United Airlines Holdings Inc. grounded aircraft nationwide due to an unspecified equipment outage, according to the Federal Aviation Administration’s website.

The Chicago-based carrier requested the grounding for its main operation and subsidiaries on Tuesday, according to the posting. United aircraft were unable to contact flight dispatchers using normal means, the FAA website showed.

The FAA confirmed in an email to Bloomberg the airline asked it to pause departures nationwide, but declined further comment.

Spokespeople for United didn’t immediately return a call seeking details.