The work was confined to the air data computers, according to a spokesperson for United. The maintenance, identified as a modification, was undertaken to ensure that pilots and other qualified members of the flight crew would have the capacity to override an over speed or stall warning that may have sounded in error.
A spokesperson for the Federal Aviation Administration (FAA) called the action on United's part "voluntary," adding that the FAA would "follow up as necessary."
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It was also not indicated if the directive issued in 2004 pertained to compliance that was voluntary in nature only or if the airline voluntarily grounded its 757 fleet en masse yesterday to affect the modification all at once. The FAA did not indicate if it pressured United to comply, given the date of the original directive.
In an unrelated aside, the FAA predicts there will be one billion fliers a year in the US by 2021—a mere ten years out. Domestic air travel is expected to double from current levels within 20 years, the FAA said.