United Airlines has temporarily grounded no fewer than 96 of its Boeing 757 airplanes for what officials deemed as "unscheduled maintenance," CNN reported this morning. Although 15 flights were cancelled February 15—the day of the grounding—the airline indicated that operations should return to normal today.
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."
However, the spokesperson also said that the modification was required to bring the airline into compliance with an airworthiness directive issued by the FAA in 2004. The spokesperson gave no indication as to why the airline took more than six years to comply. United did not give an indication as to why such a high number of planes all had to be grounded on the same day, forcing cancellations.
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.