Bell Helicopter International, which literally keeps the Iranian army flying has begun evacuating even its "essential" employes from Iran and soon will be reduced to a token operation here.

As a result, much of the armed forces fleet of nearly 1,000 combat helicopters will eventually be all but grounded for a lack of maintenance and pilot training, according to U.S. sources.

In time, military sources said, the effectiveness of Iran's airborne infantry is certain to suffer as increased numbers of aircraft sit in hangers awaiting maintenance.

"It will be a cumulative problem. Their capabiilites are bound to suffer," said a U.S. source close to the Iranian military.

Bell, which in October had 14,000 employes in Iran, had handled nearly all maintenance and pilot training for the Iranian armed services. Similarly, but on a smaller scale, Grumman Aerospace Corp. has serviced the F14 fighters it sold to Iran.

Contracts with both firms are to be renegotiated following the Iranian government's decision to slash defense deals with the United States by up to $10 billion and drastically reduce the number of defense contractor personnel in Iran. Bell, with about 2,000 employes left here, plans to leave a force of only about 500 technicians and pilot trainers, at Iran's request.

Helicopters require even more maintenance manpower than the most sophisticated jet fighters. Newer jet aircraft are component assembled, and when a particular system malfunctions a technician can simply remove the defective component and replace it. Helicopter maintenance, however, requires long hours of manual labor, industry officials said.

Meanwhile, the exodus of foreigners from Iran continues with the United States leading the pace at an estimated 400 evacuees daily. The U.S. Air Force's military airlift command has been flying out expatriates in four C141 transport planes.

U.S. officials said about 7,000 Americans remain in Iran, most of them defense contract personnel, out of about 45,000 here before Iran's political turmoil began slightly more than a year ago.

Sporadic operations at Tehran's Mehrabad International Airport have slowed the evacuations. International airlines have canceled some flights because of inadequate air traffic controls.

For six weeks, air traffic controllers have been on strike in support of the Islamic-inspired revolution, while demanding that all U.S. airlines be banned from the airport.

As a result, Mehrabad Airport's ground guidance equipment has shut down and Iranian Air Force technicians manning the control tower simply give clearance to land visually. Witnesses have described several near misses during landings in cloudy weather and commerical pilots have described landing at Tehran as "terrifying."