The House Transportation Appropriations Subcommittee, seeking to avert a financial crisis by Washington's Metrorail system, has voted to grant a $29.3 million federal subsidy of an interest payment due July 1 on Metro bonds.

A total of $36.6 million interest must be paid that day to investors in the first batch of revenue bonds that were sold in 1972 to raise money for subway construction. The sum approved Thursday by the congressional unit is 80 per cent of the total due, with the balance presumably to be paid by Washington area political jurisdictions.

Metro's general manager, Theodore C. Lutz, warned the subcommittee in February that Metro would default on the payment unless it received federal assistance.

If Metro defaults, the Department of Transportation is required by federal law to draw the needed money from the U.S. Treasury and pay the bondholders.

The need for such an action on a stopgap basis probably will remain, since it is not likely that the Transportation Department appropriation bill - the one that contains the Metro money - will be enacted by the July 1. payment deadline. The appropriation bill is timed to supply funds for the federal government's fiscal year, which starts Oct. 1.