A twin-engine air ambulance lifted off from National Airport at 1:20 p.m. yesterday, carrying a single patient, Timothy J. Tiernan, on a 300-mile journey that medical experts say will provide him the best chance of ever awakening.

Until last Aug. 7, Tiernan, 14, was an athletic, strong-willed and sometimes mischievous teen-ager from Annandale who collected hubcaps and antique bottles and who insisted on pizza or hotdogs instead of turkey at Thanksgiving.

On a rain-slicked road in West Virginia, that past life ended. Tiernan suffered severe and largely irreversible brain damage in an automobile accident. He has remained in a coma-like state for 32 weeks, most of it spent at Children's Hospital in Washington, where he remained longer than any neurosurgical patient in recent memory.

His eyes opened in September, and his limbs move on occasion. Otherwise he is motionless, silent and seemingly oblivious to his surroundings -- despite countless hours that his mother, Patricia, a former nurse, has spent at his bedside trying to evoke responses.

Tiernan's survival represents an achievement of modern American medicine, which kept him alive only through advanced technology, powerful drugs and a dramatic 13-hour emergency brain surgery last October at Children's.

Tiernan also represents what medical authorities call a growing and silent epidemic: patients whose injuries once would have killed them, but who now are kept alive -- only to be doomed to indefinite periods of twilight. His condition poses a nagging question: What is to become of this growing number of coma victims, many of them young, strong and capable of surviving for years?

Yesterday's flight was part of the Tiernan family's last slim hope of regaining the son they knew, or at least part of him. The air ambulance carried Timothy to the Lake Erie Institute of Rehabilitation in Erie, Pa., one of only about six coma treatment facilities in the U.S. that together have capacity for only about 200 coma patients, according to the National Head Injury Foundation of Framingham, Mass.

Tiernan was transferred to LEIR only after four months of rejections from hospitals, nursing homes, Blue Cross, and Medicaid officials whom the Tiernan family fruitlessly approached for help. However, his placement is only temporary and the Tiernans may soon have to face the same frustrations again.

Every year, an estimated 700,000 Americans suffer head injuries requiring hospital care, most often because of automobile accidents, according to the head injury foundation. Of those, roughly 50,000 to 90,000 injuries are severe enough to cause coma.

Many die and many recover at least partially. Perhaps 5,000 or more remain in coma for more than a month, including many who will linger indefinitely, according to Richard Friswell, a psychologist who is director of the foundation, which was formed in 1980.

The treatment goal, usually unattainable, is to awaken or partially rehabilitate patients such as Tiernan through lengthy and intensive sensory stimulation, according to head injury experts.

Because of the scarcity of services, Friswell said, "patients are being cranked through the medical system and then just discharged," either to their homes or to nursing homes, where they languish and often die of complications related to inadequate care, he said.

Virtually no hospital in the United States would take Timothy Tiernan, according to Carolyn Johnson, the Children's Hospital social worker who tried unsuccessfully to find treatment for him once his medical condition stabilized in December. She compared her effort to "traveling through uncharted territory."

Moreover, complicating the problem, most medical insurance companies will not pay for such care because they consider it custodial, rather than active medical treatment. As a still small but growing number of coma victims accumulates -- in some cases the patients lingering for years and occasionally for more than a decade -- the social and financial questions raised by these cases have not yet been confronted, those familiar with the field agree.

Meanwhile, the Tiernan case remains one family's ongoing nightmare, involving not only an eerie and permanent injury, but the added burden of facing an uphill bureaucratic battle to assure their son the best care.

It began about 10 p.m. on a foggy Friday night that was supposed to be the start of a summer weekend outing. Robert Tiernan, 48, a Washington lawyer, was taking Timothy and two teen-aged family friends to a lakeside cottage near Romney, W.Va.

They had just stopped for groceries and loaded them in the back seat of the rented Chrysler, meaning that Timothy and his friend, Danny Cappello, 13, had to move into the front seat, where they did not fasten their seat belts, Robert Tiernan recalled in an interview.

The two others had fallen asleep, but Timothy and his father were singing "Elvira" along with the car radio. Tiernan said he had complained that the car was handling oddly on the wet roads, and said he was only driving about 30 miles per hour when the car skidded through an S-curve on Rte. 50 and slammed into a tree.

Robert Tiernan hit the windshield. Ann Hillgren, 19, a friend of Timothy's older brother, was thrown clear of the car and escaped serious injury. Danny Cappello fractured both legs when the front seat slammed forward into the dashboard.

Timothy was thrown out the right front door of the car, but his legs remained pinned inside, his father said, creating a horrible whiplash. He suffered a "severe tear of the brain stem," according to Dr. M. Kathryn Hammock, the chief pediatric neurosurgeon at Children's. The injury irreparably damaged the stem, which essentially controls the ability to remain awake, alert and aware of the environment, doctors said.

"I thought he was dead right then," Tiernan said, as he sat red-eyed in his law office, recounting the start of the months-long ordeal. He said he often is tormented by the thoughts that if only it hadn't rained, if only they hadn't stopped for groceries, then it somehow would never have happened.

"Sometimes I feel like jumping out of my skin, and sometimes I cry," he said, "And I know it sounds terrible to say this, but I think sometimes he would be better off dead."

In some ways, the old Timothy is dead, a fact that becomes evident listening to his mother and father talk about him in the past tense, describing the boy who once was.

