SAN FRANCISCO (AP) _ Lightning detectors will be used to track ash clouds spewed by Alaskan volcanos, allowing jetliners to bypass the clouds and avoid clogged engines, which nearly caused an airline disaster last year.

''The lightning-detection system will help us warn the airline industry so they can divert airplanes and prevent crashes,'' said Tom Miller, scientist- in-charge of the U.S. Geological Survey's Alaska Volcano Observatory.

Volcanic ash plumes generate lightning bolts. Scientists don't know why, but it means lightning detectors can be used to find and track ash clouds from Redoubt Volcano and others in Alaska.

''That's useful information at a volcano you can't see most of the time because of bad weather and short winter days,'' said Rick Hoblitt, the USGS Cascades Volcano Observatory volcanologist who dreamed up the idea of using the detectors to protect jetliners.

The new $100,000 system of sensors will be installed in February to protect heavy traffic at Anchorage International Airport, Geological Survey researchers said Thursday at the American Geophysical Union's fall meeting.

The detection system consists of direction-finding antennas and radio receivers that sense radio signals from lightning, and a central computer that uses the information to pinpoint the lightning.

The sensors will be installed at Palmer, Homer, Lake Iliamna and Squentna, Alaska, not at the Anchorage airport itself.

Even in good weather, the pilot of a KLM Boeing 747-400 jumbo jet failed to detect an ash cloud from Redoubt Volcano last Dec. 15, a day after the 10,197- foot volcano started erupting for the first time since 1968.

The plane, traveling from Amsterdam to Tokyo via Anchorage, flew into the cloud and all four engines stalled.

For nine terrifying minutes, the plane and its 245 passengers and crew members plunged in a powerless glide from 28,000 feet to 13,000 feet. The engines were restarted just in time to prevent the jet from hitting 11,000- foot-tall mountains in the area.

Redoubt, 110 miles southwest of Anchorage, repeatedly spewed ash up to 40,000 feet since last December. A snow-covered lava dome in its summit crater threatens to trigger steam eruptions and send more ash skyward any time, Miller said.

After the KLM jet's near-crash, a Boeing 727 and two 737s also flew into Redoubt's ash clouds and sustained minor damage last December and February. Three jumbo jets had close calls similar to the KLM incident when they flew into ash clouds from Indonesian volcanos during the 1980s.

Thunderstorms are rare around Anchorage in winter, so there won't be a problem of confusing storm lightning with bolts from the volcanic clouds, Miller said.