Rudder System Recommendation Could Portend More to Come

Air Safety Week, June 7, 2004

A recommendation to modify the Airbus A300-600 rudder travel limiter (RTL) to prevent excessive aerodynamic loads could be the first of more calls to improve the airplane's rudder control system.

The National Transportation Safety Board (NTSB) issued a May 28 recommendation based on its findings that the RTL did not limit rudder movement as designed when an American Airlines [AMR] A300-600 involved in a 1997 stall event experienced rapid airspeed changes that outran the limiter's ability to keep up. As a result, the tailfin was stressed beyond ultimate load, which eventually required replacement of the composite tailfin. Ultimate load is the 50 percent safety factor added to limit load, which represents the greatest force expected to be encountered in service. The incident airplane, operating as Flight 903 at the time, experienced rudder reversals as a consequence of four rudder inputs in opposing directions. Reversals, or rapid rudder deflections from one side to the other without stopping at the neutral position, can induce yawing that places great stress on the tailfin, rapidly building aerodynamic forces the fin was not designed to withstand.

Another American A300-600 operating as Flight 587 experienced a series of rudder reversals in November 2001, leading again to ultimate load and, this time, to fin separation. The airplane crashed into Belle Harbor, N.Y., killing all 260 aboard and five persons on the ground. It was one of the deadliest crashes in North American aviation history. NTSB sources close to the Flight 587 investigation tell ASW that additional rudder design recommendations are likely.

The Flight 587 crash has locked American Airlines and Airbus into a heated dispute. Airbus claims the copilot flying the aircraft mishandled the rudder system by imparting a series of rapid and inappropriate reversals. American counters that the reversals were the inevitable result of an overly sensitive and flawed rudder control system design (see ASW, March 29).

The NTSB recommendation focuses on the Flight 903 event, saying that in cases of rapid airspeed changes the RTL cannot keep up with the rate of change. Therefore, the rudder could travel beyond the 14.5[degrees] deflection limit established for an airspeed of 220 knots (the deflection limit decreases with an increase in airspeed). The Flight 903 stall upset involved an airspeed change from 190 to 220 knots in the span of some three seconds. That is an airspeed change rate of 10 knots per second. In the 165-220 knot speed range, the limiter can "handle" airspeed changes of 2.4 knots per second and restrict rudder movement. However, the 10 knots per second airspeed change in the Flight 903 event was a rate some four times faster than the rudder travel limiter could respond.

Because of this, the NTSB said, "The rudder was allowed to travel in excess of its RTL design limit for approximately 20 seconds."

Therefore, the NTSB wants the Federal Aviation Administration (FAA) to require Airbus to modify the RTL so that it can respond "effectively to rapid airspeed changes."

A footnote in the NTSB's recommendation letter said the safety issues emanating from the Flight 903 event were "not a factor in the Flight 587 accident."

In effect, the NTSB appears to be disassociating Flight 903 and Flight 587 as two distinct and different events. Airbus officials have publicly seconded this point of view, claiming in response to the NTSB letter that the RTL modification has "nothing to do" with the Flight 587 accident.

However, the acceleration parameter is not viewed by all the participants as the dividing line between the Flight 903 upset and the Flight 587 crash. In its statement reacting to the NTSB letter, American Airlines said its rudder training - revised after the Flight 587 crash - "addresses specific characteristics of the rudder travel limiter system ... that this NTSB recommendation involves."

"These limitations, along with others related to the sensitivity of the system, are precisely the concerns American addressed in its final submission and recommendations to the NTSB regarding the Flight 587 accident," the airline's statement said.

The Flight 587 accident is unique in the sense that some American Airlines A300-600 pilots have organized themselves into a group, and they have issued various papers and recommendations during the investigation outlining their point of view (see ASW, June 17, 2002). In response to this latest development, the pilot's group declared, "As for the limiter failure being 'separate' from the AA 587 incident, this attempt to 'parse' this deficiency waters the eyes it is such a transparent obfuscation." The A300-600 pilots group asserted that the investigation has uncovered evidence that the limiter failed twice on Flight 587, "including on the fifth rudder movement."

One of the most contentious issues concerns the series of opposing rudder movements imparted in the space of some seven seconds on the Flight 587 accident aircraft. Airbus has contended that American's upset recovery training predisposed pilots to make excessive use of the rudder.