MEXICO CITY, July 22 (Reuters) - Delta Air Lines ( DAL )

and Aeromexico requested a four-week extension to respond to a

U.S. Department of Transportation proposal which would impact

their joint venture, a filing seen by Reuters on Tuesday showed.

The DOT on Saturday proposed withdrawing the joint venture's

antitrust immunity to address competitive issues. It also put

out orders which could impact Mexican airlines' flight

schedules.