An Air Force plane crashed early yesterday in the Sequoia National Forest near Bakersfield, Calif., killing the pilot and prompting the Air Force to clamp a lid of secrecy over the episode.

The Air Force, according to government officials, told the nation's air traffic control system before the crash that the plane was an A7 attack plane.

But after the crash, at 2 a.m. PDT, sources said, the Air Force asked the Federal Aviation Administration to ban flights over the crash site, prompting speculation that the plane was some kind of Stealth fighter.

However, the plane was not the new Northrop Flying Wing Stealth bomber, Air Force officials said, pointing out that the plane that crashed had only a one-man crew. The Flying Wing is thought to have more than one.

Edwards Air Force Base, where experimental aircraft are tested, is 80 miles from the crash site. The Air Force has been building and flight-testing experimental Stealth fighters in secret, and some have crashed, sources said.

Government officials said a commercial airline pilot reported seeing a midair explosion near the crash site, raising the possibility that the unidentified plane had blown up.

U.S. military pilots fly Soviet MiG fighters and other captured Soviet aircraft out of Tonopah, Nev., to determine their capabilities and vulnerabilities. The crash of such a plane would trigger the kind of secrecy the Air Force imposed yesterday.

Air Force officials would not identify what kind of plane crashed or say whether its pilot was civilian or military. The crash touched off a 150-acre brush fire brought under control six hours later, according to reports from the scene.

Meanwhile, wire services reported, an Army UH1H helicopter crashed in California's San Bernardino Mountains, killing three soldiers and injuring two, officials said.

The helicopter, which apparently crashed Thursday night, was discovered yesterday morning southeast of Big Bear Lake, said Capt. Margaret Sutter of George Air Force Base near Victorville. Rescue efforts were under way, she said.

An Army statement said the helicopter, from Fort Sill, Okla., was on a mission in advance of the Gallant Eagle military exercise to be conducted in southern California later this month. The crash occurred on the east side of San Gorgonio Mountain between the 6,000-foot and 7,000-foot level.