A military court judge, questioning the impartiality of a hearing officer in a previous court investigation, yesterday ordered a new probe into the case of Cmdr. Donal M. Billig, the former head of cardiothoracic surgery at Bethesda Naval Hospital who has been ordered to face a trial on involuntary manslaughter charges.

The judge, Lt. Col. Hugh S. Atkins, agreed to a request from Billig's attorneys for a new pretrial investigation and a review by a military hearing officer who has had no involvement with the Billig case.

Commodore James Sears was the hearing officer who reviewed testimony given in September that led to a decision last week that Billig, 54, should face a court-martial on five counts of involuntary manslaughter and 24 counts of dereliction of duty.

Sears, however, was also a member of a formal board of inquiry that earlier this year investigated Billig's tenure as a surgeon and recommended a court-martial.

Defense attorneys have argued that Sears should not have been involved with the second investigation, in which Billig was attempting to have the charges dismissed.

Atkins said yesterday that he agreed with defense attorneys that Sears "could not be impartial because of his involvement with the original board."

Spokesmen for the Naval Medical Command said that Capt. Alvin S. Transeau, a military judge from Norfolk, will be assigned to review the formal board's findings and the first pretrial investigation.

Navy spokesmen said they did not know when Transeau might make a recommendation about the existing testimony. Lt. Neil Worden, one of Billig's attorneys, said he expects to recall some witnesses who testified in September, in the hope that additional statements from them could lead to the charges being dropped or reduced.