MUSICIAN Alexander Schneider, or Sasha, as the world calls him, lolls at his ease on a Delta Air Lines flight. He has polished off his homemade picnic of fruit, salami and cheese. His 1723 Guarneri violin is tucked safely at his side. "Excuse me," asks a stewardess, "are you Stern?" "No!" teases the 77-year-old Schneider, "I am Jascha Heifetz." Ten minutes later, another stewardess taps the fiddler on the shoulder. "Excuse me, sir, but are you Stern?" "No!" winks Schneider, "I am Heifetz." Five minutes later, yet another stewardess: "Are you Stern?"

"Vy? Vot do you vant from me?" barks the suddenly cantankerous Schneider in his idiosyncratic concoction of Russian, English and Yiddish.

"We have one meal left over," the stewardess replies. "It's a kosher platter and it's for a Rabbi Stern."

THE STORY, a favorite of violinist Isaac Stern, is one of the few "Sasha stories" that are printable, say Schneider's friends. Known as an enfant terrible of classical music, he is also an impresario straordin,ario, impassioned fiddler, conductor, Casanova, guru, provocateur and, for a half- century, bon vivant of American musical life in Washington, New York and around the globe. To know Sasha is to be charmed by Sasha, bullied by Sasha, infuriated with Sasha, forgiving of Sasha, forever linked with him musically through his indomitable spirit, energy and passion.

Schneider's music is like nobody else's in these days of pyrotechnics and academic purists. No matter what he plays or conducts, there is a looseness of phrasing, a purposeful lack of concern over various points of articulation, and a disregard for so- called "authenticity of style." What he does is make other people make good music. This gift arises from his need for intimacy, a communication among his musicians, his audiences and himself. "You cannot take music out of the freezer and bake it," he says. "There are a million vays to do it. Ve must create together."

There comes a point, he says, "ven you ask: Vat do you vant in life? To do it better, more perfectly, follow all the notes? That is silly. The mistakes help you. My God, Schnabel, Rubinstein, Hubermann, Casals, they made mistakes but didn't care. Ve should all make such vonderful mistakes."

For those fortunate enough to hold tickets (they'll be sold out by the time you read this), the quintessence of Sasha Schneider's longtime love affair with audiences will be on view at the Kennedy Center Tuesday at the annual "A Night in Old Vienna" concert featuring the ensemble of "Alexander Schneider and Friends." It's one of the city's most unusual New Year's Eve fetes, a concert -- this year featuring chamber music of Mozart, Schubert and Mendelssohn -- that is followed by audience waltzing at midnight in the center's Grand Foyer. For the dancing, Schneider and the ensemble -- a familiar pickup group of some of his oldest friends -- mount a small stage and fiddle away at Viennese melodies. Participants -- not only concertgoers but anyone attending one of the evening's performances at the center -- appareled in everything from blue jeans to ball gowns and white tie and tails twirl on the crim- son carpet, quaff champagne and occasionally nip from bottles they've hidden in the ivy on the terrace.

The Schneider New Year's Eve party, putting in its 13th appearance this week, is one of Sasha's three regular stings in Washington. In February he'll be back with the young players of the Brandenburg Ensemble. Last Thursday at the Kennedy Center marked the 17th appearance here of his New York String Orchestra, 58 musicians between the ages of 15 and 22 who performed Haydn, Mozart and Johann Strauss under Schneider's beckoning hands.

The ensemble, as usual, had practiced together only 11 days before the concert, the final event of a Christmas season marathon. The marathon begins in New York with nine hours a day of practicing and rehearsing under the supervision of Schneider and whichever of his musical friends he can rope in to help, and it culminates in two concerts at Carnegie Hall in New York followed by the Washington appearance. Schneider travels around the country every year auditioning players for these concerts. Among the best-known New York String Orchestra alumni are cellist Yo-Yo Ma and violinists Schlomo Mintz and Cho-Liang (Jimmy) Lin. Lin speaks for many young disciples of the frizzy-haired conductor when he says, "Sasha has the ability to make every person in the orchestra feel he is the only one playing."

INSIDE THE FRONT DOOR of Schneider's duplex loft in Manhattan, to the right and trailing up the wall, is a large, bold banner printed in red: "Here's to Sasha's next 75 years of Schmoekadores."

