A proposal to open the Metro system at 5 a.m. on weekdays won preliminary approval yesterday, but it could take several months of hearings and committee meetings before the idea is translated into policy.

Metro spokeswoman Beverly Silverberg said the proposal was approved by the Metro board's operations committee after reviewing staff estimates of cost and ridership.

The staff estimated that opening the gates an hour earlier would cost an additional $3.27 million a year, but commuters could generate $1.27 million a year in fares if they paid peak rates during that hour.

The additional $2 million a year subsidy could be a sensitive issue at a time when local governments are reviewing Metro's latest request for increasing subsidy contributions.

Metro General Manager Carmen E. Turner has proposed a fiscal 1989 operating budget for Metro that would require local governments to pay $257.3 million, or $22.1 million more than they paid this year.

The board's budget committee is reviewing Turner's proposal, which includes no money for longer hours.

The operations committee voted to ask the budget committee to study the proposal to open at 5 a.m.

Because the early opening would require changes and additions in bus service, a series of public hearings would have to be conducted before the full Metro board takes up the proposal.

If the proposal is approved, the change could not take place before fall at the earliest, Silverberg said.