A U.S. Air Force F15 "eagle" jet fighter that crashed in northern West Germany Tuesday was the forth of these front-line warplanes to have crashed here since April.

Although widely billed as the best new fighter-plane in the world, the first 76-plane tactical wing of the twin-engine jets to be deployed to Europe seems to be losing aircraft at an alarming rate.

Some West German reports claim that the U.S. F15 force here is being decimated at a faster than West German pilots suffered when they first put the U.S. built F104 "Starfighter" into service with the West German Air Force some 17 years ago.

The West Germans eventually bought some 900 starfighters, but over the years about 180 of them crashed for reasons that are still debated, killing about 80 pilots.

An Air Force spokesman, Maj. Fred Morgan at Ramstein AFB, said in a telephone interview that no official Air Force statement on the cause of the latest crash would be made until a board of investigation had completed its work. He said investigators were still looking into the other three crashes - two into the North Sea in April and June and one elsewhere in West Germaby in July - but that details on what caused these crashes would not normally be made public anyway.

Maj. Morgan said, however that each of the four accidents appears to have happened for different reasons and there is no trend which indicates there is anything grossly wrong with the aircraft." He said the planes here, which are all based at Bitburg AFB in the hills of western West Germany, were not grounded or under any flight restrictions. One pilot has been killed in the four crashes.

Speculation in both the U.S. and West German pres, however, centers on the plane's two fan-jet engines. The development of these highly sophisticated engines and the methods under which they were tested for compliance to specifications were a matter of considerable controversy in the mid-1970s.

If the planes are suffering from engine problems, the situation becomes doubly serious for the pilots and the Pentagon because aside from the 727 twin-jet F15s slated to be eventually produced, the new F16 single-engine jet, of which more than 1,000 will be build, uses the same engine.

The $15 million F1ks here in West Germany, which arrived in April 1977, are supposed to be the vanguard of the American air defence of Western Europe. They are eventually to work in conjunction with and protect the multi-billion dollar airborne warning and control planes designed to signal the first sign of enemy air attack.

The F15s at Bitburg are supposed to be the supposed to be the first off the runway to challenge invading planes from the east. At the moment, however, U.S. Air Force plans call for stationing only about 100 F15s in Europe.

In Washington, a U.S. Air Force spokesman said the F15's accident rate is the lowest of any Air Force jet fighter. According to an Air Force study of crashes, during each of three fighter aircraft's first 100,000 flying hours, there were six F15 crashes, 46 crashes of F104 Startfighters and 24 F4 Phantom crashes.

Of the current total of eight crashes of F15s, the Air Force spokesman said, four were in-air or landing accidents, three were caused by mechanical failures and the cause of the eighth is still being invistigated.)$