IF YOU THINK it doesn't make much sense for the government to pay $56,364 to fly a single congressman down to Rio -- even if that congressman is accompanied by his daughter and four other guests and attended by a doctor and four military escorts on a 42-passenger C9 transport -- if you think that doesn't make much sense, you're certainly not alone. Even Rep. Bill Alexander (D- Ark.), the congressman who made such a journey last month, probably agrees with you by now. So, surely, do most of his colleagues. Yet it's not clear that either Mr. Alexander or his colleagues understand why.

Mr. Alexander evidently thinks the problem is that he failed to make it clear that he was not just off on a pleasure junket. He is at pains to explain that he went to Brazil to study gasohol; and it's true that Brazil -- burdened with foreign debts, too much sugar cane and too little oil -- uses gasohol extensively as a gasoline substitute. Put aside for a moment any nagging suspicion that gasohol as a substitute for gasoline is an expensive boondoggle of little interest to any Americans except the rice-growers in Mr. Alexander's district: the purpose is public, if foolish. Even so, there's still no reason Mr. Alexander and his party couldn't have used commercial planes to get from the United States to Brazil. Hundreds of passengers do so every day of the week.

Some of Mr. Alexander's colleagues think the problem is that he didn't stick closely to House rules and custom. They require that military planes be used only for bipartisan groups of at least five congressmen; and Mr. Alexander admits that at least some of the four colleagues whose names he submitted to Speaker O'Neill had told him they weren't interested in going. But why should the magic number be five? The real question is whether it's cheaper to fly traveling congressmen on commercial or military planes. That depends more on the destination than on the number of congressmen involved.

Unfortunately, Congress won't straightforwardly vote itself the travel funds it needs. That gives members and the leadership an incentive to rely on military planes -- whose costs are buried in the big Pentagon budget -- rather than to use commercial flights, the money for which comes out of Congress' budget and is reported (though not summarized in any readily accessible way) in the Congressional Record.

Of course, it is possible for abuses to occur under any system. But straightforward accounting would reduce the chances of embarrassments such as this flight by Mr. Alexander which harm not only his reputation but that of congressmen generally.