Tiny cracks have been discovered in the metal cap at the end of a main wing brace on seven older McDonnell Douglas DC9 jetliners, the Federal Aviation Administration said yesterday.

Although the FAA is ordering that other DC9s be inspected and any cracks repaired, FAA officials said there is no safety problem or plan to ground the DC9 fleet. About 400 DC9s are in U.S. service; about 900 fly worldwide.

The FAA took the unusual step of attacking a newspaper account in yesterday's Phoenix Republic that said the DC9 fleet might be grounded and that the crack could cause a crash. The FAA called the article erroneous, and said "The reporter was advised of this by several different FAA offices yesterday before he had finished his story . . . . "

A routine inspection of an older DC9 by Republic Airlines turned up the first reported crack in the wing spar cap about two months ago, the FAA said. McDonnell Douglas was notified, and it alerted other DC9 users.

Since then, six more spar caps with cracks have been discovered. The largest of these cracks was 1 1/2-inches long, the FAA said. Some "weeping" of fuel from the wing tanks was associated with the cracks, the FAA said.

"This is typical of what happens in an aging metal structure, and is exactly why we inspect they way we do," said Tony Broderick of the FAA's aviation standards office.

"Even of one of the spar caps was totally cracked through, there would be no immediate danger to flight safety," the FAA statement said.

A spar is one of the six major braces that runs the length of the DC9 wing. The cap is a metal plate at the end of the wing. Repair takes several days, but airlines said they expected no interruption of service.