A FUNNY THING happened to the administration's proposals to make those who use the nation's airways pay the costs of operating them. The proposals ran into something called the aviation lobby and were transformed overnight. The 20 percent tax on aviation fuel suddenly became a flat 12 cents (or less) a gallon. The 80 percent increase in the tax on airline tickets slimmed down to a 30 percent increase.

The official explanation of this change is that the original proposals would have raised more money for the airport and airways trust fund than the administration wants the fund to spend. That's because the fund now has a surplus of more than $3 billion and not because the lower tax rates will put the airways system on a pay-as-you-fly basis.

The truth is that general aviation -- that great category of activity ranging from single-engined puddle-jumper flights to corporate jet traffic -- has been getting away with murder for years. The Congressional Budget Office estimates that these flyers accounted for about $740 million of the capital and operating costs of the airways system last year and paid $80 million in user taxes. Guess who paid for the rest of it?

According to the administration's first version, the proposed taxes would have turned the airways into a break-even proposition although passengers would still have been subsidizing general aviation. In the latest version, that subsidy -- presumably from the trust fund's surplus until it is exhausted -- will be much larger. A 20 percent tax on fuel would have brought in about $400 million. A 12-cents-a-gallon tax will bring in about $150 million. (The administration does hope to get the $150 million up by gradually raising the tax over the next five years.)

So sharp a tax increase is pretty hard to take, and we don't blame the general aviation folks for protesting lustily. But comparable protests about the end of subsidies from people who ride trains or use food stamps or benefit from other programs aimed at the near-poor haven't received such a favorable response from the White House.

Reporter Douglas B. Feaver put it into perspective in an article in Sunday's paper. "What we had to do was to disabuse [the administration] of the notion that everybody in general aviation wears a baseball cap and is called 'Ace,'" he quoted the president of the Aircraft Owners and Pilots Association as saying. That association has 261,000 members with an average income of $44,000 -- "right in the guts of the Reagan constituency," as another association official said. The corporate jet, by the way, gets away with paying even less of its share than do those little planes. So much for real user fees and pay-as-you-go theory.