West German airliners have been given permission to fly special evasion patterns in taking off and landing at U.S. airports as a precaution against terrorist threats to blow up three Lufthansa jets in flight, American officials said yesterday.

At the same time, the U.S Coast Guard confirmed that its vessels were patrolling the waters off the international airports in New York, Boston and Philadelphia, checking out "suspicious-looking craft" that could be carrying terrorists armed with portable Soviet-made SA-7 heat-seeking missiles.

the security precautions, which Lufthansa put into effect in West German last Friday, were instituted after the airline received letters from purported members of the Red Army Faction threatening to blow up three of its jetliners in flight starting today.

The letters, promising to avenge the prison deaths of three terrorists found in their Stuttgart cells Oct. 18, declared:

"For each comrade assassinated, we shall blow up a Lufthansa plane in flight. There will be no way to prevent that. Therefore, everyone should know that if he boards a German plane on Nov. 15 or afterward, death will be on board."

Both the West German and the U.S. governments are taking the threat seriously. Since airport security measures in both countries make it extremely difficult for passengers to smuggle explosives aboard a plane, concern has focused on the possibility that terrorists might try to shoot down an airliner as it was landing or taking off.

The West German Federal Flight Security Institute said yesterday that Lufthansa pilots were using "random approach and departure flight procedures" at West German airports to decrease the threat from missiles.

Spokesman Peter Graf said special air controller teams are directing Lufthansa planes in takeoff and landing patterns whose direction and angle of climb or descent constantly vary in order to confuse any possible attackers.

The Federal Aviation Administration yesterday declined to discuss what type of special flight procedures had been authorized at airports served by Lufthansa in the United States.

"Local arrangements have been made for special measures if requested," declared FAA spokesman Fred Farrar. Sources confirmed, however that special air traffic control procedures had been requested and authorized for at least some of the six U.S. cities - New York, Boston, Philadelphia, Chicago, San Francisco and Los Angeles - that Lufthansa serves.

The sources said these measures include altering normal landing and take-off patterns, and authorizing the Lufthansa jets to ascend and descend more rapidly than normal for regular passenger flights.

In addition to making it more difficult for a terrorist to know the route a Lufthansa jet would use in approaching or taking off, these procedures are designed to minimize the time the large passenger jet fly at a relatively low altitude.

The SA-7, the weapon that most concerns airline security officials, can only reach planes flying under 5,000 feet.

Authorities recalled yesterday that two West Germans reportedly connected with the Red Army Faction were among the five terrorists involved in January 1976 in the only confirmed attempt to shoot down a civilian airliner with SA-7 heat-seeking missiles.

That incident took place at the airport at Nairobi, Kenya An El Al plane, carrying 100 passengers en route from Johannesburgh to Tel Aviv, was about to land when three Arabs armed with Soviet SA-7s were arrested by Kenyan security forces near the airport fence. The two West Germans, accused of being accomplices, were arrested later.

All five suspects in that incident were subsequently spirited off to Israel, where they remain in prison.

The West German newspaper Bild Zeitung reported yesterday that German terrorist have more of these SA-7 missiles.

Defending against the SA-7, moreover, poses a formidable task. Despite its altitude limitation, the SA-7 has a "slant range" of about 6 miles.

"With that kind of a range, you'd have a very difficult time securing an American or European airport." a U.S. Air Force source said. "A terrorist could sit out there a long way off the runways."

Beyond that, the SA-7 can be shoulder fired, and is highly portable. "You can pack them into a stationwagon," many's Frankfurt airport, the planes land and take off right over the Autobahn."

While U.S. officials were taking no chances at American airports served by Lufthansa, most felt that this kind of missile attack was unlikely in the United States.

"We have no indication any such missiles are on the loose in this country now, and no indication that there ever have been any," FAA spokesman Farrar said.

Lufthansa officials, meanwhile, confirmed that passenger cancellations were "higher than normal" because of the terrorist threats. They said, however, there were no immediate plans to cancel any scheduled flights.

The airline - second biggest outside North America - has 92 passenger jets, and serves 111 airports in West Germany and other countries with some 150,000 flights a year.