Standard for Tail Fins Probed in N.Y. Crash
The crash of a jetliner in New York last year after its tail fin ripped off is prompting investigators to question whether government structural standards are tough enough, according to sources close to the inquiry.
Boeing Co. has informed federal investigators that the tail of its comparable Boeing 767 airliner would not have broken under the forces experienced by the Airbus A300-600, said three sources familiar with the investigation.
Both the Airbus and Boeing planes meet Federal Aviation Administration structural standards. But Boeing imposes two additional requirements for its tail fin design, beyond four that are mandated by the FAA, a company document indicates.
American Airlines Flight 587 crashed shortly after takeoff from New York’s John F. Kennedy International Airport on Nov. 12, killing all 260 on board and five people on the ground. The pilots lost control soon after the Airbus crossed the wake of a larger plane ahead of it.
The National Transportation Safety Board has reached no conclusions yet about what caused the accident. It was the second deadliest airplane accident in U.S. history, after a 1979 disaster in Chicago that killed all 271 aboard and two people on the ground. Bernard Loeb, the agency’s former aviation safety director, said the Airbus accident raises “significant issues” about the adequacy of the FAA standards.
“It is critically important that the NTSB get into these issues, because it is the only oversight that exists on the part of the public,” Loeb said. “I have concerns about the closeness of the FAA and the industry. There is no oversight of the design and certification process if the NTSB isn’t there.”
Airbus said in a written statement that comparing Boeing planes with its own “can easily be misleading.” The company did not respond to the specific question of whether its tail fin design goes beyond minimum FAA standards, citing trade secrets and NTSB rules barring disclosure of accident investigation data.
“The bottom line is this,” the Airbus statement said. “While our designs are proprietary, the [tail fin] on the A300-600--as on all our aircraft--was designed and demonstrated to meet or exceed all certification criteria established by the FAA and European certification authorities.”
Airbus, a European consortium, is vying with Boeing for dominance of the world airliner market. It is unusual in the field of accident investigation for one manufacturer to get involved in a case dealing with another company’s plane.
FAA spokesman Les Dorr defended the agency’s standards. “There is no information that has come out of the accident investigation to date that would cause us to think any change needs to be made to the requirements. Obviously, if any such information does come to light, we would review it and determine what the proper course of action should be.” Dorr also warned against comparing planes built by different manufacturers.
The tail fin is one of the strongest parts of an aircraft. Also known as the vertical stabilizer, it helps keep the nose of the plane pointed in the direction the pilot wants to fly. The loss of a tail fin would render a plane uncontrollable.
“The vertical fin is very much like the feathers on an arrow,” said aviation safety consultant Barry Schiff, a retired airline captain. “If you remove one or more of those feathers, the arrow becomes no more than a stick and just wobbles through the air. This airplane was an arrow without its feathers.”
NTSB experts are analyzing Boeing’s findings about the strength of its 767 jetliner to see if the comparison is valid. They are “trying to understand the nuances and differences ... to see if [Boeing is] talking apples to apples,” said one source close to the investigation.
No conclusions have been reached.
“Boeing may have [an airplane] that is designed way above the standard, and Airbus may have one that is designed to the standard,” said the source, who asked not to be identified. “There are design standards which people are supposed to meet. If someone breaks a tail, then there is an issue with those standards.”
The FAA standards are intended to safeguard the tail fin from such forces as extreme gusts of wind or pilots aggressively using the aircraft’s rudder.
The rudder is a movable panel attached to the rear edge of the tail fin. It helps to keep a plane on course when landing or taking off in a crosswind, or when an engine fails. Jetliners have big, powerful rudders, and pilots use them sparingly in normal flight.
Flight 587 experienced a series of rudder movements in opposing directions during the final seconds before the pilots lost control. The NTSB now believes those movements were most likely the result of pilot actions, but it remains unclear why the pilot would have moved the rudder back and forth.
The FAA standards address sharp movements of the rudder. The standards are highly technical, but essentially, they represent individual steps of a hypothetical flight maneuver. The rudder is turned fully in one direction, held and then suddenly returned to neutral.
The FAA’s hypothetical maneuver is supposed to represent a worst-case scenario. A manufacturer must certify that its design is at least 50% stronger than the hypothetical worst case. This added safety margin is called “ultimate load.” Engineers rely on mathematical calculations and data from various kinds of tests to certify to regulators that their designs will hold up under ultimate load.
Airbus has told NTSB investigators that its calculations indicate that Flight 587 experienced forces beyond the A300 design limits, its “ultimate load,” said another source familiar with the investigation. But Boeing told the NTSB its engineers tested several scenarios that involved manipulating the rudder of its 767, and “it would appear to them that their loads were below ultimate,” the source added. “If you are below ultimate, you wouldn’t break the tail.”
According to this source, Boeing obtained data on the forces experienced by the Airbus plane and then calculated the effects of those forces on its plane. The NTSB then asked Boeing to do additional tests.
“They tried to get the worst possible scenario, and they [were] getting loads below ultimate,” said the source.
The Boeing standards raise the bar on what the FAA requires.
For example, the company calls for its aircraft to withstand full rudder in one direction followed by full rudder in the opposite direction. (The FAA only requires a full turn in one direction, followed by a quick return to neutral.)
“The net result of this approach is that there has been no catastrophic structural failure of a Boeing airplane due to pilot control input in 40 years of commercial operations involving more than 300 million commercial flights,” Boeing said in a recent bulletin to pilots.
The NTSB has scheduled hearings in October on the crash. Agency officials have stressed that they are still considering a number of factors, including the actions of the pilots, the design of the tail fin , the operation of the rudder controls, possible preexisting problems with the plane and the performance of the advanced composite material used to build the tail. The NTSB has rejected a call by some American Airlines pilots to recommend grounding all Airbus A300s.
“There are a lot of unanswered questions at this point,” Loeb said. “This is a very complicated investigation. The issue is not solely the structural capacity of the tail. It is a combination of that and the rudder control system.”
If the crash leads to tougher design requirements, manufacturers literally would have to go back to the drawing board to plan costly changes to many existing aircraft. Another option would be to impose higher requirements on new aircraft, while advising pilots to exercise caution in flying older models.
One surprising caution for pilots already has emerged from the investigation. The NTSB warned that, even under normal flying conditions, abrupt, repeated, opposite-direction rudder commands can break the tail on any plane. Pilots were traditionally instructed that they could make full use of controls at normal speeds without damaging the plane.
“I think there has been a miscommunication between the people who design airplanes and the people who fly them,” Schiff said. “Either we have been trained incorrectly, or this fin had a structural weakness that should not have been allowed to exist.”
