WASHINGTON, Feb 2 (Reuters) - A key U.S. pilot messaging

system resumed operating on Sunday morning after it experienced

an outage, the U.S. Federal Aviation Administration said.

The FAA's "Notice to Air Mission," or NOTAM, system that

provides safety notices to pilots went down late on Saturday,

prompting the agency to set up a hotline to send notices every

30 minutes with updates to airlines.

"We are investigating the root cause of the outage," the FAA

said on Sunday.