and overhaul of the air traffic control system."