Canadian investigators have released a factual update of their ongoing investigation into the loss of the rudder on an Air Transat A310, which indicates that the rudder broke off while the airplane was in straight and level flight with no input from the crew.
This makes the case different from the rudder and fin separation involved in the loss of an American Airlines A300-600, which has the same rudder, as pilot inputs to the rudder were involved in the breakup sequence (see ASW, Nov. 1, 2004). The latest case is also different from that of a May 1997 event involving another American Airlines A300-600, in which the rudder and fin were not lost but which experienced high loads because of rudder reversal input by the crew. The latest event is also different in this respect from a 1991 case involving a German Interflug A310, where crew-induced rudder reversals contributed to high loading (see ASW, Nov. 25, 2002).
In the latest incident, which occurred March 6, the airplane was on a flight from Varadero, Cuba, to Quebec City, Canada, when the rudder separated. A flight attendant suffered minor injuries during the event. The airplane was near Ft. Lauderdale, Fla., at the time, but the crew elected to return to Cuba, less than 100 miles distant.
The crew did not input rudder reversals, or back and forth movements, to the rudder. In fact, no crew inputs to the rudder were given throughout the flight. Yet the rearmost lugs holding the tailfin to the fuselage were damaged. The rear lugs, of the six that attach the fin to the fuselage, generally endure the greatest loads. But how were such loads generated in this case?
According to Mark Fernandez, the investigator in charge for the Transportation Safety Board (TSB) of Canada, “We believe the rudder pulled the fin back, so the loads were from front to back.” In other words, they were not side-to-side, as occurred in previous cases resulting from rudder reversals.
There is no evidence of lateral loading beyond certain limits, which are considered within the normal range.
But how the loads were imparted to the rudder and fin are under intense study, and that study is confounded by the fact that the flight data recorder (FDR) features the use of filtered data (a problem which also complicated the American Airlines crash investigation).
Fernandez said four persons are working to de-filter the FDR’s raw data: a representative from the TSB, from Airbus, from the French accident investigation board, the Bureau Enquetes d’Accidents (BEA), and from the U.S. National Transportation Safety Board (NTSB).
Even so, the break happened so fast that the event was not recorded on the FDR. Fernandez explained that the FDR records the G forces at a rate of about 1/400 of a second, and the rudder was lost in less time than that. Nevertheless, the information captured on the FDR, once de-filtered, is expected to lend valuable insights regarding loss of the rudder and forces on the remaining fin. To be sure, how and when the flight attendant was injured is also germane to the case.
“Everything is to be verified,” Fernandez said. “You need 72 percent delamination to get complete destruction of the rudder,” he added, so how the rudder delaminated to that extent is a major point of concern.
In addition, the fin was removed from the incident aircraft and shipped to Bremen, Germany, where it was made, for detailed study. In particular, the attachment lugs are under intense scrutiny.
Carrier Air Transat affixed a replacement fin and rudder to the airplane to fly it out of Cuba; Airbus provided a new fin and rudder, although cost-sharing details are to be worked out. The airplane is now in Canada and is expected to be back in service within days, according to an airline official.
Photos taken by plane spotters of the airplane in 2003 show two vertical lines or wrinkles on the rudder’s surface, suggesting delamination of the composite. A photograph taken in 2001 shows no such line. Fernandez said the TSB has obtained “five or six” of the original negatives for analysis. In this day and age, there is the danger that photographs can be altered, but with the original negatives he is confident they will get to the truth. A 3-D conversion of the photos is being done to better understand what they portray.
Among various scenarios, three are under investigation:
1. If the various layers of the rudder were damaged by oil or glycol leaking onto the composite material.
2. If damage was caused by lightning during a previous flight.
3. If damage occurred to the inside of the composite layers, which cannot be seen from the outside.
In addition, the entire A310/A300-600 fleet is being checked with a tap test to determine if any rudders or tailfins are delaminating, in accordance with airworthiness directives (ADs) issued by various regulatory bodies to capture the worldwide fleet. Those tap tests are now under way. About 37 percent of the fleet has been examined, and there are no major findings, Fernandez reports.
In addition, Airbus has conducted so called ELCH tests, or elastic checker tests, on the fins and rudders of groups of A310s – specifically five older aircraft, five middle aged aircraft and five brand new aircraft. In this test, the composite material is examined every five inches in a test that takes eight hours per airplane. So far, those tests too are negative.
“Was it the guy on the ramp who damaged the rudder? We don’t know,” said Fernandez. Given the amount of delamination required for the rudder to separate, that possibility seems remote, as the damage would be visible. At this point, investigators are only at the beginning of their analysis.
Factual Update on the Rudder Separation Of Air Transat Flight 961 (excerpts)
“While at an altitude of 35,000 feet, the flight crew heard a loud bang with simultaneous vibrations that lasted a few seconds. The aircraft entered a periodic rolling and yawing motion known as dutch roll that 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 from systems monitoring.”
“The investigation team observed that only the lower rudder spar and the base rib of the rudder were remaining. Less than five per cent of the total rudder surface actually remained attached to the spar.”
“On 19 March 2005, the vertical tailplane (VTP) and the rudder were transported from Cuba to Bremen, Germany, for further examination. The VTP, to which the rudder is attached, is bolted to the top of the fuselage by six attachment lugs. The VTP was subject to ultrasonic inspection, which revealed delamination damage to the two rear attachment lugs.”
Source: http://www.tsb.gc.can/en/reports/air/2005/A05F0047/A05F0047_update_20050504.asp