DAVID LEARMOUNT / LONDON
Early analysis indicates that sideslip and rudder movement could have caused November's AA587 crash in New York
The vigorous use of rudder in large aircraft under certain conditions can generate enough force to break off an undamaged vertical tail fin, the US National Transportation Safety Board (NTSB) is warning airlines. The alert is based on findings from the NTSB's investigation of the American Airlines Airbus A300-600R crash last November. French tests, using different aerodynamic load calculation methods, have come up with similar results, says the NTSB.
The NTSB has recommended to the US Federal Aviation Administration that it should publish advice to all pilots of large transport aircraft - not just those flying A300-600s - about the limitations on safe rudder use. The NTSB was due to reveal its findings as Flight International went to press.
The NTSB says that, under certain sideslip conditions - which could be temporarily generated by slipstream vortices or windshear - vigorous rudder use could generate loads on the fin "high enough to cause concern", and that in these circumstances rudder deflection limiters do not provide enough protection.
The tests, being carried out on A300s by the NTSB, have involved "wiggling the controls", measuring results, and projecting them mathematically. The aerodynamic results indicated that loads strong enough to break a new tail could be generated by rudder use under certain circumstances involving sideslip.
Meanwhile, working independently, Canada's National Research Council believes that it may have discovered a contributory factor in the American Airlines accident. While researching the characteristics of slipstream vortices, principal research officer Dr Miroslav Mokry has discovered that wingtip vortices can be strengthened by windshear. Because windshear with height was present when the accident occurred, Mokry believes that this could have been a factor.
Applying recently researched mathematical models to the vortices of the 747 ahead of the American aircraft as they would have been affected by the prevailing meteorological conditions, Mokry found that the second of the wingtip vortices that the A300 hit would have been strengthened.
Source: Flight International
