Boeing Considers Safety Improvement For 737s
Boeing 737 operators may be asked to install rudder movement limiters as a safety improvement, a company spokeswoman said Sunday.
Susan Bradley of Boeing said the possibility of a retrofit for the 2,800 twinjets in use worldwide had been under study for months.
The purpose would be additional protection against unintended rudder movement that could cause a plane to go out of control and crash.
Bradley explained that the aerodynamic design is intended to restrict the movement of Boeing 737 rudders according to airspeed. The proposal would add a mechanical or hydraulic device for further protection.
Newer models of the 737 that have yet to begin rolling off the assembly line in Renton include a hydraulic limiter because greater engine thrust and rudder size required the change, Bradley said.
In considering a retrofit, company engineers have yet to decide whether to go with a mechanical limiter, another hydraulic device or drop the idea altogether, nor is there any timetable, she said.
“It’s still just a concept,” she said.
Consideration of a limiter gained new impetus as a result of National Transportation Safety Board recommendations that were issued Oct. 16 as part of its continuing investigation into two unsolved crashes.
“That was probably the first step toward any required action” to develop and install limiters, Bradley said.
As of now, however, Boeing officials know of no reason for such a move, she said.
The 737 rudder mechanism has been a prime focus of investigation in the death of 25 people in a United Airlines 737 crash in Colorado Springs, Colo., on March 3, 1991, and 132 people in a USAir 737 crash near Pittsburgh on Sept. 8, 1994.
The device under consideration would be a step beyond the inspections - and, if necessary, replacements - of the 737 rudder power control unit ordered Friday by the Federal Aviation Administration.
That order, following closely along the lines of a service bulletin or recommendation issued by Boeing, gave airlines 10 days in which to test the secondary slide or valve in the unit. The slide is a backup device for use in case the primary slide became jammed.
Tests conducted by Boeing indicated that if both slides became jammed, stomping the rudder pedal in the cockpit could cause fluid to enter the wrong chamber and move the rudder “hard over” - to the extreme position - in the opposite direction of what the pilot intended, the FAA said.
A mechanical or hydraulic limiter would prevent a rudder hard-over even in the event of a jam.
“We’ve never, ever had a report that the secondary slide has jammed in more than 69 million flights,” Bradley said.