The US air accident investigation body has called for immediate inspection and mandatory repair of some Airbus A300, A310, A330, and A340-series rudders to prevent possible in-flight failure and separation.
The request by the US National Transportation Safety Board (NTSB) follows several reports of a weakening of the adhesive connecting the rudder’s honeycomb core and the surrounding composite skin.
In its recommendation, the NTSB says operators must follow Airbus’s own recommendations that require repair if there is any evidence of a separation between the core and skin, and immediate grounding if more than one separation, also known as a disbondment, is uncovered.
The agency, however, says that Airbus’ flight cycle requirements - that range from 2,500 cycles for disbondments less than 130mm (5.1in) to general inspection every 500 cycles - be replaced with a mandate for immediate inspection. The safety board is recommending that the US Federal Aviation Administration mandate all operators of Airbus A300 series aircraft to immediately (possibly before further flight) comply with four Airbus all operators telexes (AOT), dated 2 March 2006.
“Any disbonding to the rudder skins that occurs in the presence of hydraulic fluid contamination should be repaired or the rudder should be replaced as soon as possible, well before the 2,500 flights specified in the AOTs,” the FAA notes, adding: “This is an urgent recommendation.”
The FAA is also being advised to establish a repetitive inspection interval for these so-called premodification rudders, which are those configured with the skin panels attached to the front spar of certain A300 and A310-series aircraft, at a frequency below 2,500 flight limit required by Airbus.
Two incidents are cited in the NTSB’s report. One involves a FedEx A300-600 (N717FE) that was damaged November 27 during routine maintenance. “To assess the extent of the damage, the rudder was shipped to the manufacturer’s facility and examined. In addition to the damage that occurred during maintenance, the examination found a substantial area of disbonding between the inner skin of the composite rudder surface and the honeycomb core, which is located between two composite skins.”
It adds: “Further examination of the disbonded area revealed traces of hydraulic fluid. Hydraulic fluid contamination between the honeycomb skin and the fiberglass composite skin can lead to progressive disbonding, which compromises the strength of the rudder. Tests on the damaged rudder also revealed that disbonding damage could spread during flight.”
The NTSB also cites an incident involving an Air Transat A310-300, C-GPAT, which experienced inflight rudder separation during a March 6 2005 flight between Varadero, Cuba and Québec City, Canada.
While the Air Transat flight returned safely to Varadero, the NTSB notes: “Upon landing at Varadero, the crew discovered that most of the airplane’s rudder had separated inflight with only the bottom closing rib and the spar between the rib and the hydraulic actuators remaining. Further examination of the vertical stabiliser determined that its two rearmost attachment lugs were damaged due to the high stresses associated with the rudder failure and separation.”
The NTSB also warns: “These high stresses may have been dangerously close in magnitude to those that caused the inflight separation of the vertical stabilizer during the 12 November, 2001 accident involving American Airlines flight 587” which resulted in the death of all onboard.
“A subsequent inspection of the vertical stabiliser noted the aircraft’s rearmost attachment lugs were damaged due to stresses associated with rudder failure and separation,” notes the US safety regulator. “The Board believes that this urgent recommendation, if acted upon quickly, will go a long way to prevent a catastrophic failure of the rudder,” says NTSB acting chairman Mark Rosenker.
About 400 Airbus are equipped with the rudders in question, before Airbus redesigned the rudder skin panels and front spar interfaces, says the NTSB. No premodification A330s or A340s are listed in the US registry, the agency adds. Canada’s safety board is expected to issue a similar recommendation later today.
JOE SINGLETON / WASHINGTON, DC
Source: Flight International
