Some U.S. airlines have begun to voluntarily change the way they fly Boeing 737s during landing approaches to give pilots a better chance of countering potential uncontrolled movements of the aircraft's rudder.
Airlines and safety experts are also considering special training for commercial pilots on how to right a 737 suddenly flipped upside down by a sudden, uncontrolled rudder movement.
Such a rudder movement is widely suspected by pilots and safety experts of causing the Sept. 8, 1994 crash of USAir Flight 427 near Pittsburgh. The Boeing 737-300 inexplicably flipped out of clear, calm skies on approach to landing and dove into a wooded ravine, killing all 132 on board.
A United 737-200 crashed in a similar manner on approach to the Colorado Springs airport on a windy morning, March 3, 1991, killing all 25 on board.
Despite exhaustive detective work by Boeing and investigators from the National Transportation Safety Board, the cause of both crashes officially remain unsolved.
Neither the 737 flight change nor the special training is being required by the Federal Aviation Administration, despite concerns raised at a meeting held by the FAA last week. More than 50 airline representatives and officials from Boeing, the safety board, the FAA and the Air Line Pilots Association gathered for an update of the crash investigations and to discuss continuing reports of uncontrolled rudder movements on 737 flights.
The flight change involves pilots flying 737s slightly faster as the aircraft descends toward landing. That would better enable wing panels, called ailerons, to offset any severe, inadvertent rudder swings.
USAir made the change in December and Alaska Airlines is in the process of doing so. Several other airlines that operate large fleets of 737s, including United, Southwest, America West, Continental, Delta, Midway Express, SAS and Aloha, are considering whether to follow suit.
During last week's meeting, the pilots' group called for crews to be formally advised of the possibility that a 737's rudder could swing "hard over" in flight, to the extreme left or right, without being commanded to do so.
The rudder is the large, hinged panel on the vertical part of the tail that controls the aircraft's right-to-left direction of travel. A rudder hard-over would cause one wing to sweep ahead of the other and roll the aircraft into an inverted dive.
The 737 has fewer safeguards against inadvertent rudder hard-overs than do other models. The 737 has one large rudder controlled by a single power control unit, or PCU.
The Boeing 727 and 747 models use a split rudder in which two rudder sections are controlled separately by two PCUs. Thus, a rogue command issued by one unit can be offset by the proper operation of the other unit.
Boeing 757s, 767s and 777s use a single rudder controlled by multiple PCUs, another way to minimize danger caused by a rogue signal issued by one of the control units.
McDonnell Douglas jetliners use a device called a "limiter" that physically prevents the rudder from extreme deflections in flight.
Concern about the potential for a rudder swing at low altitudes, when there may be only a few seconds to make the proper recovery maneuvers, led the pilots' group to call for special aerobatic training for 737 pilots. Such training would complement the approach speed change some airlines are now embracing, the group contends.
Boeing officials declined interview requests for this story. But sources who attended the safety summit said Boeing, in a two-hour presentation, defended the 737 rudder as sound and described how pilot error or unusual turbulence is more likely to put a 737 into an inverted dive.
Even so, Boeing officials agreed special aerobatic training for 737 pilots was a good idea, and did not object to airlines changing the approach speed to give pilots more leeway to recover from a sudden dive, sources said.
The pilots' group had been pushing for such safeguards for several months before the summit.
"The most important thing is giving pilots some information about what to do if they encounter an unusual situation," said John O'Brien, director of engineering and air safety for the pilots' group. "The added air speed gives you more time and capability to respond properly. But unless you make your pilots aware of what can happen - and train them what to do - you're not going to get the proper response."
Developments at the FAA's safety summit underscore growing concern about rogue rudder movements on 737 flights, and highlight mounting awkwardness in the aviation community about how to deal with an emerging safety concern that affects more than 2,600 airplanes worldwide. Fixing such a problem could cost Boeing and its airline customers hundreds of millions of dollars.
The FAA concluded the meeting by indicating it will not mandate any safety improvements now.
Dave Harrington, the FAA official, who directed the meeting, was ill this week and unavailable for comment. FAA spokesman Bob Hawk said no one else from the agency would answer questions about the summit.
Instead, the agency issued a news release yesterday describing the meeting as "an opportunity . . . to exchange information in general about 737 aircraft."
Sources said Harrington did not see any specific benefits in ordering airlines to make the approach speed change or to develop special pilot training programs.
Suggested safety measures for the moment remain voluntary and focus on what pilots can do to recover from a rudder hard-over. Little discussion has taken place about what can be done to prevent rudder swings in the first place.
"We've taken care of the front end of the airplane, but nobody is doing much about what's going on in the back end," said a summit participant. "The FAA isn't going to order any change to that aircraft until the NTSB makes some specific recommendations, and then they'll respond to the recommendations."
Yet 17 months after the crash in Pittsburgh, the safety board continues to say little publicly except that it has no evidence the rudder malfunctioned on the ill-fated flight.
Privately, however, two safety-board investigators, Tom Haueter and Greg Phillips, several months ago drafted recommendations calling for costly design changes to the 737's flight-control systems, according to sources close to the safety board.
Haueter and Phillips reportedly want the FAA to order Boeing to install a limiter on the 737 rudder, to prevent it from fully deflecting in flight.
Their draft recommendations also call on the FAA to order Boeing to change the 737's wing-panel controls to reduce the rudder's dominant influence on the aircraft's overall position in the air at lower speeds, sources said. But such changes would entail heavy expenses in retrofitting costs and lost airline revenue.
Boeing was asked at the summit whether it is in the process of upgrading the rudder system of the latest versions of the 737, the -600, -700 and -800s series now under development. It indicated some rudder components may be changed but declined to be specific.
Southwest Airlines is scheduled to take delivery of the first of the new-series 737s next year, but Boeing still has not advised the Dallas-based carrier if the rudder controls will be modified, said airline spokeswoman Beth Harbin.
In the past, the safety board has not hesitated issuing recommendations for safety improvements during the course of a crash probe - and putting the onus on the FAA to enact the safety measures.
Safety board spokeswoman Julie Beal acknowledged that "there are some recommendations staff is evaluating and working on" but said the suggestions did not involve pressing safety concerns.
"If we had recommendations of an urgent nature, we would issue those, and not wait," Beal said.
Aviation records obtained by the Seattle Times show pilots have reported hundreds of cases of rogue rudder incidents on 737 flights over the past three decades. Since the Pittsburgh crash, airlines have reported more than 45 such incidents, including more than a dozen cases on USAir 737s alone, records show.
The USAir jet that crashed in Pittsburgh got into trouble as it was flying at 190 knots, with wing panels, called flaps, extended slightly to help keep the plane aloft at a relatively low air speed.
Flight tests conducted last fall as part of the NTSB's crash probe revealed that, flying at 190 knots with flaps slightly extended, a 737's ailerons have negligible ability to counter a rudder hard-over.
Ailerons are the panels on the outer, rear sections of the wing that help to steer the plane. They move in opposing directions when the pilot turns the cockpit wheel.
USAir subsequently ordered its pilots to make comparable landing approaches at 200 knots, a speed at which the ailerons are thought to regain ability to offset a rudder hard-over. Following last week's summit, other airlines are considering whether to adopt the same policy.
"It's a cushion of 10 knots in case you did have a full rudder hard-over and had a jet upset," said Capt. Milt Painter, director of flight standards and training at Southwest Airlines. "The added air speed could help, it might not, who knows? By having it 10 knots higher, it might give you authority to recover. . . . That's what you have to look at."
Painter said Southwest, which operates an all-737 fleet of 226 jets, will decide in the next few weeks whether to make the approach-speed change.