Cal Fire aerial bombers played a critical role Sunday in slowing down a fire that was burning in steep, rugged terrain east of Ukiah.

As of Sunday night, the fire was only 15 percent contained and had consumed 4,100 acres in an area bordered by Scotts Valley Road, Highway 20 and the north Cow Mountain Recreation Area.

Three hundred homes and 40 other buildings were threatened.

"It is like in a bowl, and it went up the hill and nothing could stop that, but we stopped it at the top of the ridge," said Jerome Laval, a Cal Fire pilot who on Sunday was flying aerial bombers for a third day against the fire.

The air attack involved seven air tankers and five helicopters that were dropping a reddish-colored retardant in an effort to keep the fire from spreading.

Thirty-eight miles away in Santa Rosa, at the Cal Fire Air Attack Base, a well-choreographed routine kept the tankers in the air and fighting.

At intervals of 15 to 30 minutes throughout the day, four Grumman S2 tankers took off and landed at the Charles M. Schulz-Sonoma County Airport. The tankers last saw duty as carrier-based submarine hunters in the 1960s.

The planes taxied to the Cal Fire facility and within a few minutes took on 1,200 gallons of retardant, underwent cursory safety inspections and were back in the air.

The retardant is made of salt, water, a thickening agent, red coloring and phosphates, creating a fertilizer-like mixture that releases water vapor when it hits flames.

It is held in 20,000-gallon tanks and pumped at a rate of 500 gallons per minute into the aircraft as it sits idling on the taxiway.

A former British Airways jetliner converted to firefighting by the U.S. Forestry also was serviced at Santa Rosa, then was diverted to a fire in Nevada and two more Grumman S2s were called up.

"Aircraft are critical to wildland firefighting. They are an integral part of it,"Cal Fire Battalion Chief Chris Jurasek said.

"Most people don't see this, but without these folks loading the planes and the dispatch, no one else can work," he added.

The base was especially busy Sunday, adding to what has already been a busy season with a large number of wildland fires.

"It has been like this since July," Jurasek said. "Every couple of weeks there has been an extended fire. Since 2008, this has been the busiest."

Fire officials warned residents along Scotts Valley Road in Lake County on Sunday morning to prepare to leave the area if wind conditions worsened. The road was open only to residents, as the fire was burning just about one mile from the community, according to Cal Fire.

Both the Scotts Creek and Mill Creek entrances to the recreation area were closed. The Mendo Rock Road access to North Cow Mountain Road was closed at Mill Creek, according to the Bureau of Land Management.

A red flag warning was issued Sunday, and firefighters were holding the fire at bay south of Highway 20, west of Scotts Valley Road and east of Cow Mountain Road.

Cal Fire officials said firefighters were strengthening their lines around the fire, but gusty winds and low humidity during the day threatened to feed the flames.

"The fuels out there are really dry. Between that and winds and steep terrain, it all plays in together," Cal Fire spokesman Kevin Colburn said.

Additional manpower was ordered to the area, swelling to 1,340 the number of firefighters battling the blaze that broke out at 1:23 p.m. Friday.

Also involved were 131 fire engines, 39 fire crews, 26 dozers and 20 water tenders.

You can reach Staff Writer Bob Norberg at 521-5206 or bob.norberg@ pressdemocrat.com.