The invasion of Grenada last October was not the classic operation the Pentagon has implied but a poorly planned venture that raises "disturbing" questions about U.S. military tactics and performance, a study released yesterday by Rep. James A. Courter (R-N.J.) concludes.

An initial invasion plan developed by the Navy's Atlantic Fleet headquarters was "overruled by the Joint Chiefs of Staff, who demanded that all four services be involved--just as in the Iran rescue mission" in 1980, according to the analysis prepared by William S. Lind, a legislative assistant to Sen. Gary Hart (D-Colo.) and a founder of the newly established Military Reform Institute.

The Army was particularly eager for "a piece of the action" to justify its request to Congress for a third Ranger battalion, Lind said, but the resulting "pie-dividing contest" allowed the relatively small number of Cuban defenders on the island "to form and maintain a fairly effective defense."

A Pentagon spokesman said: "These are very complicated issues and nobody's ready for this and nobody's going to comment on this . . . . The Army has and is still conducting a detailed lessons-learned study."

Lind praised the Rangers' performance in rescuing American medical students, the Marines' ability to "adapt swiftly to circumstances as they changed," and the 82nd Airborne Division's avoidance of civilian casualties. But among his numerous criticisms:

* The elite military units in the invasion, including Navy SEAL commandos and a Delta Force anti-terrorist squad, "failed in much of what they attempted." For example, the SEALs failed to knock Radio Grenada off the air because they "attacked the wrong building" after finding the station compound. Several SEALs drowned because of "poor weather forecasting."

* The 82nd Airborne advanced with such caution across the island, partly because estimates of Cuban strength were inflated, that Gen. John W. Vessey Jr., chairman of the Joint Chiefs, complained, "We have two companies of Marines running all over the island and thousands of Army troops doing nothing. What the hell is going on?"

* Of "approximately 100 U.S. helicopters used on Grenada, nine were destroyed and a number of others were damaged" although the Cubans lacked antiaircraft missiles. The study asks, "What does it suggest our helicopter losses would be, for example, in a war in Europe?"

Shortly after the invasion, Maj. Gen. H. Norman Schwarzkopf, deputy commander of the U.S. task force, acknowledged that "the helicopter is a very vulnerable object when it goes against a well-organized air defense."

Army Chief of Staff John A. Wickham Jr. has enthusiastically praised the performance of the Army's Blackhawk helicopters in Grenada for what he said was an ability to stay airborne despite being repeatedly raked with gunfire.

Lind said the source for most of his findings was "bar talk" from military officials involved in the planning and execution of the invasion. He is generally seen as one of a group of military "reformers" who emphasize innovative military tactics over high technology.

The study, Lind said, "is not an attempt to say we did poorly. The troops fought magnificently, the courage was excellent." Rather, the issue is whether the Pentagon will profit from the Grenada experience because "lessons learned in combat are worth more than their weight in gold because they're paid for not with gold but with blood," he added.

Courter, a member of the House Armed Services Committee and chairman of the Military Reform Caucus, called for a House investigation because "the Grenada operation was not the complete success" Pentagon officials have suggested.

"The deficiencies in this case resulted in loss of life," he added.

Eighteen U.S. servicemen died in the invasion.