"That's my vord," he says, "I invented it." Indeed, such an original word could not possibly belong to anyone other than this original man. On the first floor of the loft, big enough to accommodate a full orchestra, are a slew of antiques, whatnots, a rocking chair, a nine-foot French country table, a Steinway draped with a mirrored Indian throw, sculpture, posters and paintings here and there. He has a signed Calder, a portrait of Casals inscribed "to my dearest faithful friend," and a Saul Steinberg drawing of Sasha in baseball togs, framed with a check to pay up a 1954 wager when the Giants won the World Series. On it Steinberg has penned: "Abrasha Schneider, Sensational Southpaw from Vilna." There's a signed photo of Irene, princess of Greece; an antique saxophone; a chef's hat; a dart board; a photograph of Sasha wearing a blond wig; another inscribed "Sasha's Nude Time Band"; and, past the recordings of Heifetz, Stern, Rudolf Serkin, Bruno Walter and Richard Tauber, amid the clutter, a framed engraved invitation to perform at the White House Nov. 13, 1966, and a letter signed by John F. Kennedy. Pasted on the walls of a bathroom, over an electric-blue sink, are hundreds of snapshots.

Upstairs, past the gold cupid on the banister, is a serious kitchen not far from an equally serious four- poster brass bed surrounded by ruby velvet drapes. "Thomas Jefferson had his bed right next to his kitchen, just like me," Sasha says, setting out a glass of tea for his guest on a bass drum that serves as a coffee table. "Add some Jerusalem cinnamon," he suggests. "The Arabs believe it is good for your sex life." Schneider constantly throws sexy references into his conversations.

"First ve have dinner, zen ve make love, zen ve do interview, maybe," he teases playfully, as if detailing a recipe. He pads back and forth from the canary yellow refrigerator to a case of olive oil from his farm in Provence to his wine collection to the stove over which he hovers, stirring boiled beef in a blue enameled pot. He pokes at noodles fresh from Chinatown, a mackerel in wine sauce, and he hauls out Long Island clams for inspection. He thinks for a moment about old age, which he dreads. "I could cope with being 90 if I could do everything my way, eating, drinking, cooking," and making love. "But vat vould I do if I got gaga?"

ROMANCE has led Sasha thrice into marriage, most recently to actress Geraldine Page, a union that ended in 1957. He will not speak of his wives as he will not speak of his home-sharing relationship with the late photographer Margaret Bourke- White. The closest he has come was a remark made to Time magazine some years ago on the failure of his three- year marriage to Page. "Homogeneity is the worst thing in music," he said. "It is not so good in marriage either. The first five bars sound vonderful, but afterwards you are very bored because everything sounds the same."

A generous variety has characterized Schneider's pursuit of his craft over the years as violinist, conductor and musical godfather. He first achieved fame as a member of the Budapest String Quartet, which was the nation's best known chamber ensemble during the 23 years (1938- 1961) it was in residence at the Library of Congress. From the Budapest era Schneider went on to participate in the organization of the Albineri Trio and the Schneider Quartet. Later, he spurred the formation of the Guarneri, Cleveland and Vermeer quartets; he promoted the creation of the Prades festivals that helped to make the cello music of Pablo Casals a familiar sound; he conducted summer outdoor concerts in New York and winter indoor concerts in Washington; in all, he became a key figure in the revival of chamber music in the United States.

Over the years, Schneider has admitted to "crushes" on various composers -- Vivaldi, Handel, Haydn, among others. Haydn is a longtime obsession: he formed the Schneider Quartet specifically to record all 83 of the Haydn string quartets for the Haydn Society. Fifty were completed before money for the project ran out.

ONE CANNOT DISCUSS Sasha Schneider without involving Pablo Casals. "I arrived in Prades, a village in the Pyrenees near the Spanish border, in 1947," recalls Sasha, "hot, dirty, exhausted. Casals asked me to play. He then paid me my greatest compliment. After I finished, he played for me. That cemented our friendship."

For two years after that meeting, Schneider hounded Casals about a festival in America. Repeatedly he was turned down. "In 1949 we played quartets, Casals was in a good mood, I asked vonce again. He refused, saying not as long as the United States recognized the Spanish government, so I got angry. I said, 'Maestro, I varn you, if you can't come to us, ve vill come to you.' Prades Festival was born.

