Aviation Safety Recommendations

Background

On 06 March 2005, an Airbus A310-300, serial number 597, registration C-GPAT, operated by Air Transat as Flight 961, departed Juan G. Gomez International Airport in Varadero, Cuba, for Québec/Jean Lesage International Airport, Quebec, with 2 pilots, 7 flight attendants, and 262 passengers on board. While at an altitude of 35 000 feet, the flight crew heard a loud bang followed by vibrations that lasted a few seconds. The aircraft entered a repetitive rolling motion, known as dutch roll, which decreased as the aircraft descended to a lower altitude. Once the aircraft reached about 19 000 feet, the flight crew had no indication of any abnormalities. The flight returned to Varadero where an uneventful landing was carried out. The Transportation Safety Board of Canada (TSB) investigation (A05F0047) into this occurrence is ongoing.

On arrival at Varadero, it was discovered that the aircraft rudder was missing. The rudder is made of composite sandwich construction, consisting of a nomex honeycomb core with carbon fibre face sheets. It had separated from the aircraft except for its bottom closing rib and the length of spar between the rib and the hydraulic actuators. Only small residual amounts of rudder side panel remained attached. An examination of the vertical tail fin of the aircraft, to which the rudder is attached, determined that the two rearmost fin attachment lugs were delaminated, likely the result of stresses that existed during the rudder separation.

The TSB investigation into this occurrence is being supported by the Bureau d'Enquêtes et d'Analyses pour la Sécurité de l'Aviation Civile (BEA) of France, the National Transportation Safety Board (NTSB) of the United States, and the Federal Bureau of Aircraft Accidents Investigation (BFU) of Germany. Technical advisors from Transport Canada, the Federal Aviation Administration (FAA), Airbus, and Air Transat are also participating.

Action Taken as a Result of the Occurrence

Based on the initial information uncovered during this TSB investigation, Airbus, on 17 March 2005, issued an All Operators Telex (AOT) for the inspection of all aircraft equipped with part number A55471500 series rudders. This one-time visual and tap-test inspection involved 222 Airbus A310s, 146 Airbus A300-600s, 6 Airbus A330s, and 34 Airbus A340s, for a total of 408 aircraft. In addition, a more detailed inspection of rudder side panels on over 20 aircraft was conducted using the elasticity laminate checker (ELCH) test method. Finally, the attention drawn to rudders by the occurrence resulted in operators examining their rudders more closely during maintenance. These various inspections found examples of disbonds, damage around hoisting points and trailing edge fasteners of the rudder, corrosion and abrasion at hinges, seized hinges, hinges with excessive free play, water ingress, and hydraulic fluid ingress.

Maintenance Standards in Place at the Time of the Accident

The following are the scheduled inspections of the rudder that were required at the time of the occurrence:

• Daily and transit checks consisting of a general visual inspection of the rudder are required. These are the only inspections of the rudder that are conducted during the 30 month period between 2C checks and are relied upon to detect any damage that could grow to a critical size within that period. The checks, which are conducted from the ground, would only detect significant damage because the rudder is as high as 15 metres in the air and the view of the rudder is partially blocked by the horizontal stabilizer.
• A 2C check is conducted every 30 months. It involves a general visual inspection of the rudder and empennage conducted at arm's length and does not require prior cleaning, additional lighting, or a magnifying glass. This type of inspection would only result in the detection of severe disbonds or delaminations that are externally visible.
• A tap test of the rudder side panels is conducted as part of the five-year inspection. The scope of this tap test is limited to a 40-millimetre-wide strip along the front edge of the rudder side panels, and to a similar narrow strip along the lower part of the trailing edge. Any disbond damage to the side panels outside these areas would not be detected. In addition, the tap test has severe limitations in its ability to detect disbonds on the interior face sheets. Finally, the tap test's effectiveness relies heavily on operator interpretation of results. A detailed visual inspection of the hinge arms, hinge fittings, and front spar is also conducted during the five-year inspection.

Although the investigation to date has not conclusively identified the events that led to the rudder separation, it was determined that, during the portion of the flight preceding the loss of the rudder, the occurrence aircraft had not been subjected to abnormal forces. It is important to note that the subsequent fleet inspection found some examples of disbond damage between the core and the face sheets, and a recent laboratory test by Airbus confirmed that disbonds could grow. This was the first occurrence of an in-flight rudder separation involving an Airbus aircraft in the 20 years that this rudder design has been in use. There are 408 aircraft in service that share this design of rudder, and they all share the same inspection program. Many of these rudders have now been in operation for close to 20 years.

Recent Rudder Damage Found on a Similar Aircraft

On 27 November 2005, during repair work being conducted on the rudder of a Federal Express (FedEx) Airbus A300-600 in the United States, delamination was found on the lower left lateral panel of that rudder. The NTSB investigation into this occurrence has determined that the initial delamination was the result of the infiltration of hydraulic fluid in the honeycomb material of the rudder, which appears to be linked to the vulnerability of rudders built before modification 8827, affecting about 370 Airbus A300-600/A310 and 40 Airbus A330/A340. Tests of this rudder in a depressurisation chamber resulted in significant further growth in the damage. Although a direct correlation between the Air Transat and the FedEx event has not been established at this time, the event confirms that significant delamination of these rudders can progress unnoticed, in spite of the present maintenance standards in place.

Board Recommendation

The separation of the rudder from Air Transat Flight 961 and the damage found during the post-occurrence fleet inspections suggest that the current inspection program for Airbus composite rudders may not be adequate to provide for the timely detection of defects. In addition, the recent discovery that disbonds could grow undetected and the increasing age of the composite rudders suggest that increased attention is warranted to mitigate the risk of additional rudder structural failures. The consequences of a rudder separation include reduced directional control and possible separation of the vertical tail plane.

Therefore, the Board recommends that:

Assessment/Reassessment Rating: Fully Satisfactory

The concern regarding the safety of affected aircraft is such that the BEA of France has recently issued a similar recommendation. As well, the NTSB is contemplating similar recommendations based on its investigation into the FedEx occurrence.

As the investigation continues, the Board may make further safety recommendations should additional safety deficiencies be identified.

Wendy A. Tadros
Acting Chair
on behalf of the Board