HONOLULU (AP) _ Fifty years ago today, and two years before she vanished, Amelia Earhart took off from Hawaii to begin her solitary conquest of the Pacific sky.

On Jan. 11, 1935, after a flight lasting 18 hours, 16 minutes, Miss Earhart's Lockheed Vega landed in Oakland, Calif. ''Lady Lindy'' had become the first to fly alone between Hawaii and North America.

''She always wanted to do something for aviation, to find out something,'' Muriel Earhart Morissey, 84, says of her sister. ''In those days, you just had to go by trial and error.''

The flight is commemorated in a bronze plaque at Wheeler Air Force Base on Oahu, to be unveiled at 4:44 p.m. (9:44 p.m. EST) today, 50 years to the minute after her plane rolled down the tarmac a half century ago.

Mrs. Morissey traveled from West Medford, Mass., to the Hawaii ceremonies with Grace McGuire, 36, who hopes to retrace and complete Miss Earhart's ill- fated 1937 around-the-world attempt later this year.

Martin Jenson, 84, the sole living survivor of the 1927 Dole Derby civilian aerial race between Oakland and Hawaii, also is expected.

''I'm tired 3/8'' were Miss Earhart's first words as she climbed from her plane in Oakland after 2,400-mile flight. ''I've been sitting down a long time.''

An estimated 5,000 people were on hand for her landing.

Although it had been eight years since an airplane made the first Hawaii- California flight, Miss Earhart's feat was front-page news.

It was a first among many. Amelia Earhart was the first woman passenger on a trans-Atlantic flight in 1928 and, in 1932, the first woman to fly the 1,800-mile route from Newfoundland to Ireland alone. She was the first to fly solo across the Atlantic and the Pacific.

''By successfully spanning the ocean stretches between Hawaii and California, following your successful transatlantic flight of 1928, you have shown the 'doubting Thomases' that aviation is a science which cannot be limited to men only,'' President Franklin Roosevelt said in a letter which reached the aviator a week after the trip.

''Because of the swift advances in this science of flight, made possible by government and private enterprise, scheduled ocean transportation by air is a distinct and definite future possibility,'' he said.

Before she left San Francisco to prepare for the flight, Miss Earhart had said: ''Anything I can do to help close the gap between Hawaii, as an integral part of the United States, and the mainland, will be a work into which I can throw myself wholeheartedly.''

Two years after the Hawaii-Oakland flight, on May 20, 1937, Miss Earhart and navigator Fred Noonan left Oakland aboard a Lockheed Electra headed east, hoping to girdle the globe. The flight, nearly completed, ended 850 miles short of Hawaii, under circumstances which may never be known.

The Wheeler plaque has been placed near the the main entrance to the base in central Oahu.

''After a three-hour rain delay, at 4:44 p.m. on Jan. 11, 1935, Amelia Earhart took off from Wheeler Field in her Lockheed Vega,'' the plaque reads. ''Eighteen hours and 16 minutes later, she landed in Oakland, Calif., thus becoming the first person to fly solo from Hawaii to the mainland. This also made her the first person to have flown solo across both oceans.''

For today's ceremonies, the Air Force had also hoped to display some Earhart memorabilia buried beneath a monument in Diamond Head Crater.

Beneath the 2,000-pound stone monument, dedicated to Miss Earhart less than four months before she vanished, is a copper box believed to contain her flight plan between Oahu and Oakland, some newspaper clippings, two poems and other mementos.

Air Force Col. John R. Johnson said the contents had ''great, significant historic value and the fact is that the material, if left in there, may deteriorate to the point of no historic value.''

The Air Force offered to preserve the items and renovate the monument if the box's contents could be briefly displayed, but the Honolulu City Council said no.

Council Chairwoman Patsy Mink said the box was a ''time capsule,'' not intended to be opened.

Amerlia Earhart's sister wished the council had said yes.

''I think it would have been fun to see what she put in there,'' Mrs. Morissey said. ''We'll have to leave that for the future.''