A number of grounded Southwest Airlines Boeing 737 MAX 8 aircraft are shown parked at Victorville Airport in Victorville, California, U.S., March 26, 2019.

Barclays lowered its rating of Boeing shares to equal weight from overweight on Tuesday, saying the firm's survey of airline passengers found that many people will avoid the 737 Max "for an extended period" once the grounded aircraft is flying again.

"In order to gauge perception of the 737 MAX, we surveyed 1,765 fliers in N America and Europe, reflecting a broad mix of age groups, income levels and frequency of airline travel," Barclays said in a note to investors.

"Nearly half won't fly MAX for year or more." Barclays said. "If given the choice between a MAX and another aircraft type on otherwise identical flights, 52% would choose the other aircraft type."

Boeing shares fell 1.3% in premarket trading from Monday's close of $371.60 a share.