UPDATE 1-U.S. lawmakers say Boeing 737 MAX 8 grounded for at least 'weeks'

U.S. lawmakers said after a briefing with the acting head of the Federal Aviation Administration on Thursday that Boeing 737 MAX 8 and 9 planes will remain grounded for "weeks" at a minimum, until a software upgrade could be tested and installed in all of the planes.

FAA Administrator Dan Elwell told reporters on Wednesday the software update will be ready within a couple of months. The FAA said Monday it planned to require the upgrade it termed "design changes" by April. An FAA spokesman confirmed Thursday that the FAA will not unground the airplanes until the software patch is approved and installed. Boeing and the FAA did not immediately comment. Representative Rick Larsen said after the briefing the software upgrade would take a few weeks to completed and installing on all aircraft would take "at least through April." (Reporting by David Shepardson; Writing by Makini Brice Editing by Chizu Nomiyama)