A 71-year-old cancer patient at Walter Reed Army Medical Center asked a federal judge yesterday to overrule Army policy and allow doctors to remove her from a life-support system "to die a dignified and natural death."

U.S. District Judge Thomas Penfield Jackson scheduled a hearing for this morning on the request by Martha A. Tune of Falls Church. The court case apparently is only the second in the nation involving a terminally ill patient at a federally owned and operated hospital facility.

In a case involving a Veterans Administration hospital in Los Angeles about two years ago, a federal judge ordered that a patient be allowed to die.

Opponents of the ruling then won a 15-day stay. Before the stay ran out, the patient suffered cardiac arrest, was judged to be brain dead and was permitted to die.

A Walter Reed spokesman, Peter Esker, said Tune, the widow of an Army lieutenant colonel, was admitted Feb. 21 and is currently listed in the hospital's most serious condition category.

"She's a [mentally] competent, terminally ill patient," Esker said.

A 1982 District law, the D.C. Natural Death Act, permits removal of life-support equipment at a patient's request. As a U.S. military facility, however, Walter Reed is not subject to the local statute.

A two-page, typewritten court document signed by Tune said "physicians at Walter Reed . . . are sympathetic to [Tune's] desire to die a natural death." But Army policy, the court papers said, "is that withdrawal of life-support from a patient in a military treatment facility is not permitted."

Esker, the hospital spokesman, said that Army policy permits military doctors to "withhold" life support assistance at a patient's request.

"But we are not permitted to withdraw it once started , meaning to pull the plug," Esker said.

Tune's options, according to Esker, were to be transferred to a civilian hospital in the District or to go to federal court "and have the federal court order us to do it."

Assistant U.S. Attorney Royce C. Lamberth said the government will try to ensure at today's hearing "that the judge considers all the relevant factors."

Jackson yesterday appointed a lawyer, James P. Schaller, to act as legal guardian for Tune at the hearing.

Schaller, who visited Tune last night at Walter Reed, said, "She is a very feisty, spirited lady who is very lucid. But she is also tremendously ill and is dying and she knows it."

Tune's court papers described her as having two grown children and four grandchildren. Lamberth said that Tune's son, Cecil C. Tune, prepared the court papers and took them to his mother for signing.

According to the papers, Tune entered Walter Reed suffering from shortness of breath and chest discomfort. During a subsequent test by Army doctors, it was discovered that she was suffering from adenocarcinoma of the pericardium -- or cancer of the dense, fibrous sac surrounding the heart, the papers said.