|
Background
The technical discipline known
as crash dynamics focuses on technologies to improve
the structural crashworthiness of aircraft and
the potential survivability of occupants. The scope
of interests includes the measurement and understanding
of structural and passenger loads experienced during
crashes, studies of the energy-absorbing characteristics
of new aircraft materials and assembled components
such as subfloors and seats, the development and
validation of analytical design methods, and the
impact of crashes on special aircraft equipment
such as emergency locator transmitters. A key goal
in this area of research is to provide enhanced
survivability with little or no increase in aircraft
weight or cost.
Crash dynamics first evolved
as a world-wide technical discipline in the 1940s.
In the 1950s and early 1960s, a series of full-scale
crash tests of transport aircraft with instrumented
dummies by the National Advisory Committee for
Aeronautics (NACA) and the Federal Aviation Administration
(FAA) provided valuable information on crash/fire
characteristics. The NACA tests were performed
by accelerating an airplane along a guide rail
and crashing it into an earthen mound. Later, NACA
studies on the dynamic response of seat structures
to impact loads resulted in a key Civil Aeronautics
Administration update of static seat-strength requirements.
Other organizations, such as the U.S. Army, also
conducted extensive pioneering crash research,
which culminated in a Crash Survival Design Guide
published in 1967 by the Army. In the 1970s, the
NASA Langley Research Center converted its existing
Lunar Landing Research Facility to a unique crash
dynamics testing facility, which has been continuously
used since that time for numerous key research
projects focused on full-scale aircraft and components.
Langley has also led the development of sophisticated
analytical methods to analyze and predict structural
and human tolerance data.
The emergence of new materials
for aircraft structures, especially advanced composites,
has resulted in new challenges for the analysis
tools, technologies, and design methodologies used
in the field of crash dynamics. The application
of composite materials to aircraft structures offers
potentially significant reductions in weight and
improved corrosion resistance when compared with
metal aircraft structures. However, the different
physical characteristics of composites, and their
failure characteristics in high-energy impacts,
have required significant modifications to existing
design methods and, in some cases, existing criteria
for crashworthiness. In view of the emphasis on
the weight-savings potential of composites, meeting
the crashworthiness criteria with minimal impact
on weight has become even more critical for modern
aircraft.
The Langley Research Center
has been an international leader in crash dynamics
technology, using its close working relationships
with industry, other government agencies such as
the FAA and the National Transportation Safety
Board (NTSB), Department of Defense (DOD), and
academia to aggressively pursue technological advances
in this critical area. The main area of contributions
by Langley for civil aircraft of the 1990s has
centered on general aviation aircraft; however,
structural crash test investigations of commercial
transports have also been conducted, and extensive
research programs (not reported herein) have resulted
in contributions to U.S. Army rotorcraft and the
design of the U.S. Air Force F-111 crew escape
capsule.
Langley Research and Development
Activities
Langley researchers have
conducted and maintained a broad spectrum of research
programs directed at crash dynamics technology.
Historically, projects have included studies of
aircraft structural integrity during ditching at
sea as well as detailed studies of the effects
of ground impact for various types of soil/concrete
materials. Most of Langley’s contributions
to crashworthiness characteristics of modern aircraft
have resulted from the ground impact studies; however,
a brief review of Langley’s ditching studies
is presented for completeness.
Ditching
NASA, and its predecessor,
NACA, have been cognizant of the challenges of
ditching an aircraft since the early days of World
War II, when ditching at sea was a major problem.
About 60 different airplane configurations were
subsequently investigated with subscale models
in the water-filled towing tanks of the Langley
Research Center. One of the most spectacular Langley
ditching investigations occurred in late 1944,
when a modified full-scale B-24 bomber was intentionally
ditched in the James River near Newport News, Virginia,
in order to obtain instrumented data for correlation
with subscale model testing in the Langley towing
tanks. This particular aircraft included a reinforced
bottom in response to a U.S. Army Air Corps request
to investigate structural strengthening for potential
improved ditching characteristics for the B-24.
The aircraft was landed by an expert Army pilot
in the James under ideal conditions and smooth
water. The landing was fairly smooth and the crew
was unharmed, but very severe damage occurred to
the airplane, even though the bottom was very much
stronger than that of a normal B-24. Water pressures
as high as 60 psi were recorded. This early experiment
provided evidence that airplanes are never designed
to have fuselages that are undamaged in a ditching
event. As a matter of fact, the undamaged shape
of a typical aircraft is not very good for a water
landing.
