Officials at Fairfax Hospital say the county's two 18-month-old police helicopters are great ambulances but are too small to transport the most critically ill patients.

So the hospital has offered to solve the problem.

It intends to buy a sophisticated $750,000 helicopter and lease it to county police for $1 a year.

"For taxpayers, this certainly represents quite a benefit," said Fairfax County police chief Col. Carroll D. Buracker.

"I think Fairfax County and Northern Virginia are coming out on the good end," he added.

For the hospital, according to senior vice president Don Harris, it means better service to the critically ill and "additional patients into Fairfax Hospital."

Buracker said he will ask the County Board to permit him to trade in one of the county's two smaller helicopters this spring for a larger chopper, a move he said might cost $150,000.

The new helicopter will essentially be a gift from the Fairfax Hospital Association, the nonprofit group that operates the hospital, a level one trauma center to which the county's helicopters bring critically injured patients.

When the new helicopter is not being used as an air ambulance, the police department will be free to use it for routine police work. The county will pay the copter's $140-an-hour operating cost.

"The current aircraft is ideal for an individual who is quite stable . . . who doesn't have significant medical needs en route," said William J. Cassidy, chairman of the hospital's emergency department. "But it simply does not have the power or the space" for seriously ill patients in need of life support equipment or more than one medical attendant, he said.

The new helicopter, expected to be delivered in late March or early April, will be a Bell Jet Long Ranger capable of carrying a pilot and five passengers or a pilot, two patients and two medical attendants.

Since August 1983, the police have had two Bell Jet Rangers, each of which holds a pilot and three passengers. The flying unit cost $1.5 million to establish and costs slightly more than $500,000 a year to maintain.

Buracker said he will ask the county board to authorize trading in one of the county's smaller helicopters for a larger one early next year.

He said the 13-person helicopter unit, soon to be based at a new $271,000 heliport near Fair Oaks Mall, will not need to be expanded.

In the first 15 months of its operation, police said, the two helicopters flew 179 medical evacuation missions, which took up only about 5 percent of the flight time. The helicopters also are used for rush-hour traffic control.

The agreement being ironed out between the hospital association and county officials is scheduled to be put before the county board for approval Jan. 7.