When it comes to sheer bravado, nobody beats the American airplane passenger. The old confidence that skilled people all over the country are alertly noting weather conditions, tracking flights, sorting out landing patterns, monitoring mechanical works, pooling information on parts that have begun to break and standing ready in the event of an emergency is now known to be unrealistic.

Knowing about cracks in engine supports, iced wings, forgotten bolt fittings and empty fuel tanks hasn't stopped the frequent flyer. The only breach I've noticed in the defiant courage of passengers is a sharp increase in the number of landings that induce spontaneous cabin applause. Yea, we made it this time.

Deep down we still believe in the system: the National Transportation Safety Board will find out what goes wrong, and when it does, the Federal Aviation Administration will fix it. It's a faith belied by a growing weight of contrary evidence.

The 1979 American Airlines DC10 crash killed 273 persons. It probably wouldn't have happened if American had known that the DC10's out-of-sight engine supports cracked during routine maintenance. Continental Airlines knew. But, as Douglas Feaver of The Post noted in a recent news story, "The Continental discovery was not widely shared among the airlines." The FAA didn't know it either. That was probably because, as Feaver reported, the FAA hands over "much" of its maintenance responsibility to the airlines.

The safety board sensibly recommended that the FAA do the obvious: have its inspectors pool their knowledge of important, inherent and maintenance-induced flaws in airplane models and pass the information on to companies that use them. It was sensible advice, and it was ignored.

On May 20, 1983, almost four years to the day after that plane crashed in Chicago, it was learned that the FAA isn't pooling or sharing such information and has no intention of doing so in the future.

J. Lynn Helms, the man who runs the FAA, seems to reason that since no other planes have fallen out of the sky and killed people because of similar model and maintenance problems, pooling and sharing will cost too much. The safety board thought his decision "disappointing and perplexing." It's more than that. It is dumb and dangerous.

But Helms must be thinking so far, so good, if he's not just waiting for his (and our) luck to run out. And luck it is. He must have been the gladdest man in Washington when the Nassau- bound Eastern Airlines plane that lost all three of its engines didn't have to crash-land in the Atlantic Ocean.

The engines stopped working because mechanics didn't put one essential part on each of them. The same thing had happened before on Eastern and Pan American World Airways. There was so little pooling and sharing of information that Eastern was as surprised to learn it as everyone else.

It's often said that the people involved with the commercial airlines are "only human." True enough. But that is no excuse for failure to perform basic duties that prevent the loss of life. It is not "only human" to run out of gas, forget to put the landing gear down or fail to notice that the wings are covered with ice.

In the matter of air traffic control, being "only human" is a bona fide factor. Two years after the president fired the striking air controllers, the system is still running with patchwork personnel. The safety Board has just sent its second report and list of recommendations to the FAA on this subject.

Circumspect it is; reassuring it is not. "Based strictly on the absence . . . of a significant number of accidents . . .the ATC (air traffic control) system has been operated safely" (italics added). The safety board notes that air traffic is still being directed by a mix of "trainees," nonstrikers and supervisors who should be supervising and not directing traffic. They are exhausted, have too many overtime hours, few vacations and are subject to great stress. This is bad business.

Controllers must be fresh and alert because their judgments are crucial. They should not be in training; they should be trained. Add to this the understanding that in the busiest airports the supervisors are not up and about helping the trainees, not coordinating the work of the controllers and not free to handle emergencies. They are acting as on-line controllers, committed to the screen or microphone where they sit. Nobody is supervising, in fact.

Air transportation systems are a mess. So it is that the normal, lily-livered traveler has been thrust into unexpected bravado. Intrepid we passengers may be, but it is time for us to recalculate the safety odds. It is also probably time for Americans to travel as Pope's Dying Christian did, "trembling, hoping, ling'ring, flying."