ABOARD THE BONHOMME RICHARD - Here are a few of the concerns weighing upon mechanics who service the Harrier jets flying bombing and surveillance missions high above Iraq:

Engines can't fail. Radar must work. Fuel tanks can't come loose. Planes can't get blown into the sea when landing vertically onto the deck. Ejection seats must eject, but not accidentally.

Plastic canopies must give way if pilots eject and come crashing through. Parachutes must open, and fliers can't fall into the jet stream.

Although it is the pilots whose lives depend on sound aircraft, it is the mechanics who toil from 12 to 14 hours a day to ensure their safety. None of this amphibious ship's 24 Harrier jets has experienced severe problems in the 3-week-old war, and every pilot has come back safely, but mechanics will surely receive much of the blame if anything goes wrong. Yet they are a serene group, seeing poetry in what they do.

"We are the first and last eyes on the plane when it leaves and comes back," said Marine Sgt. Ricardo Alanis, 24, an engine and fuel-tank mechanic from Lynwood, Calif.

"I know my job is important," said Marine Sgt. Chris Levinski, 21, a seat mechanic from Emmett, Idaho. "We're the last thing between a pilot living and dying."

Maintainers

In Navy lingo, mechanics are maintainers. They fix - and maintain - everything from worn engine bearings, which can send a plane nose-diving into the sea, to titanium heat shields, which protect the planes from burning from jet engine exhaust.

They work in the dimly lighted cavern - the hangar - below the flight deck where aircraft are lowered on an open-air elevator. On one end, mechanics can be seen crawling into the scoop of a Harrier's turbine engine to see whether the fan blades are pitted, cracked or bent. Elsewhere, they are removing a plane's wings, the only way to gain access to an engine in need of removal.

All around, maintainers are crouching under fuel tanks, squeezing into cockpits and sprawling over fuselages. There is no auto shop where mechanics are as specialized as these and few where the fix-it folks regard their charges in more nearly human terms.

A 'kick in the ass'

Levinski takes care of seats, nothing else. He makes sure that the ejection system will be armed when the pilot depresses a steel lever upon entering the cockpit. He sees to it that a pilot in trouble can easily pull the metal ring between his legs that triggers ejection. Twenty-eight explosive charges beneath the seat will fail to blast the pilot out of the cockpit if they have exhausted their shelf life.

Even if all of this comes off right, he knows that ejection will be gruesome unless the rubberized cord that holds the acrylic canopy together like lead seams in a stained glass window breaks up the instant the seat comes crashing through with the force of 13 Gs.

"It's like a huge kick in the ass," said Levinski, who hasn't experienced it but understands the physics well enough to know that the pilot should fly about 150 feet before going into a free-fall that is broken by his parachute.

With each ejection, a pilot suffers a spinal compression that wins him a trip to a chiropractor. Anyone unlucky enough to eject three times is scrubbed from future missions for his safety. But ejection is one of the jet's principal safety features, so Levinsky tests each part on a different cycle set by a maintenance schedule.

The arming lever must yield to 15 to 20 pounds of pressure. The parachute must survive an air test, and its cords cannot be frayed. Even if everything passes muster, each part has a replacement schedule. Rockets under the seats get changed every eight years, the canopy cord every five.

"If there is anything I'd worry about it is his canopy not breaking," said Levinsky, who insists he hasn't yet had reason to lose sleep.

For the power plant mechanics, any damaged engine part could mean catastrophe. In their daily inspections of planes, these mechanics crawl inside the turbine scoop to inspect the blades for problems. Flaws merit a temporary grounding of the Harrier, so the mechanics can pull out the engine and send the turbine to a metal shop for retooling.

Periodically, the mechanics chain a Harrier to the flight deck and rev its engines at full blast so they can test the engine's vibration. Any unusual vibration detected by an electronic sensing device could mean a worn bearing or engine mount, either one of which could cause the Harrier to seize in midflight and nose-dive to earth.