The Jan. 11 ground stop forced a halt to all U.S. passenger departing traffic for almost two hours, the first such action since the Sept. 11, 2001, attacks.

Acting FAA Administrator Billy Nolen will tell the senators he took the action "to maintain safety and preserve predictability," according to written testimony seen by Reuters and added the FAA has since taken steps "to ensure that bad data from a database cannot affect a backup database."

The computer system outage disrupted more than 11,000 U.S. flights. Nolen told lawmakers last month the FAA found that the outage occurred when contract personnel "unintentionally deleted files."

Last month, the FAA told lawmakers it had revoked access to a pilot messaging database by contractor personnel who unintentionally deleted files in the Notice to Air Missions (NOTAM) database. The NOTAM system provides pilots, flight crews and other users of U.S. airspace with critical safety notices.

The FAA is working on efforts to modernize the NOTAM system. "The nearly decade-long modernization work includes transitioning away from the legacy portion of the system," Nolen's written testimony says. "We expect that a significant portion of the modernization work will be complete by mid-2025. We continue to assess the feasibility of accelerating the current schedule."

Nolen's testimony said on Jan. 10, the NOTAM system became unreliable and technical experts sought to address the issue by switching to a NOTAM backup database.

By early the next day, "the system appeared to have been restored, but formatting issues persisted. To resolve this, FAA’s air traffic leadership directed the rebuild of the database."

Nolen said as the morning air traffic rush approached, and system work continued, he ordered a ground stop at approximately 7:15 a.m. ET and it was lifted at 9:07 a.m.

The FAA will need support from Congress to fund FAA "modernization needs," Nolen's testimony says.