Nine rules proposed by the Federal Aviation Administration to make the flight controls of Boeing 737 jets safer do not include a key change sought by some pilots and federal safety-board officials.

That change would involve restricting the amount the rudder could move once the plane was airborne. It would eliminate the possibility of a sudden, acute rudder swing believed to have caused a United Airlines jet to twist into a fatal nosedive in Colorado Springs on March 3, 1991.

The similar crash of a USAir 737 descending routinely into Pittsburgh on Sept. 8, 1994, also involved a sudden, acute rudder deflection, investigators have said.

The rudder is the large, hinged panel on the 737's vertical tail, which controls the aircraft's left-to-right direction of travel.

Concerns about the rudders were raised in both crashes, although a final cause was never pinpointed.

The two crashes killed 157 people.

Last fall, pilots' representatives began discussing with National Transportation Safety Board investigators what it would take to require Boeing to restrict the rudder's range of motion, which would involve installing a mechanism called a limiter on the rudder's main power-control unit.

Then last January, NTSB investigator Greg Phillips drafted a lengthy proposal calling for a wide range of 737 safety measures, including installation of a rudder limiter on all 737s, said sources close to the Pittsburgh investigation.

Normally the safety board recommends measures to the FAA, which then drafts and enforces specific regulations. But the safety board has declined to issue Phillips' proposal as a formal safety recommendation.

Boeing opposes limiter

Boeing opposes installing a limiter on the 737's rudder, said Jean McGrew, Boeing's chief project engineer for the 737.

Even so, industry sources say FAA officials are now discussing the possibility of requiring Boeing to include a rudder limiter on the new series of 737-600s, 737-700s and 737-800s Boeing will begin delivering next year.

"We have studied that, along with other things as we go through the accident investigation. At this point, we do not see a need for that," McGrew said, noting that Boeing has been unable to find evidence that a rudder malfunction caused the Pittsburgh crash.

The FAA yesterday began calling for repetitive inspection and eventual upgrading of the 737's standby rudder power-control unit, long known to be prone to malfunctions. The agency proposed mandatory inspections for the standby PCU once before but withdrew the directive in 1994 after airlines protested.

The FAA also proposed ongoing inspection and eventual upgrading of a device called the yaw damper, which makes hundreds of small rudder adjustments during flight - and sometimes commands slight rudder deflections when none are needed.

The standby PCU and the yaw-damper directives are intended to fix malfunctions that can cause a 737 to swerve briefly off course, an occurrence pilots are trained to safely deal with, safety officials said.

A half-dozen of the FAA's directives involve making mandatory relatively minor systems changes Boeing has been recommending for years - upgrades which airlines voluntarily carry out.

Tom McSweeny, FAA director of aircraft-certification service, characterized the rule changes as "improvements to a system that we believe is a very good system. Many are fairly insignificant changes, but ones we think are prudent having looked at the airplane."

Boeing spokesman Russ Young said the company viewed the FAA's proposed directives as "product improvements." Boeing often challenges FAA directives, but in this case will make no objections during a 60-day comment period.

Some of the new requirements affect just 737-100s and 737-200s, while others cover newer 737-300s, 737-400s and 737-500s as well. There are 1,037 U.S.-registered 737s, which represents about a third of the worldwide fleet of 2,830 737s. Foreign regulators are expected to adopt the FAA directives.

$10,000-$5 million price tag

Estimated costs range from $10,000 to $5 million to accomplish the directives on the U.S. fleet, with deadlines ranging from 90 days to 18 months. USAir spokesman Paul Turk, said the airline, which operates 203 737s, "would comply fully and promptly" with any directives adapted by the FAA. Airlines typically request to extend or remove FAA-imposed deadlines made in connection with such directives.

The FAA's action is the latest government response to the crashes in Pittsburgh and Colorado Springs.

After the Colorado Springs crash, NTSB investigators gave serious consideration to a Boeing theory that a freak gust of wind caused the United 737-200 to flip nearly upside down and straight down into a city park from 1,000 feet.

As part of the Pittsburgh crash probe, investigators conducted a special $1 million fight test to explore a Boeing theory that wingtip turbulence from a jet flying 4.2 miles ahead of the USAir jet caused the 8-year-old 737-300 to flip over and careen 6,000 feet into a wooded ravine.

Since the Pittsburgh accident, pilots have reported more than 50 incidents of 737s swerving briefly off course because of rudder malfunctions, including a series of incidents last May and June on an Eastwind Airline 737 that led to a 22-day grounding of the jet.