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FEA Makes Airframes Safer PUBLIC ACCESS

Data Gathered during the Controlled Destruction of a Boeing 747 airliner are helping Engineers to Identify ways of Strengthening Aircraft to make them less Vulnerable to an Internal Explosion.

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Mechanical Engineering 120(01), 64-65 (Jan 01, 1998) (1 page) doi:10.1115/1.1998-JAN-3

Abstract

This article focuses on data gathered during the controlled destruction of a Boeing 747 airliner are helping engineers to identify ways of strengthening aircraft to make them less vulnerable to an internal explosion. Even though it may not help engineers understand the specific events in the crash of Flight 800, a controlled explosion of a Boeing 747 by the British Defence Evaluation Research Agency (DERA) , based in London, could point the way to controlling the damage from airliner explosions in the future. The 747-1 00 had been an attraction at the Bruntingthorpe Aerodrome in Leicestershire, England, before it was purchased by the British Ministry of Defence and the US Federal Aviation Administration (FAA). Initially, the meshes were too coarse for the dynamic analyses to be used in the test, so engineers refined them accordingly in the blast area. Using new features in MSC/DYTRAN 4, the team will model the airframe as a series of layers, which is representative of the lining concepts to be tested.

Article

MILLIONS'OF DOLLARS and well over a year have been invested in the investigation of the crash of TW A Flight 800, yet engineers still have their work cut out for them in re tracing the events that led to the explosion of the Paris-bound Boeing 747 airliner after taking off from New York's Kennedy International Airport in July 1996. Even with mountains of physical evidence, establishing why vapor in the jet's fuel tank ignited remains a daunting task: As is the case with so many disasters, engineers can 't determine with confidence exactly what events occurred, and in what order, through all the time increments spanning the event.

Even though it may not help engineers understand the specific events in the crash of Flight 800, a controlled explosion of a Boeing 747 by the British Defence Evaluation Research Agency (DERA), based in London, could point the way to controlling the damage from airliner explosions in the future. The 747-1 00 had been an attraction at the Bruntingthorpe Aerodrome in Leicestershire, England, before it was purchased by the British Ministry of Defence and the U.S. Federal Aviation Administration (FAA). Following the explosion of Pan Am Flight 103 over Lockerbie, Scotland, the British Civil Aviation Authority announced a research project to advise its Safety Regulation Group (SRC) on the effects of explosive devices in baggage on aircraft, possible design and operational methodologies to mitigate these effects, and tests and specifications that could be used in future designs. In the United States, the FAA has also had a program under way to reinforce the baggage containers on wide-bodied aircraft.

Both government agencies wanted to conduct controlled explosions to test how well a variety of cladding and other reinforcement schemes would limit the damage caused by small amounts of explosives. The project was commissioned by the SR C, in cooperation with British Airways, Daimler-Benz Aerospace, and other partners, as part of an effort to develop liners for an airliner's luggage hold and to strengthen the hold structures of existing fleets.

This project is the culmination of five years of DE RA research aimed at reinforcing airliner luggage holds. One of four charges was set against a protective lining for the luggage hold, which DERA engineers developed to protect the skin of the aircraft. Another was detonated inside a specially designed luggage container provided by the FAA. A third charge tested another liner- design concept. The fourth charge demonstrated the effects of a blast on an unprotected airliner. All blasts occurred simultaneously.

In addition to providing data on how well the current design concepts performed, the tests should also yield information that will aid engineers creating models of explosive events in the future. Over the past five years, DERA researchers have investigated the density of packed baggage and its effects on modifying the shock and blast of an explosion a well as other blast scenarios. Drawing on all of this information, engineers and researchers at DERA expect to model and predict the effects of an internal explosion on any aircraft.

The data provided by tills research project also tested the ability of digital simulations to accurately analyze complex nonlinear events involving interactions between fluids and solid structures. However, the many simulations performed were useful before the controlled explosion, ensuring that only the most viable alternatives were physically tested. (The up-front tests were also intended to determine how closely the digital simulations predicted what actually happened as measured by test data.)

To perform the digital simulations, the DERA team used MSC/DYTRAN-general- purpose, three-dimensional software from the MacNeal-Schwendler Corp. in Los Angeles that uses finite- element-analysis techniques-that simulates the dynamic response of structures and fluids. The software can be used to analyze structure-to-structure contact (as in collisions), material flows (as in forging), and fluid-structure interactions (as in the airliner explosion). MSC/DYTRAN was chosen after a proprietary advanced third-order Euler analysis code for studying the behavior of fluids verified the accuracy of MSC/DYTRAN's own first-order Euler technology.

