Quote:
Originally Posted by alvrb211
This statement leads me to believe you are unfamiliar with the unusual 737 rudder design and resultant fatal crashes.
Al
I will answer this on this thread because a memory of that chapter will help us keep this mishap (and the response to it) in perspective. As qouted in this article: (
Clear Agreement Elusive On Fixing 737's Rudders - The New York Times): "
The unforgivable sin in aviation is not killing a planeload of passengers -- it is killing two planeloads because of the same cause. That is one reason the Valujet crash last May near Miami was such a crisis for the aviation agency; it was not the first time that hazardous material had burned in a cargo compartment with no smoke detectors or fire suppression system. The tragedy of the Pittsburgh crash will seem worse if investigators eventually conclude that the one in Colorado, three years earlier, had the same cause.
If both crashes were caused by a rudder problem, the odds of another rudder malfunction is, quite literally, about one in a million. There are about 1,100 Boeing 737's in service in this country and 1,600 abroad, and the airlines worldwide have about 75 million hours of experience flying them over the last 30 years, the manufacturer says. Since the 737 makes mostly short flights, that comes out to about 75 million flights."
What we are hearing, and are going to hear, is that the Airbus has quite the safety record and that the aircraft was designed to meet or exceed all applicable requirements. That is all a smokescreen to deflect attention and blame - there will be a huge fight to avoid the finger pointing and the safety of subsequent flights will be compromised.
I am quite familiar with the history of the 737 rudder system. It consisted of one PCU that was served simultaneously by two hydraulic systems. While no "smoking gun" was ever found in the wreckage that confirmed a rudder hardover or reversal, it was shown that a rudder reversal "could" occur with a misalignment of guide springs/etc that could allow an actuator to extend too far and allow hydraulic fluid to travel down an unintended path. Overall, there were two crashes and several other incidents that were most likely attributable to the rudder - but the lack of a clear "direct link" allowed the FAA and Boeing wiggle room to avoid immediate action. Specifically, the FAA was wary of grounding up to 1600 airframes (worldwide) and several carriers that were flying the fleet exclusively. Eventually, the FAA agreed to a generous timeline for retrofit after Boeing finalized a re-design.
The crime was not that the engineers overlooked a potential single-point failure - there is no evidence that they were aware that the concentric dual servo valve's spring, spring guide, and end cap could be forced out of alignment and allow the hydraulic reversal when the system was designed. They felt they had designed the required redundancy into the system and the regulations at the time clearly allowed such a system. In a similar fashion, JAR's allowed Airbus to avoid the use of traditional cotter pins and crown nuts in lieu of a proprietary spring locking system. Amongst the A300/310 mechanics that I know, there is unanimous opinion that it is a very common occurrence to find these connectors securing flight control surfaces and actuators to have de-torqued. The regulations don't force traditional methods to be used, they look for equivalent levels of safety that don't always work in the real world over the long haul.
There is no literature from the NTSB calling for any sanctions against the original designers or changes to the certification process - they focused on getting the newly discovered fault fixed. For that reason - I am hesitant to call the design "unusually poor", but I am willing to call the reaction by the FAA and manufacturers "unusually poor". It is in this reference that I will be looking at the Airbus, JAA, and FAA response to this latest accident.