Water-impact accidents of
commercial transports have varied greatly in severity
and fatalities to occupants, largely dependent
on sea state, ditching procedures, and certain
aircraft characteristics including floor structure
and wing location. No ditching investigations were
conducted by Langley for a number of years following
World War II because of the similarity of most
transport airplanes at that time with respect to
ditching behavior. Ditching tests of a subscale
model of the Boeing 707 were conducted at Langley
in 1955 to produce information for the first generation
of swept-wing jet transports. Then, in the 1970s,
the emergence of a new family of large jet airplanes,
such as the jumbo jets and the C-5 military transport,
renewed ditching investigations. The results of
the subsequent Langley tests showed that as the
size of the aircraft increased, the ditching behavior
became less violent. This result was, in part,
due to the large size of the aircraft relative
to typical waves.
The damage that is expected
in a ditching event will be severe, and it is extremely
difficult to accomplish an optimal design for minimal
ditching impact. However, the Langley testing provided
extremely valuable guidelines for airline operators
and the military regarding recommendations for
forced ditching maneuvers, including the preferred
landing attitude, flap settings, and aircraft orientation
relative to wave crests during water impact. The
critical nature of the design of the floor structure
was also highlighted.
Although Langley’s
contributions in ditching research have proven
very significant and widely recognized, an even
more impressive array of contributions to aircraft
technologies has resulted from research studies
of aircraft during land crashes.
Langley Impact Dynamics Research
Facility
Full-scale aircraft and component
crash testing is performed by NASA and its industry,
DOD, FAA, and academia partners at the Langley
Impact Dynamics Research Facility (IDRF). This
facility is the former Lunar Landing Research Facility,
which Langley modified following the Apollo Program
for free-flight crash testing under controlled
test conditions. The basic gantry structure is
240-ft high and 400-ft long, supported by three
sets of inclined legs spread 265 ft apart at the
ground. An 8-in-thick reinforced concrete impact
surface is centered under the facility gantry and
is approximately 396-ft long and 29-ft wide. A
movable bridge with a pullback winch for raising
the test specimen spans the top and traverses the
length of the gantry. In a typical test, the aircraft
is suspended from the top of the gantry by two
swing cables and is drawn back above the impact
surface by a pullback cable. An umbilical cable
used for data acquisition is also suspended from
the top of the gantry and connected to the top
of the aircraft. When the aircraft is released
from the pullback cable, it swings as a pendulum
into the impact surface. The swing cables are separated
from the aircraft by pyrotechnics just prior to
impact, which frees the aircraft from restraint.
The umbilical cable remains attached to the aircraft
for data acquisition, but pyrotechnics also separates
it before it comes taut during skid out. The flight-path
angle is adjusted from 0∞ to 60∞ by
changing the length of the swing cable. The initial
height of the aircraft above the impact surface
at release determines the impact velocity, which
can be varied from 0 to 60 mph. For some tests,
the flight-path velocity has been increased to
about 100 mph by using wing-mounted rockets to
accelerate the aircraft on its downward swing.
After the aircraft is released with the rockets
ignited, the rockets continue to burn during the
downward acceleration trajectory, but they are
expended by the time of impact. Data acquisition
from full-scale crash tests is accomplished with
extensive photographic coverage and onboard strain
gauges and accelerometers. Instrumented anthropomorphic
dummies are onboard the aircraft for full-scale
tests.
Aircraft suspended from
Langley Impact Dynamics Research Facility in preparation
for crash test.
Postcrash photograph of
dummies used in IDRF testing.
Since 1974, numerous types
of aircraft have been successfully crash tested,
including helicopters, high- and low-wing single-
and twin-engine general aviation aircraft, and
aircraft fuselage sections. Test scenarios have
included vertical drop tests to simulate aircraft
cabin sink rates experienced in a crash, as well
as swing tests conducted using various impact surfaces,
including dirt and concrete.