The particular heartbreak of Timothy's condition is that he looks so healthy, said Patricia Tiernan, 44, who was divorced from her husband three years ago and has custody of Timothy and two other children, Robert, 18, and Amy, 20.

"The killing part of a head injury like this is that he looks like he is about to roar up and get up and run . . . But he can't," she said. Last August, she quit a real estate job to devote full time to her son.

Nancy Reilly, one of Timothy Tiernan's nurses, said he has maintained good muscle tone and avoided common skin problems largely because of the countless hours his mother has spent exercising his limbs, massaging his facial muscles, brushing his teeth and rubbing vitamin lotion into his skin.

His mother talked to him constantly during her six months of daily visits to Room 4114 at Children's. She read to him. She used a battery-operated toy ray-gun that made weird high-pitched noises. She also played a tape that included the barking of Timothy's dog, Pooba, and the tape-recorded greetings of his friends, some of whom were too frightened to come see Timothy.

"Hi, Tim," the tape begins. The message tells Timothy about all the things he has missed; about school, his friends' Pac-Man scores, their bicycle trips. Then it concludes: "Please wake up and start getting well . . . If you wake up, everybody'll be happy again."

Through it all, despite all the attempted stimulation, Timothy lay with eyes closed or with a fixed focusless stare, his eyes moving mostly at random.

Once, at about 5:30 p.m. on Feb. 26, while "I Love Lucy" was on television in Room 4114, Reilly and Mrs. Tiernan saw Timothy smiling. Moments like that filled his mother with renewed hope, she said. But the smile was not repeated and his doctors say they have seen no tangible evidence of progress in months.

Patricia Tiernan has kept a meticulous record of the entire tragedy, in the form of a calendar written to her son. "From today, we will keep a daily log," she wrote last September, "It's impossible to tell all of the agony, despair, anger and denial we've gone through. That we will tell you as you awaken, and have the same feelings. Just remember our love."

The calendar chronicles the medical crises: Two bouts with meningitis. Cerebral bleeding forcing installation of "shunts" to relieve pressure. Fevers, seizures and surgery.

Darkly written entries dominate October: "Timmy, Timmy, you certainly have had better days . . . Keep fighting babe . . . You are one sick human being . . . You have us all so scared."

Then the worst was over: "Timmy, Timmy you are a heartbreaker. Today again you look so wonderful. Temperature down. Relaxed and rested-looking. Please God let it be uphill now."

Christmas Day: "How we all miss the old you . . . There was an electricity in the house that is missing with you gone."

The calendar recalls the changes of season and the events Timothy has missed: a December visit by Nancy Reagan, the heavy snowfall of January, the Air Florida plane crash, the day a nurse took Timmy outside and rubbed snow on his face in a futile attempt to get a response.

Throughout the calendar, there is also stubborn hope. "I keep hoping you will awaken . . . I keep hoping and praying . . . Somehow you seem alert . . . I keep hoping you will awaken."

The brain, more than any other organ, remains largely a mystery, said another of Timothy's doctors, Joel W. Ray. Current brain research attempts to learn whether parts of the brain can be trained to take over functions of damaged parts, he said. "There are still too many unknowns," Ray said. "It would not be fair to him" to declare Timothy hopeless.

The success rate with patients such as Timothy is extremely low and gets lower the longer coma persists, according to medical literature. The "miracle" awakenings make headlines, but the overwhelming majority of long-term patients make little measurable progress, said Patricia Lojewski, executive director of LEIR, which began its head trauma program in 1980 and now plans to convert the entire 114-bed facility to that purpose.

"There are so few options for people like Timothy because so few people really understand head injury," Lojewski said. LEIR, whose basic room rate is $150 per day, provides an intensive program with therapists who will stimulate Timothy daily, much the same way his mother did, she said.

Blue Cross initially refused to cover Timothy's placement because the "state of the art" of medicine has not yet demonstrated enough results to show that such costly long-term care is anything more than custodial, said Barry P. Wilson, Blue Cross vice president for public affairs.

"It is a social policy question" whether society can afford to pay large amounts for care with uncertain outcome, Wilson said, "And I don't think it has been addressed yet . . . This is a growing issue, with growing numbers of patients and their families. But it is still a relatively small population."

He compared the situation to hospice care for the terminally ill, a coverage not provided until the medical establishment recognized the benefit and enough people demanded it.

In the Tiernan case, Blue Cross reversed its decision two weeks ago when Robert Tiernan arranged to present his case to the insurance company, and Lojewski flew here from Pennsylvania for the meeting. Because LEIR is licensed as both a nursing home and hospital and because Timothy still needs some medical care, Blue Cross agreed to fund the treatment temporarily.

Medicaid normally will not cover the cost of care at places such as LEIR, Lojewski said, and Patricia Tiernan said her attempts to get answers from the government made her feel "like Alice in Wonderland, at the Mad Hatter's tea party."

Tiernan said he was thankful for Blue Cross's reversal, but said most coma victims are not as fortunate. "Timmy is getting his best shot at recovery and I don't know how many kids ever do," he said. "They all deserve it. They don't all get it."

Instead, many end up at home with families that are not capable of caring for them, or in nursing homes that are not properly equipped, said psychologist Friswell.

Timothy's mother tries to cope by writing to Timothy. She includes poems, like the one she wrote on the back of a calendar page:

"You were a gleam of joy. How I loved to see you shine. You were a dazzling boy, and you were mine, for a time, for a time. Just for a time."