Schneider's tenacity in pressing the vision, once it came to him, was virtually unbounded. "Everyone called me nuts: 'Leave the old man alone!' 'Vhere vill you raise the money?' 'Who vill go all the vay to France?' later, people killed themselves to visit Prades." For two decades, amidst chronic turbulence, Schneider administered the festival. "I always loved Casals, not only as the world's greatest cellist but as a man," he says. "Casals, Schveitzer, Einstein, they were leaders of humanity. Casals taught me vibrato (the pulsing quality an oscillating string finger can instill in a violin note), even Stern he taught vibrato. It is the single most important thing in music: the strings must sing."

"I was a cellist of 13 when I first met Sasha," recalls Marta Casals Istomin, Casals' widow (now married to pianist Eugene Istomin), who is artistic director of the Kennedy Center. "He called the maestro Don Pablo. In the ensuing years, when the maestro and I were together, our life with Sasha was en famille: Sasha was there during good times, sad times, but always with exuberance. Once we all flew first class on a long plane trip, but Sasha insisted on economy fare. He pinched a stewardess, or whatever

he does to charm them, and, suddenly, he was given a special bottle of wine. So everybody came back to him in economy class. How he adores food! He would always bring us a side of beef in San Juan, convinced we never got good meat there."

Eugene Istomin remembers meeting Schneider backstage at a Budapest String Quartet concert in 1948. "I was 24 years old, Sasha was 41," says Istomin. "I was arrogant. So was he. We had a contentious relationship, sparks flying, competitive. But now he's like a sort of a pain-in- the-neck sibling, an older brother, and we understand one another. We don't have to speak about what matters.

"Sasha ignites things but burns them up. He's in love with ignition, whether he's playing with what he calls 'the yut (youth) of America' or immersing himself in incipient love affairs. Then he tires, burns up. In the course of all this he does brave, even important things. Underneath the bluff, that scandalizing ghettolike manner of his, he has a refinement -- you hear it in his music. You see it in his taste for art, food and books. He's the thinking man's player with an illuminated lunacy mixed with the very rational, pragmatic way of making things happen."

Istomin recalled that he was estranged from Schneider for a time in the mid-1950s. Indeed, Schneider has broken with many of his friends and colleagues at one time or another, though reconciliations are also common. Even Casals and he broke for a time. "It was Casals who understood him best," Istomin says. "He was paternal in a way, and Sasha had veneration with him at all times. He was never feisty, never vulgar, never disrespectful to Casals, yet they were very emotional with one another."

As to Sasha's quarrels, Istomin shrugs. "I have learned to humor him. If you really fight ith him, and I did once, he is surprised. He doesn't believe he can anger or hurt anybody because he feels he is after the truth . . . For Sasha, to overdo things is human, to him life is one's soul and one's tears . . .

"I owe Sasha a great deal, most of all, Casals. In the maestro was a voice imbedded to reach the heart, a powerful signal for good, and Sasha knows that voice. That's why we are like brothers."

Casals spent his last years in Puerto Rico. When his body was exhumed in 1979 after Franco's death for reburial in Casals' native Catalonia, Schneider went to the memorial service in San Juan. Conductor-violinist Jorge Mester, for eight years music director of the Casals Festival in San Juan, recalls the event. "There was to be played a tape of Casals performing 'The Song of the Birds,' a folk tune that was, in essence, his own signature piece," says Mester. But by mistake the tape had been wound backward, and when the tape player was turned on, cacophony rather than melody squawked from the speakers. "Without a thought, Sasha grabbed a violin that was on hand, Eugene (Istomin) went to the piano, and the two completed 'The Sound of the Birds.'

FOR AMERICANS, the Sasha Schneider story begins when he and his cellist brother, Mischa, violinist Joseph Roisman and violist Boris Kroyt, the four members of the world-renowned Budapest String Quartet, arrived in the United States in 1938, refugees from Nazism. Soon after they were offered a permanent musical residence at the Library of Congress.

Says Felix Galimir, a 40-year friend and colleague, "The sheer excellence of the Budapest made them a household word; there is a cartoon that shows guys with masks coming out of a bank holding violin cases, and the caption has the cops saying: 'Let them go, fellas, they're the Budapest Quartet.'