General Aviation Crashworthiness
Studies
In 1972, Langley embarked
on a cooperative effort with the FAA and industry
to develop the technology required for improved
crashworthiness and occupant survivability in general
aviation aircraft. The research included extensive
analytical and experimental work as well as structural
concept development; this research was directed
at enabling future general aviation aircraft designs
to have enhanced survivability under specified
crash conditions and little or no increase in weight
at acceptable cost. The program was divided into
three areas. The first area, environmental technology,
consisted of acquiring and evaluating the field
crash data to support and validate studies conducted
under controlled full-scale crash testing. The
goal of this area was to define a crash envelope
within which the impact parameters allow human
tolerable acceleration levels. The second area,
airframe design, was to assess and apply existing
analytical methods to predict structural collapse
and develop and validate new analytical techniques.
The average acceleration time histories (crash
pulses) in the cabin area for each principal direction
were calculated for each crash test. Airframe design
also included the validation of new load-limiting
concepts for use in aircraft subfloor designs.
The third element, component design technology,
consisted of exploring new and innovative load-limiting
concepts to improve the performance of the seat
and occupant restraint systems by providing for
controlled seat collapse while maintaining seat/occupant
integrity.
Early in the Langley research
program, critical quantitative information began
to be contributed by full-scale tests. The test
parameters included in the Langley full-scale crash
tests could not possibly encompass all crash scenarios,
but the data obtained were found to adequately
represent some of the more serious but potentially
survivable general aviation airplane crash situations.
The data were used in a number of applications
to make reasonable estimates of the critical accelerations
and survivability issues for postcrash analysis.
Langley researchers who led
research efforts on crash tests for the general
aviation program included Victor L. Vaughn, Jr.,
Emelio Alfaro-Bau, Claude B. Castle, Susan M. Williams,
and Edwin L. Fasanella. The objectives of the studies
were to determine the dynamic responses of the
aircraft structure, seats, and occupants during
simulated crashes; to determine the effect of flight
parameters at impact (i.e., flight speed, flight-path
angle, pitch angle, and roll angle) on the magnitude
and pattern of structural damage; to determine
the failure modes of the seats and occupant restraint
systems; and to determine the loads imposed upon
the occupants.
This information proved invaluable
to crash investigation assessments and technology
development. An example of this application involved
an accident that occurred on August 30, 1978, when
a twin-engine Piper Navajo Chieftain aircraft,
carrying a pilot and nine passengers, crash-landed
in the desert shortly after taking off from the
Las Vegas airport. All 10 persons on board were
killed. A comparative study of this crash and a
Langley controlled-crash test was made to compare
damage modes and estimates of acceleration levels
in the actual accident and to assess the validity
of Langley’s full-scale crash simulation.
The Langley crash test utilized the velocity augmentation
method, wherein the aircraft reached a flight path
velocity of about 93 mph at impact. The aircraft
pitch angle was 12∞ with nose down and the
wing was rolled about 5∞ at impact. Although
the aircraft tested was a Piper Navajo, which was
slightly smaller (6 to 8 passengers versus 10),
the similarity of the damage and failure distortions
of the seats enabled the investigators to estimate
that the peak pelvic accelerations of passengers
were probably in excess of 60g normal to the aircraft
longitudinal axis, 40g longitudinal, and 10g transverse.
The Langley data were also extremely valued for
studies of human tolerance characteristics, especially
the development of prediction criteria for spinal
injury and the correlation of such injuries with
various formulations for impulse-momentum relationships
during crashes.
Typical crash sequence
for twin-engine aircraft.
Aircraft Subfloor and Seat
Technology
The development of structural
concepts to limit the loads transmitted to the
occupants of the aircraft is an ongoing element
of the Langley program in determining crash loads
and identifying structural failure mechanisms during
aircraft crashes. The objective of the research
is to mitigate the loads transmitted by the structure,
either by modifying the structural assembly, changing
the geometry of the elements, or adding specific
load-limiting devices to help dissipate the kinetic
energy during a crash. The focal point of these
efforts has been the development of crashworthy
subfloor systems. These subfloor systems provide
a high-strength structural floor platform to retain
the seats (and resist overturning) and a crushable
subfloor zone to absorb energy, distribute the
loads evenly across the fuselage, and limit vertical
loads by “stroking.” In the general
aviation program, several subfloor concepts were
defined that showed a significant reduction in
cabin decelerations over a representative unmodified
subfloor. Laboratory and full-scale crash tests
provided substantiating data to validate the effectiveness
of these concepts. In the design of load-limiting
seats, Langley research highlighted the critical
importance of available stroke in determining the
load-attenuating characteristics of different configurations.
