The "silent alarm" system that Metro has installed on its buses so drivers can signal when they are in trouble was "not in operational order" on the bus that was the scene of a sexual assault on a woman driver last Suday, Metro officials said yesterday.

According to a report submitted to the Metro board, the bus driver was warned by her two assailants, one of whom was armed with a gun, not to press the silent alarm activation switch. However, after she had been assaulted and her assailants had fled the scene, she pushed the silent alarm button and tried to use the on-bus radio to summon help.

The bus driver waited five minutes for a response, tried to stop a passing vehicle without success and then started driving the bus in search of a police officer. She found a D.C. police cruiser within three blocks.

"A check of the bus silent alarm and radio was made and both were not in operational order," the report to the board said, "Flashing lights were operational." Some Metrobuses are also equipped with a switch that flashes exterior lights as a signal to police.

Metro board chairman Jerry A. Moore, who said after the incident that he would conduct his own investigation, presented the Metro staff yesterday with a three-page letter setting out questions about the incident and the operation of the silent alarm. There have been no arrests in connection with the assault, but a composite sketch of one of the assailants is being distributed by police. The assault occurred near the 400 block of F Street NW.

Thomas S. Trimmer, Metro's director of bus operations, said yesterday that the radios and silent alarms are checked randomly at a rate of about 100 a day. There are about 1,800 buses.

Bus drivers, Trimmer said, are supposed to check the radio to see if a light is on before they leave the garage. The silent alarm is a part of the radio. When activated, it transmits a signal telling Metro headquarters the number of the bus in trouble.

The silent alarms were installed after a woman bus driver was raped last May while her bus was parked at a Southeast Washington turnaround point. That incident led to a one-day wildcat bus driver strike that ended after Moore promised the drivers that there would be no retaliation for striking and that Metro would increase its efforts to make bus driving more secure. Additional police patrols on buses and other security measures, including the installation of silent alarms, have been taken.

"As a result of what happened the other day there has been some talk of another strike," Moore told the board.

He told reporters later that "I think the radios should be checked out every day."

Trimmer said that the check-out procedure had already been revised to assure that buses operating late at night did receive a complete radio test.

In another Metro Board matter yesterday, accountants Ernst and Ernst told the board that an internal Metro administrative problem might be costing the transit authority some interest income on federal operating grants.

William Boleyn, Metro's assistant general manager for finance, promised a full report to the board on the allegation, but said that "I am certain we have not lost any possible interest income."