Upgraded 737 Parts Questioned -- Two Incidents: On Ground At Sea- Tac, In Air
Seattle Times Staff Reporter
Two incidents in the past week - one on the ground at Seattle-Tacoma International Airport, the other in the air over the Atlantic seaboard - have raised fresh questions about replacement rudder-control parts ordered by the FAA for Boeing 737s.
Last Friday, pilots doing a preflight check of a United Airlines 737 at Sea-Tac reported the rudder pedals felt sluggish. Mechanics confirmed that the pedals could not be depressed smoothly. They removed a mechanism called the rudder power-control unit, or PCU.
On Tuesday, a 737 flown by USAirways' discount carrier, Metrojet, started an uncommanded roll while cruising at 33,000 feet over Maryland.
The pilots took a number of steps but did not regain control until they shut off all hydraulic power to the PCU. The plane, carrying 117 people, made a safe emergency landing in Baltimore.
Industry and safety officials are paying close attention because each jet recently had been equipped with an upgraded rudder servo valve intended to prevent rudder malfunctions on 737s.
More than 900 planes have been upgraded. These are the first two with reported rudder problems.
Investigators from the National Transportation Safety Board traveled to the Parker Hannifin plant in Irvine, Calif., yesterday to examine and test the Metrojet's rudder PCU. Parker Hannifin designed and manufactures all 737 rudder PCUs. After that, the investigators will test the United jet's PCU.
"It's obviously of considerable interest to us," said Boeing spokesman Russ Young. "We want to understand what's going on. And if we need to take any action, we will."
Why servo valve is a concern
NTSB officials declined to comment on the incidents or tests, beyond describing the Metrojet incident and noting that investigators would be testing the suspect PCUs.
The safety board has given Boeing a copy of the Metrojet flight data-recorder readouts. Boeing will analyze the data to see if the plane's position as it moved through the air, as captured by the recorder, matches the pilots' account of what happened.
The Seattle and Baltimore incidents follow the recent discovery that the upgraded servo valves - among FAA-mandated improvements Boeing is making to the rudder-control systems of more than 2,700 737s - are prone to cracking.
There is no evidence as yet of cracking in the past week's incidents; in fact, there is nothing so far to indicate the replacement parts figured in the incidents at all.
But jamming of the servo valve became a concern after the 1991 crash of a United 737 in Colorado Springs and the similar sudden nose dive of a USAirways 737 in Pittsburgh in 1994.
The servo valve directs pressurized hydraulic fluid to move the rudder, the hinged tail section that moves a plane left or right during flight.
The NTSB has scheduled a meeting next month to rule on the probable cause of the Pittsburgh crash and, possibly, the Colorado Springs crash.
A complex part
The 737 is the only large jetliner with a single rudder PCU. Other models have a split rudder or multiple PCUs.
In the eight years since the Colorado Springs crash, investigators have learned that the servo valve at the heart of the PCU can jam and inadvertently deflect the rudder to acute angles, quickly twisting the aircraft into a steep dive.
The servo valve is a complex part that can jam in a number of ways with varying results. In late 1996, Boeing discovered in lab tests that a certain kind of jam could cause the rudder to reverse, or move in a direction opposite that called for by the pilot.
As a result of that finding, Boeing designed the upgraded servo and agreed to install it on all older 737 models - the 737-100s, -200s, -300s, -400s and -500s - by August.
How pilots overcame deflection
According to the NTSB's account of the Metrojet incident, the 737 had left Orlando on Tuesday bound for Hartford, Conn. It was flying on autopilot, a routine procedure when cruising.
Around 11:15 a.m., the crew noticed the control wheel twist left as the plane began to bank to the right.
At the same moment, the right rudder pedal depressed without either pilot touching it.
Metrojet is a subsidiary of USAirways, which has intensively trained its pilots how to handle rudder problems. The crew deactivated the autopilot and tried to center the rudder pedals. But the pedals did not respond.
Next, they disengaged the yaw damper, a device that automatically makes small rudder adjustments in flight. Boeing is taking steps to upgrade the yaw damper on all 737s because the device can sometimes call for rudder deflections when none are needed. Airline procedures call for switching off the yaw damper if the plane banks.
But the bank persisted.
Next, using a technique recommended by Boeing in early 1997, the pilots shut down the hydraulic system supplying pressurized fluid to the rudder PCU. The plane straightened, with no further problems.
The Sea-Tac incident
The United incident at Sea-Tac was reminiscent of an incident in Chicago in 1992 during which United Capt. Mack Moore declined to fly a 737 whose rudder pedals were difficult to depress. At the time, the NTSB was trying to solve the Colorado Springs crash.
Subsequent tests of what has become known as the Mack Moore PCU led investigators to their discovery that the servo valve could inadvertently reverse the rudder.
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