Vertical stroking of general aviation seats was
found to be more critical than horizontal stroking
from the allowable human tolerance standpoint.
This finding was especially significant because
little crushable structure in the vertical direction
is normally available in most subfloor structures
of single-engine general aviation airplanes. Langley
developed energy-absorbing seats that utilized
seat linkages and wire roller trolley assemblies
that were especially effective in attenuating the
vertical loads transmitted to the seat to a human
tolerable value.
Analytical Methods
The extensive experimental
crash tests and analyses conducted in the Langley
program were complemented by the development and
applications of computer-based analytical studies
that model the complex, nonlinear responses of
aircraft subfloor sections to crash loads. A nonlinear,
finite-element, structural dynamics program known
as DYCAST was developed by Grumman Aerospace Corporation
(now Northrop Grumman) under contract to Langley
and the FAA to accurately analyze load-limiting
subfloors. Static crush tests of simplified components
that characterize the nonlinear load-deflection
behavior of the crushable elements of the subfloor
are used in the model to predict dynamic behavior.
Utilization of this computer program was successful
for certain configurations and these efforts also
showed the validity of using statically determined
crush data for dynamic analyses. However, the results
also indicated that the analyst must have some
assurance that the static deformation behavior
will approximate the dynamic deformation behavior.
Analytical efforts were also
directed toward developing methodology for the
design of load-limiting seats. The DYCAST code
was applied to numerous studies of the dynamics
of seat configurations, including the characteristics
of wire-bending load limiters, shoulder harnesses,
lap belts, seat back stiffeners, and pelvis stiffness
properties. The computer model proved very useful
for detailed modeling of load-limiting seats with
a hybrid finite-element approach. This approach
builds on a database of static crush characteristics
of component seat structures to help predict mathematically
the dynamic behavior of the seat occupant restraint
system response. The analyst can start with a simple
seat and occupant and provide increasing sophistication
of the representation as needed for the specific
task.
In 1992, Huey D. Carden published
the results of a key analytical study made to (1)
provide comparative information on various crash
pulse shapes that potentially could be used to
test seats under conditions included in Federal
Regulations Part 23 for dynamic testing of general
aviation seats, (2) show the effects that crash
pulse shape could have on the seat stroke requirements
necessary to maintain a specified limit loading
on the seat/occupant during crash pulse loadings,
(3) compare results from certain analytical model
pulses with approximations of actual crash pulses,
and (4) compare analytical seat results with experimental
aircraft crash data. Carden’s study derived
structural and seat/occupant displacement equations
in terms of the maximum deceleration, velocity
change, limit seat pan load, and pulse time for
five potentially useful pulse shapes; from these,
analytical seat stroke data were obtained for conditions
as specified in Federal Regulations Part 23 S 23.562(b)(1)
for dynamic testing of general aviation seats.
Emergency Locator Tests
Since the early 1970s, general
aviation aircraft have been required to carry an
Emergency Locator Transmitter (ELT) to help identify
the location of crashed aircraft by automatically
activating a transmitting distress signal in the
event of a crash. Unfortunately, early systems
displayed a high rate of false activation and failures
to activate as desired in a crash situation. Langley
assisted the FAA and industry through a special
committee of the Radio Technical Commission for
Aeronautics (RTCA) by studying the ELT problems
to help identify solutions.
Langley replicated and demonstrated
the ELT problems by mounting a sampling of ELT
devices in full-scale crash test aircraft and in
a test apparatus used for dynamic seat tests. Evaluation
of the test results indicated that one of the key
factors in failure was the vibration sensitivity
of the ELT switches and sensors. Testing showed
that the critical resonant frequencies of most
commercial crash sensors fell within the range
of structural vibrations that existed on general
aviation aircraft. However, the frequency range
of crash pulses that needed to be detected was
on the lower end of the frequency spectrum. Thus,
the sensors were too responsive to the local structural
vibrations during aircraft operations, causing
unwarranted activation in some cases and nonresponsive
action in other cases. Langley conceived and demonstrated
improvements by redesigning a typical ELT device
to have a much lower resonant frequency and superior
response characteristics for crash alerts.
