A career FBI agent who once was one of the bureau's top experts in court-ordered secret break-ins and safecracking went on trial yesterday in U.S. District Court here on charges of attempting to rob the FBI's credit union.

In opening arguments, Assistant U.S. Attorney John P. Hume told the jury that the evidence would show that H. Edward Tickel, a 13-year veteran who was fired after his indictment in November, needed money because his "income was less than his bills."

Hume said Tickel entered the darkened credit union office on the eighth floor of the J. Edgar Hoover Building shortly after it closed on April 16, 1980. That was the day before payday at the bureau, Hume said, when about $260,000 was in a safe inside a vault. Hume said Tickel was discovered behind a counter in the credit union by a janitor and the door to the vault was open.

Kenneth M. Robinson, Tickel's lawyer, argued that Tickel, who also served as locksmith in the headquarters, was in the credit union because he had received an anonymous phone call telling him the door to the credit union was open. Robinson said Tickel was in the credit union investigating when the janitor arrived.

Robinson said that the vault was unlocked when Tickel arrived and that witnesses would testify that they were on the phone with Tickel in his basement office at the time prosecutors say he entered the credit union.

The woman who managed the credit union at the time testified yesterday that she locked both the vault and the front door before leaving that night.

Earl Thornton, the janitor, testified that he unlocked one of the glass front doors when he arrived and saw the vault door open after he discovered Tickel. But he acknowledged under cross-examination that he did not check the other front door.

The trial is expected to end late next week.

Tickel is scheduled to stand trial in Alexandria later this month on other unrelated charges.