THE TROUBLE with airplane food is not just what it tastes like. It's that we're all so likely to it anyway.

The only real defense is the brown bag. If you're heading for Europe or the Caribbean or a holiday visit, add a picnic lunch to the carry-on luggage. Or a thermos jug dinner. Bring it from home. Bring it from one of the classier carry-outs. Just bring it.

Otherwise, up there at 30,000 feet, with the blue blankness glaring outside and the gray blankness of the hours droning by inside, which of us is not vulnerable to distraction? There are magazines, of course, and headsets and briefcases full of work. There could be a serendipitous seatmate. But chances are that, when the time comes, even those strong enough to say no to the clattering drink cart will be sufficiently steeped in ennui to accept the plastic tray.

The justification is the optimistic fancy that maybe this time there will be something good. That, and the notion that it's always okay to eat a salad.

The brute reality of soft meat and mucilaginous sauce, not to mention vegetables frozen and microwaved into complete characterlessness, wipes out the last irrational hope. The salad, with its pale pink tomato and bright orange "french" dressing is as bad as everything else. It goes without saying that no one with either a palate or a minimal awareness of calories and sugar would touch that square of heavily frosted cake.

Except that people often do. Yes, otherwise discriminating, prudent, self-disciplined people often eat quite a lot of the dismal food they're served on airplanes.

Meanwhile, we could all be discriminating, prudent, self-disciplined and hedonistic besides. Let the stewardess confine her role to opening the wine bottle and perhaps pouring hot water for a superior cup of tea. We're eating pa te' de campagne on homemade bread, to be followed by a perfect pear and a chunk of jarlsberg -- the kind with dill in it.

A wide-mouthed thermos that won't add much to the baggage burden can accommodate lentil soup or minestrone or any number of congenial potages -- pleasing hot supplements to cold roast pork with mustard or cold roast beef with horseradish. And no traveler should overlook the potentialities of oriental food. Packed in insulated bags, it can stay reasonably hot to Dulles and beyond.

The personal traveling table knife, fork and spoon were standard equipment for the peripatetic in the early days when such implements were recognized as better than fingers and poniards, which is to say, as the Renaissance was giving way to the Baroque period.No reason why a fastidious person should be limited to the plastic, paper and base metal typical of airplane dining appointments. An honest wine goblet at least is worth toting, along with the drinkable wine. Wrap it in a nightgown or sweater in the carry-on luggage.

Fifty years ago, when people who went to Europe got there by ship, the fruit basket and bottle of champagne were standard going-away presents for departing travelers. The champagne is still a good idea, but limit the fruit to the two or three pieces likely to be consumed by the time of arrival at Heathrow or Charles de Gaulle. The rest of the basket space will be needed for the smoked salmon, not to mention the french bread and cheese.

Provisioning an airborne picnic, for yourself or someone else, can be done personally or by way of some of the professionals who put together box meals for tailgate consumption at Wolf Trap, bicycle trips and miscellaneous on-the-grass summer eating. It can cost a modest $4 or $5 a person or, with the help of a little beluga caviar, run into serious money.

The Capitol Hill Wine & Cheese Shop regularly supplies the sky-bound with a tidily-boxed assortment of meat and cheese and crusty bread plus an appropriate wine and an after-dinner mint. Sometimes there's a salad, too -- customer's choice. Price: around $4.99.

Sutton Place Gourmet does the same sort of thing for as little as $3.99. That's only the beginning, though. Jeff Cohen, who's in charge at Sutton Place, reports that one party traveling to Puerto Rico managed to spend $100 a person on food for the trip. Caviar, smoked goose breast, foie gras and top-drawer champagne explain the stratospheric tab.

Suzanne's has a two-person package called "the hamper." For $10.95, you get cheeses, pa te's, wine, brownies and other delectations. It will also tailor individual boxes to personal taste.

The Watergate Gourmet shop also specializes in a la carte assortments for individual air travelers. Prices are from $5 up.

Here are a couple of soup-stews to put into that wide-mouthed thermos, enticing alternatives to the stuff under the plastic wrap. They are also to be commended as quick and easy enough to fit into a pre-trip schedule. LENTIL SOUP (1 serving)

1/2 cup lentils

1 pork link sausage, cubed

1 clove garlic, minced

1 carrot, sliced

1 small white turnip, cubed

1 small onion, sliced

Salt and pepper to taste

1 1/2 cups water

Simmer all ingredients in a covered saucepan 40 minutes, or until lentils are tender. Taste for salt and pepper seasoning. Pour into 2-cup thermos.

POTATO SOUP WITH LEEKS AND BACON (1 serving) 1 medium-sized unpeeled potato, scrubbed and sliced 1 clove garlic, minced Pinch dried coriander leaves Pinch dried thyme Salt and pepper to taste 1 1/2 cups water 1 leek, washed and sliced 1 slice bacon

Simmer potato, garlic and seasonings in water until potato is tender. Drain, reserving liquid. Pure'e potato in blender. Simmer leeks in reserved liquid until barely tender. Cut bacon into small pieces and fry crisp. Combine all ingredients in saucepan, and heat through, adding a little more water if too thick. Correct seasoning with salt and pepper. Pour into 2-cup thermos.