Composite Structures
During the general aviation
crash dynamics program at Langley, the efforts
in the area of subfloor designs for crash dynamics
were directed toward metal aircraft. The aggressive
emergence of composite materials for aircraft structures,
however, introduced new issues regarding the energy-absorbing
properties of composite materials and their crashworthy
characteristics. For example, because composites
typically are brittle and did not necessarily exhibit
plasticity prior to failure, changes may be required
in the geometry and designs for many composite
aircraft structural elements to protect occupants
in the event of a crash and to provide efficient
energy-absorbing mechanisms. Therefore, new concepts
of subfloor structures need to be formulated and
tested to verify the behavior of properties of
composite designs.
Lear Fan test specimen
prepared for crash test.
Test aircraft suspended
for crash test in Langley and Terry Engineering
investigation of
improved crashworthiness. Note soil spread over
impact surface for soft soil test.
One particular characteristic
of composite aircraft structures that should be
a concern to the designer is the intersections
of longitudinal subfloor beams and lateral bulkheads
which form efficient load paths or hard points
which transfer high loads to the seat and occupant
and often prevent desirable energy-absorbing failure
modes from occurring during a crash. In 1989, Lisa
E. Jones and Huey D. Carden conducted experiments
to determine the energy-absorbing characteristics
and performance of typical DuPont Kevlar and graphite-epoxy
aircraft subfloor intersections. Various concepts
for the attachment of laminated longitudinal floor
beams and lateral bulkheads were incorporated into
specimens for static testing. Quasi-static testing
was performed with a testing machine that crushed
the specimens to 25 percent of their original heights
at a travel rate of about 2 in/min. This detailed
study of failure modes and the differences observed
between Kevlar and the graphite-epoxy specimens
provided fundamental information for the design
of more efficient composite subfloors.
In 1993, Huey Carden and
Sotiris Kellas of Lockheed Engineering & Sciences
Company collaborated in a study of an energy-absorbing
beam design for composite aircraft subfloors. The
tests were one element of a broader full-scale
aircraft crash test program using a Lear Fan composite
aircraft. The composite fuselage of the Lear Fan
with its original subfloor structure had four aluminum
spars that supported the seat rails, whereas the
remainder of the structure was constructed of graphite
composite. Static tests of the subfloor section
showed that the original structure was too strong
to provide reasonable occupant loads at crash speeds
of about 30 ft/sec as recommended by Part 23 of
the Federal Aviation Regulations for aircraft seat
tests. The objective of the Langley study was to
design and test a composite subfloor structure
that would provide the desired cushioning (less
than 20g on an occupant) as a potential retrofit
to the original aluminum spar design. A sandwich
spar construction based on a sine-wave beam was
chosen for evaluation and found to have excellent
energy-absorbing characteristics. The design objective
of obtaining sustained crushing loads of the spar
for potential limiting loads of around 20g was
obtained.
In 1995, Lisa Jones and Huey
Carden reported on crash tests of a Lear Fan aircraft
with composite wing, fuselage, and empennage (but
with aluminum subfloor) at the Langley IDRF. The
test was conducted to determine composite aircraft
structural behavior for crash loading conditions
and to provide a baseline for similar aircraft
crash tests of the modified subfloor. Langley had
obtained two nonflying Lear Fan test airplane structures,
and one aircraft was tested in essentially an “as
received” condition to provide a baseline.
Avionics, seats, engines, propellers, tails, and
landing gear were not included in the test. The
scope of testing included three different energy-absorbing
seats as well as a bulkhead airbag experiment.
The results of the crash test showed that the accelerations
on the floor of the composite aircraft were much
higher than those for comparable all-metallic aircraft,
that the subfloor structure did not crush but failed
in a brittle manner, and that very little energy
was absorbed. Although the structural design was
not considered the optimum composite design for
crashworthiness, postcrash integrity and cabin
volume were maintained.
Although not covered by the
fixed-wing aircraft focus of the present document,
it is important to note that the Langley staff
has conducted extensive crashworthy research on
rotary wing aircraft in partnership with the U.S.
Army. The program has been particularly productive
regarding the characteristics of composite materials
and structures. U.S. Army helicopters are designed
to dissipate prescribed levels of crash impact
kinetic energy without compromising the integrity
of the fuselage. Because of the complexity of the
energy-absorption process, it is imperative for
designers of energy-absorbing structures to develop
an in-depth understanding of how and why composite
structures absorb energy. Karen E. Jackson and
Edwin L. Fasanella were key leaders in this research.
