INCIRLIK AIR BASE, Turkey -- Every other day, U.S. warplanes loaded with bombs and missiles cruise the skies over northern Iraq, routine patrols that are giving pilots experience that could prove invaluable if President Bush decides to go to war against Saddam Hussein.

Pilots say that after 11 years of patrols, almost every U.S. pilot has flown over Iraq, learning the terrain and carefully monitored the skills of Saddam's gunners.

"Knowing the terrain and knowing your enemy is definitely an advantage," Col. John Burgess, head of air operations over the northern no-fly zone, said Thursday. "I think it would be hard to find a U.S. tactical pilot who has not rotated through Northern Watch or Southern Watch."

The northern zone was set up in 1991 to protect rebellious Kurds from the Iraqi army. A year later a southern zone was established. Both are patrolled by U.S. and British aircraft.

$1 billion a year

Just last year, top U.S. Air Force officials were questioning the usefulness of the operation, which costs about $1 billion a year and has strained Air Force resources at a time of budget cutbacks.

But with talk of a possible war against Iraq, officials are taking another look at the operation.

"A lot of these pilots are the pilots who may be attacking Iraqi sites if there is a war," said Charles Heyman, editor of Jane's World Armies. "They may have been over Iraq dozens of times. Every mission is money in the bank as far as pilot skills go."

The flights also give the Air Force a chance to monitor Iraqi defenses on a regular basis. The flights are conducted about 18 times a month, but the schedules are constantly changed to confuse the Iraqis.

Iraqi gunners opened fire 60 times on U.S. and British warplanes patrolling the north this year. Warplanes struck back 10 times, bombing Iraqi anti-aircraft batteries or radar stations. The attacks have been more intense in the larger southern zone, with almost twice as many strikes.

The last time the Iraqis opened fire in the north was Wednesday. A mission was canceled Thursday, apparently due to stormy weather.

The shooting varies in intensity. Burgess said the Iraqis open up -- usually with anti-aircraft artillery or machine guns -- about once an hour during the flights, but sometimes shoot as much as three times an hour.

The shooting, he said, has been fairly constant during his year commanding air operations.

Although no pilots have been shot down during the history of the flights, there is always a risk.

"They shoot at us quite a bit," said "Bartdude," an F-16 pilot with the Ohio National Guard who has flown over the no-fly zone for the past month. For security reasons, pilots of "Bartdude's" 112th Fighter Squadron have been ordered to speak only on condition that they are identified by their Air Force call signs and not their real names. Pilots are rotated into the region on 90-day assignments.

"It gets close enough that we have to move the airplane around," said "Bartdude," who has also flown patrols in the south. His unit is based in Toledo, Ohio.

The pilot spoke in a concrete aircraft shelter at Incirlik air base in southern Turkey. The base is about an hour's flight from northern Iraq and hosted some 100 U.S. fighters during the 1991 Gulf War.

Behind the pilot was an F-16, which carried four 500-pound bombs. Pilots of the 112th "Stinger" squadron fly with F-15 fighters on patrols of the no-fly zone and bomb anti-aircraft batteries that threaten the fighters.

It is a cat-and-mouse game between Iraq gunners and U.S. planes.

Experts say Iraqi gunners have been hiding their weapons in fruit orchards, behind hospitals and near mosques to make it difficult for U.S. pilots to strike back.

That is especially the case in the northern zone, which is more crowded than the southern area.

"There have been times when there are targets that are found and there would be collateral damage" if they were bombed, said Brig. Gen. Robin Scott, U.S. commander of the northern zone. "We would back off."

"We will avoid certain areas at certain times to ensure the safety of our crews," he said.