My heart sagged when I read recently that USAir had lost $75 million in the second quarter of 1990. No doubt the hunt for scapegoats and savings will now begin in earnest. A likely target for extinction: the full can of soda.

When USAir swallowed Piedmont Airlines several months ago, USAir executives thought long and hard about which Piedmontisms to preserve. The cutesy color scheme of the cocktail napkins? The uniform of the flight attendants? The corny in-flight jokes of the pilots?

None of those made the cut. Only one major marketing morsel did: the longstanding Piedmont policy of providing a full can of soda to every customer who asks for any soda at all.

The full can was a masterstroke when Piedmont was still around, and it has been equally masterful since Piedmont's death. When I'm bouncing through thunderstorms, the last thing I want is some chintzy airline deciding that I'm not worth more than three gulps of Diet Coke. If I want the whole can while I'm on the ground -- and I always do -- I'll certainly want the whole can at 30,000 feet.

USAir made a considerable financial sacrifice to keep the full-can policy. According to an article in the USAir in-flight magazine written by Chairman and President Edwin I. Colodny, the carrier spends nearly $16 million a month on food and drink.

Because USAir flies lots of short routes, it serves relatively few meals (there isn't enough time to heat, serve and clear). Therefore, beverages are a considerable chunk of the $16 million (the airline won't say how considerable). This in turn means that handing each thirsty customer 12 ounces of fluid rather than four costs enough money to pay a utility infielder for at least a week.

And remember, USAir can't recover these costs through increased fares, or in any other way. If USAir installed a $1-per-flight "soda surcharge," the carrier would be deserted overnight.

However, the full can is a golden slice of public relations that almost certainly attracts more money than it costs. Here's how golden:

On a recent USAir flight from Charlotte to Baltimore, I was sitting beside an elderly woman who announced that she was 75 and had never flown before. But her son had recently moved to Annapolis, and she didn't want to lose touch, so she'd be flying up to see him maybe four times a year, she told me.

About half an hour after we took off, the flight attendants wheeled the drink cart past. The woman asked for a club soda. She got a full can of it, without a hitch.

"You mean I get the whole can and I didn't have to ask?" she said to the attendant.

"That's right, ma'am," he said.

She proceeded to tell him the story of her son. Then she said: "And you can be sure I'll fly USAir every time."

I realize the story sounds like the rough draft of a hopelessly hokey TV ad. But I swear this is the way it went. Oh, the power of a complete club soda.

On a USAir flight from Roanoke to Baltimore, I looked around and discovered that I was one of only six people on the plane.

Small wonder. This is a lightly traveled route anyway. And the flight had left Roanoke about 8 p.m. on a Saturday -- not exactly a piece of the week when business fliers come flocking. The only people aboard were a college student in denim cutoffs who was strumming a ukulele (badly), four rather dour Japanese tourists and rumpled old me.

After about a half-hour, the drink cart came around. I asked for my usual Diet Coke. I got a full can, of course. Then the flight attendant said, "You know, sir, you look thirsty."

"I do?"

"Yes, you do. I think you need another Diet Coke. I just happen to have a few I can spare." And she handed me a second one. Also a full can, of course.

Rumpled old typists aren't supposed to be swayed by such gestures. But, as the old song said, little things mean a lot. The second Diet Coke made a dull flight a little less dull.

In case you're wondering, USAir is the only carrier with a large piece of the Washington market that provides a full can of soda without conditions. According to spokesmen, United and Delta airlines will give you the whole can, but only on request. American will give you the whole can only on request, and only if there are sufficient supplies of that particular brand on board to cover the whole planeload.

Wondering how and why USAir had made such a winner of a decision, I tried to arrange an interview with Edwin Colodny. Spokesman Dave Shipley called back to say the boss declined to see me. "He thinks it's old news," Dave said. "He'd rather go on to other things."

Let's hope those other things don't include a deep-sixing of the full-can policy. No one is sneezing at $75 million worth of red ink. But if USAir starts handing out as little liquid as the other guys, I know one mom and one rumpled typist who are going to be mighty disappointed.