The inspector general for the U.S. Department of Transportation is seeking an explanation for the recent increase in reported errors by air traffic controllers who guide 70,000 planes through American skies each day.

The formal review comes at the request of Congress, which “expressed concerns about the accuracy of the reported number of operational errors, the causes for a significant increase in this number, and the related safety implications,” according to a memorandum from the inspector general’s office.

The memo cited one near mid-air collision — between an American Airlines plane carrying 259 people and a pair of Air Force C-17 transports near New York in January — and an almost 53 percent increase in the number of reported errors by controllers in fiscal 2010.

The inquiry was requested by the Senate commerce committee, which sought to verify the assertion by the Federal Aviation Administration that more errors are being recorded because of a new program that encourages controllers to report their mistakes.

Under the Air Traffic Safety Action Program, controllers are protected from punitive action if they report errors that went undetected through radar or other methods.

Those previously undetected errors are not included in the official count of reported errors. The inspector general’s memo said Congress wants to know how ATSAP “impacts the number of operational errors reported, and if it is capturing errors that could lead to breakdowns in safety.”

The air traffic control system has come under scrutiny in the past year as the number of errors has mounted. The vast majority of the 1,887 controller errors recorded in fiscal 2010 were mistakes in which planes came closer to one another than allowed but posed no serious risk of collision.

On more than a dozen occasions, however, including the New York incident and one involving a United Airlines plane carrying Rep. F. James Sensenbrenner Jr. (R-Wis.), disastrous consequences were narrowly averted.

Recent high-profile incidents in which controllers were caught sleeping on the job and a mistake which put a plane carrying Michelle Obama and Jill Biden too close to a cargo jet have caused FAA Administrator Randy Babbitt and Transportation Secretary Ray LaHood to bring additional scrutiny to the performance of controllers and change scheduling practices.