Famous though it may have been, the quartet in its last years at the library became careless to the point that Washington Post critic Paul Hume commented of the four in a 1961 review, "It is too long since they played any single quartet in tune from start to finish." "Hume was right," said the late Donald Leavitt, retired head of the library's music division, shortly before his recent death. intonation was slipping. Sasha didn't rehearse enough and would drown out Joe. It was Mischa who always played hlike an angel."

The upshot, as Schneider recalled during a triumphant return appearance at the Library of Congress arranged by Leavitt in 1982, was that "two men from the library came and told us our services were no longer desired."

"Roisman was the one out of tune," recalls Washington writer and music aficionado Henry Raymont, "but, on the whole, there was enormous musicianship. It is only in this mechanized American society that such revered musicians could be ousted so abruptly."

ISADORE COHEN is a 35-year friend who joined the Budapest from time to time, was an original member of the Schneider Quartet and was a colleague to Schneider during his long stint at the Marlboro Music School in Vermont. Cohen remembers Mischa (who died in October) as the restrained brother, Sasha as the impetous one. "Sasha was always up to something," he says. "One night at the library we discovered pornographic photos in between our music. Sasha, who else?"

"Sasha's inner and outer lives are synonymous," says Judith Cohen, Isadore Cohen's wife. "I met him in 1950 and since then he has consistently said precisely what is on his mind. Most of us have little inhibitors to sort out what should be said and what should not; they keep us out of trouble. But not Sasha. He has a child's lack of judgment and inhibition. Sasha has never grown up, and that is part of his charm."

"If unpredictable, he is always exciting," says Marjory Hanson, who was Schneider's secretary for six years. "My favorite Sasha story is when Albert Eintein was sitting in with the Budapest, and Sasha could be heard addressing him, 'Vun, two, vun, two, Dr. Einstein, ven vill you ever learn to count?' offer a clue to the violinist's personal word. "Schmoek a verb, as a noun, as an expletive, even as a term of endearment." Invitations to his 72nd birthday party, recalls Martin Feinstein of the Washington Opera, staked Schneider's claim to the word with a photograph of the host costumed as someone labeled "the original Schmoekdore Cohen supplies examples of how the word arises during practice sessions: Schneider might say, " of this Schmoekadores -- let's get back to work' . . . or 'This passage needs a little more Schmoekadores, it needs imagination . . . or 'After we finish rehearsing, let's go out and Schmoekadores a little bit' . . . or 'The best things in life are making music, making chicken soup and making Schmoekadores.' I wouldn't be surprised if it shows up in Webster's someday."

Cohen speculates on Schmoekadores in tones of enduring affection. The same tones pop up in the voices of a wide range of musicians and music lovers: lawyer Carolyn Agger, speaking of Schneider's music-making with her late husband, Associate Justice Abe Fortas; pianists Claude Frank and his wife Lilian Kallir, declaring that Sasha is the most generous person they have ever known; Lily Guest, former president of the Friends of the Kennedy Center, describing Schneider's long-ago concerts-cum-parties at Dumbarton Oaks; Judith Cohen, telling of the day an excited Schneider bent down in the middle of Fifth Avenue to kiss her very pregnant stomach.

Longtime friend Julius Levine, a bass player, is speaking in the same tones as he says that Schneider's musical secret is to be found in his insight. "One hears it in how he breathes musically, how he feels pulsation, how his timing is invariably right," Levine says. "That is because he is listening to the sound of his own voice and within this, there comes a sense of freedom. Only when that happens, when you are no longer merely involved with the music, can you feel your own sound, your voice, from your instrument. This is what he teaches the kids."

There is a dark side to Schneider as well, Levine observes: "Sasha is a genius . . . but he sometimes has a compulsion to destroy what he created. But he reaches out to us. It is impossible for him to apologize, but he says, somehow, 'Please understand who you are dealing with, please take me in that context.'

MY telephone rings. It is Sasha. "I'd appreciate a short interview to talk specifically about Washington," I say. Silence. Then, "Call me Friday at 6 a.m." I set my alarm and dial Schneider. "Sasha, when are you free to talk?"

"Never! I don't have ze time to make love, vot makes you think I have ze time for an interview?" He slams down the phone.

I finally understand the word "Schmoekadores."