The D.C. statehood convention completed preliminary approval late yesterday of a judicial system under the city's proposed constitution that would provide a two-tiered court structure with judges appointed for life but subject to periodic "retention elections" by the voters.

After four nights of often stormy debate, the convention resisted efforts by some delegates to create an intermediate court of appeals and settled simply on a trial court and a supreme court similar to the present D.C. Superior Court and D.C. Court of Appeals.

A major difference in addition to the retention elections, however, is that the nine "justices" of the proposed supreme court would sit as a group on all cases, rather than in panels of three. This raised objections by some delegates that the already crowded appeals docket may become even more jammed.

Other features of the proposed system approved last night include:

* A minimum of 44 judges in the trial court, a number chosen to accommodate transition of the 44 judges currently in the D.C. Superior Court in the event the city achieves statehood.

* Appointment of all judges by the governor from candidates provided by a nominating commission. Thereafter, trial court judges would face approval or rejection by city voters every six years and supreme court justices every 10 years in a referendum-style retention election.

A nine-member judicial nomination commission with six members appointed by the governor, two by the unified bar and one by the legislature. Up to four of the nine may be nonlawyers.

* A five-member judicial disabilities and tenure commission that can remove a judge from office who has been convicted of a felony or who has been found guilty by the commission of willful misconduct in office, willful and persistent failure to perform judicial duties or other conduct "prejudicial to the administration of justice."

The judicial system approved last night now faces two more readings by the 45-member convention before final adoption.