In the course of the analysis work, DERA engineers used finite-element meshes of airframe sections provided by its industrial partners for import into MSC/DYTRAN. Initially, the meshes were too coarse for the dynamic analyses to be used in the test, so engineers refined them accordingly in the blast area. In these analyses, engineers also used results from earlier DERA studies to determine where a bomb's most likely position would be in relation to the airframe. By understanding how the baggage was stored in the hold, engineers identified the most dangerous position as the area closest to the skin. This area was chosen as the initiation point of the explosion.

Engineers then had to take additional steps to ensure that their models faithfully replicated reality before simulating the explosion itself. They had to apply forces due to pressurization to the model before detonation, since internal cabin pressure would be a major factor in an airborne explosion. To do so, engineers wrote a subroutine for MSC/DYTRAN that prestressed the model accordingly. Plastic limits were also set for the elements representing the skin; the frame sections, which run around the fuselage; the stringers, which run along the fuselage; and the rivets that hold the structure together.

Initially, DERA engineers modeled three fuselage bays. At the start of the analysis, the explosion, represented by an expanding ball of gas, enlarged until it impacted the static structure of the frame. As the pressure front hit the structure, a line of weakness immediately appeared where the skin thickness changed.

The results offered some food for thought for airframe designers. To resist the conventional loads on an airframe, for example, the material used for the belly is slightly thicker than that for the sides of the fuselage. The line where these two different material sections meet is where the fuselage started to fail, as the analysis predicted.' From this fault line, the thinner skin tears away from the frames; the frames then snap, and the event concludes with the remainder of the skin opening out like a mailbox. The DERA team immediately saw that the fault line running along the length of the fuselage had quickly reached the edge of the three-bay model. To find out how far this tear extended, engineers enlarged the model to five bays and repeated the analysis. This next analysis predicted that once initiated, the split would virtually run the full length of the aircraft.

After completing this phase of the project, the DERA team exploded a representative section of airframe to determine how closely the digital simulation predicted real-world behavior. Using high-speed video techniques, DERA researchers filmed the explosion so that the film could later be replayed for comparison with the digital predictions_ Based on the video images, DEIU engineers concluded that the correlation had been exceptionally close.

From this point on, engineers concentrated on optimizing the design of the skin, frames, and stringers, principally by increasing material thickness. The analysis results clearly showed that relatively small increases in material resulted in significant improvements to the airframe's ability to survive an internal explosion, and this was confirmed by further physical tests on different sections of an airframe. N ear the tail section, where the bell y and sidewalls are made from thicker material, the only video evidence of an internal explosion was a small rippling of the skin during the event.

Until this point, MSC/DYTRAN analyses had been applied only to the exposed skin, frames, and stringers, which is not a true representation of an active hold. (For the physical tests, the DERA team had stripped the Boeing 747 of its seats, carpets, and paneling.) In reality, the hold also comprises a flooring system, a lining to protect the skin exposed between the frames and stringers, and the luggage. Each of these could improve or impair the aircraft 's blast resistance. The DERA team recently acquired the latest release of MSC/DYTR AN, version 4, which it intends to use to perform more-complex modeling tasks, such as incorporating these features in a dynamic simulation. These analyses should shed more light on the viability of the liner and containment devices in the explosion of real-world structures.

Using new features in MSC/DYTRAN 4, the team will model the airframe as a series of layers, which is representative of the lining concepts to be tested. As the blast destroy each layer, that layer can be removed from the model, the reduced pressure front is free to move to the next layer, and so on. Using these techniques, the DERA team will be able to model new design concepts and test them within the context of all the key features of an aircraft's hold. The results may be not only future airframes capable of remaining airborne in the event of a crash but also new liner concepts that fortify the holds of existing wide-bodied fleets like those that dominate long-haul air travel.

Data from this controlled explosion of a Boeing 747-100 airliner will help engineers model the effects of an internal explosion on any aircraft.

Grahic Jump LocationData from this controlled explosion of a Boeing 747-100 airliner will help engineers model the effects of an internal explosion on any aircraft.

Copyright © 1998 by ASME
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