A310 Loses Rudder, Prompts Fleetwide Inspections, Inquiry

Air Safety Week, March 21, 2005

An Air Transat A310 lost its rudder on March 6, prompting manufacturer Airbus to issue a directive calling for field inspections of rudders on both A310 and A300-600 models.

About 400 aircraft worldwide are affected by the inspection mandate. This is a two-fold improvement over the handling when an American Airlines A300-600 lost its tailfin, in that the manufacturer is now calling for immediate inspection of all potentially affected aircraft and using an instrumented tap hammer to do so.

The A310 Rudder Separation ex Cuba

From a pilot's point of view, it is an entirely different proposition to lose a rudder than it is to lose an entire fin and its attached rudder - such as the American Airlines Flight 587 A300-605 did in Nov. 12, 2001, over New York's Jamaica Bay. Loss of a vertical fin would remove all important directional stability, whereas loss of a rudder merely deprives the pilot of most of his ability to control the aircraft in the yaw axis.

"Most," because he can still use the yawing secondary effect of roll and can still turn, climb, and descend almost as normal - as long as both engines are operable and thrust-symmetrical. Once it's clear that the rudder is of use (and essential) only in crosswind takeoffs and landings and one-engine inoperative asymmetric flight, you can appreciate how the Air Transat Flight 961 pilots were able to turn around and land their 1991 built A310-308 at Varadero, Cuba.

Nevertheless, the event has refocused attention on A300 and A310 rear ends and stimulated attempts to relate this incident to the AA587 accident. The fact that the A300 and A310 have identical composite (carbon fiber) fin construction, rudder actuators and software does nothing to dispel the notion of a possible causal relationship. AA587's pilot was blamed by the investigators for pedaling his aircraft's rudder pedals into an involuntary aircraft/pilot coupling (APC) during a wake turbulence encounter (ASW, Jan. 31). He allegedly overstressed the vertical fin to as much as 1.93 times its design ultimate load. By comparison, the Flight 961 pilots were cruising at Flight Level 350 on autopilot when there was a sideways lurch that threw a flight attendant off her feet, a loud bang and a change in handling characteristics. Naturally, the pilots weren't aware until after they'd landed that 95 percent of their composite construction rudder had departed.

Airbus Operator Information Telex TX530526F of March 8 advised that "a portion of the rudder structure remained attached between the lower hinge and the three servo-control attachment points." The big difference between the American Airlines event and Air Transat one may be related to the A310's rudder having momentarily aerodynamically loaded up an unstressed and streamlined fin (before itself breaking) and not a fin already side-loaded by the combination of repeated out-of-phase pilot pedal inputs reinforcing a yawing cycle that was first initiated by wake turbulence. Consider also that American Flight 587 involved a 266-seat A300's fuselage length of 177'5" (54.1m) compared to a 200- seat A310's length of 153'1" (46.66m). The A300 has a 27-foot-tall fin and the A310 has a slightly shorter fin. The difference in length between the two aircraft would also vary the moment arm for stability-restoring forces acting upon the vertical fin.

Composite Past and Future

The A310 series featured the first composite primary structure to appear on non-military aircraft. The original A300s had aluminum fins. Currently Boeing's 777 and all in-production Airbuses have composite vertical and horizontal stabilizers. The Boeing 787 will be all composite and the A380 will have an all composite center wing box, aft pressure bulkhead, tail surfaces and many other composite parts. In the bizjet field, Raytheon's Premier I and Hawker 4000 Horizon both have entire fuselages of carbon fiber/epoxy honeycomb, while Visionaire's VA-10A Vantage promises to be the world's first all composite business jet.

But it's Boeing and Airbus that have a definite need to resolve this failure mechanism. If the composite tails are starting to age via a wear and tear process, it would seem to be more likely that freezing and refreezing of trapped water or moisture entrained in the matrix voids or between laminations would be a likely culprit. These aircraft move between destinations at very cold and very hot latitudes and regularly spend much time at high-level cruise in outside air temperatures down to minus 50 degrees C (-58[degrees]F). In the AA587 case, the rudder broke itself into three pieces and departed the vertical fin at some stage. An early AA587 theory suggested that the rudder may have fluttered (very rapid fullish movements), failed at the bottom hinge and then rotated laterally around the top hinge before tearing away with the overstressed fin. That theory faded into the background as being the outcome, a symptom of a catalyst triggering mechanism that had to be either the flight-control system or involuntary and oscillatory cyclic inputs via the pilot's rudder pedals. These latter inputs are referred to as reversals. If they become out-of-phase with the aircraft's yawing, they reinforce the yaw-induced loads upon the fin and can easily induce fin attachment failure. That fact was almost unknown before it was disclosed by the AA587 investigation. As few as three sequenced opposite rudder inputs (known as a triplet) could in fact overstress the fin.