The Federal Aviation Administration yesterday grounded Puerto Rico International Airlines (Prinair), a major passenger carrier in the Caribbean, for violating safety regulations.

The emergency suspension of Prinair's right to fly came at the same time a National Transportation Safety Board hearing in San Juan is examining a Prinair accident July 24 in which eight people were killed. FAA officials said the grounding order was coincidental.

In that accident, which occurred on takeoff from St. Croix, Virgin Islands, the Prinair plane was discovered to be 1,008 pounds overweight and 11 inches out of balance because of an improperly distributed load. Investigators have turned up two other recent similar incidents involving Prinair flights that did not result in fatalities.

On Sept. 26, Prinair and the FAA signed a special agreement in which Prinair agreed that each flight would be carefully checked for weight and balance.

FAA officials discovered yesterday during a special check, they said, that flights were not being checked. One plane was found seriously out of balance. Such condition is particularly critical on a small aircraft and can make it unflyable. Prinair has a fleet of 26 DeHavilland Herons, four-engine planes whuch seat about 20 people.

In addition to the weight and balance problem, FAA officials said, Prinair was found not be be complying with inspection schedules on propeller bolts and crankshaft counterweight liners.

Furthermore, two instances were discovered yesterday of pilots flying Prinair flights under instrument rules after filing a flight plan for visual flight conditions.

That means, in effect, that the pilots said they were going to do one thing, then did another.

Prinair is the largest commuter airline in the United States carrying almost 700,000 people last year. It serves the Virgin Islands and the Bahamas; among other places. Passengers flying from East Coast cities to St. Thomas are regularly bookd on Prinair flights between San Juan and St. Thomas.

According to FAA spokesman Jack Barker in San Juan, Prinair will remain under suspension until it has developed a method of complying with the airplane inspection schedule, until baggage loaders have been retrained and until Prinair's pilots have been checked out by an FAA inspector.

The Prinair suspension comes at a time of growing concern in the aviation community about the safety record of commuter airlines. Commuters have been adding service to many cities as major trunk carriers have dropped out because of airline deregulation.