A computer outage that grounded flights nationally and disrupted more than 11,000 flights was caused by a procedural error related to a data file, the Federal Aviation Administration said as US airline operations returned to normal.
The FAA said its preliminary analysis "determined that a data file was damaged by personnel who failed to follow procedures. The system is functioning properly." The FAA did not answer more questions about the specifics of the problem.
More than 11,300 flights were delayed or cancelled on Wednesday in the first national grounding of domestic traffic in about two decades.
As of Thursday night, more than 5100 US flights were delayed and over 160 were cancelled, according to FlightAware. The FAA said cancellations were below one per cent of flights on Thursday.
Delta Air Lines, United Airlines, American Airlines Group Inc and Southwest Airlines Co reported normal operations on Thursday.
The FAA computer failure prevented airports filing updated safety notices that warn pilots of potential hazards such as runway closures, equipment outages and construction, bringing flights to a temporary halt.
FAA officials earlier had traced the problem to a damaged database file in the system that provides pilot safety notices known as Notices to Air Missions (NOTAMs), but said there was no evidence of a cyberattack.
The same file corrupted both the main system and its backup, said people familiar with the review, who asked not to be identified.
The FAA last year sought $US29.4 million ($A42 million) from Congress for computer upgrades and to allow the acceleration of a sole NOTAM repository "and eliminate the failing vintage hardware that currently supports that function in the national airspace system".
A separate outage in Canada on Wednesday that temporarily prevented new safety notices being received and disseminated digitally was caused by an isolated IT system failure, an industry source told Reuters.
The FAA has been without a permanent administrator since March. The Senate has not held a hearing on President Joe Biden's pick to head the agency, Denver International Airport CEO Phil Washington.