The number of aircraft near misses more than doubled last year, but U.S. aviation officials tied the increase to new systems that report mistakes not caught previously.
There were 4,394 cases in which planes came too close to one another in the year ending September 1, 2012 -- up from 1,895 the year before, the Federal Aviation Administration said on Thursday.
Forty-one events were characterized as "high-risk events." None resulted in accidents.
The FAA had predicted an increase as it phased in a program to monitor radar and automatically report problems.
The agency says it is a victim of its own success. With commercial aircraft accidents rare -- or non-existent in some recent years -- it is difficult to use accident data to identify trends.
So the FAA has placed a greater emphasis on looking at "precursors" to accidents, such as "loss of separation" events in which aircraft come closer together than intended.
"We are covering electronically and identifying things that we never did before," said one FAA official, speaking on condition of anonymity.
The 4,394 "losses of separation" in 2012 occurred during 132 million take-offs and landings.
Put another way, that's 3.3 incidents per 100,000 operations, the FAA said.
"If you want a measure of safety, that's the number," the official said.
FAA has various separation standards, depending on air space and aircraft. At high altitudes, planes should be separated by at least five miles horizontally or 1,000 feet vertically. Nearer to airports, planes can be closer together.
The agency said it has worked with its workforce and the controllers' union to change the safety culture of the agency -- emphasizing collection of data over punishment.
Under the new non-punitive reporting system, controllers are encouraged to voluntarily report mistakes and problems.
It has buttressed that with an automated system that monitors air traffic, reporting problems to headquarters without any influence from the local facility.
As a result of five different reporting systems, the number of reports grew 10-fold in 2012.
Officials said the new system makes comparisons with previous years misleading. Information collected in 2012 and 2013 will create a new baseline for meaningful comparisons in the future.
Of the 4,394 "loss of separation" incidents, the FAA said 1,271 involved cases where the planes came 34 percent closer than allowed.
Of those, it characterized 1,048 as "low risk" events, 182 as "medium risk," and 41 as "high risk."