In the late 1990s, Langley’s
Lisa Jones and James E. Terry of Terry Engineering
led a cooperative NASA SBIR project to design and
test an improved crashworthiness small composite
airframe representative of general aviation applications.
The study was motivated by the fact that the NTSB
had studied a large number of general aviation
accidents in detail and concluded that fatalities
could have been reduced by 20 percent if the occupants
had worn shoulder harnesses and 88 percent of the
seriously injured occupants would have had less
serious injuries if shoulder harnesses had been
worn. Also, it was estimated that energy-absorbing
seats could have reduced the severity of injuries
by 34 percent and reduced fatalities by about 2
percent.
This project had also noted
that previous drop tests of both high-wing and
low-wing single-engine general aviation configurations
by Langley showed very different responses during
impact with soil (representative of off-runway
conditions) compared with impact on hard surfaces.
In particular, the results showed that traditional
single-engine aircraft tended to dig nose first
into the soil on impact, producing twice the deceleration
experienced during similar crashes on concrete.
As a result of the high deceleration loads, the
fuselage structure normally failed catastrophically
with a complete loss of cabin integrity and survivable
volume. The objective of the Langley and Terry
Engineering study was to expand the survival envelope
and reduce the severity of injuries and survivable
accidents. Particular emphasis was placed on design
of an engine mount and forward fuselage whose failure
mode would preclude “digging in,” a
fuselage floor that would minimize loads, and air
bags and load limiters designed to expand the survivability
envelope. Simulated crashes at the Langley IDRF
began in 1996 and included impact flight angles
of -30∞ and an impact velocity of about 82
ft/sec on concrete and soft soil. A soil bed approximately
3-ft deep was placed on a concrete surface for
the soft soil tests. Results of the IDRF drop tests
indicated that improved survivability was possible
(compared with prior Langley tests of conventional
airplanes) for symmetrical impact at approximately
stall speed, even for a relatively severe impact
angle onto both hard and soft soil surfaces. The
airframe weight penalty for this improved survivability
was approximately 50 lb, with airbags adding another
12 lb and load-limiting shoulder harnesses adding
an additional 1 lb.
Controlled Impact Demonstration
Program
The original NASA, FAA, and
industry joint general aviation crash program was
concluded in 1982 following a decade of extremely
productive research. At the conclusion of the program,
national interests in crash research became focused
on commercial transport aircraft, stimulated by
the introduction of wide-body jumbo jets with large
passenger complements, which represented a potential
for a substantial loss of life or injuries in a
single accident. In 1980, Langley and the FAA began
a research program to quantitatively assess transport
aircraft crashes. As part of this program, an intentional
survivable, full-scale crash test of a transport
aircraft was planned. The agencies subsequently
conducted the Controlled Impact Dynamics (CID)
Program that involved a controlled crash of an
out-of-service, mid-1960 four-engine Boeing 720
research aircraft at the NASA Dryden Flight Research
Center. Although the 720 was considered obsolete,
its structural design and construction were still
representative of narrow-body transport aircraft
in use at that time. Two major objectives were
included in the tests: (1) to test an antimisting
kerosene fuel in an FAA program to reduce the severity
of aircraft crash fires and (2) to study structural
crashworthiness. Langley’s responsibilities
were to build the data acquisition system, collect
and analyze the data from the 350 transducers onboard,
conduct crashworthiness experiments, provide onboard
film coverage, and perform finite-element modeling
assisted by the Boeing Commercial Airplane Company.
Dryden developed the flight research program, the
remotely piloted aircraft technique, and remotely
piloted the aircraft to the impact site.
Prior to the flight test
program, Langley had conducted a series of three
Boeing 707 transport fuselage section drop tests
at the Impact Dynamics Research Facility. These
tests provided data to qualify instrumentation
and test the impact tolerance of data acquisition
hardware in preparation for the CID. Analytical
correlation with the predictions from the DYCAST
code had also been completed. The FAA had conducted
an additional drop test of a complete Boeing 707
aircraft that also had provided valuable crush
and damage information for the Langley analytical
studies.
During the preparations for
the crash flight at Dryden, 14 flights of the Boeing
720 aircraft with crews were flown, with Dryden
providing superlative efforts to develop the remote
piloting techniques necessary for the 720 to fly
as a drone aircraft. The 14 flights had 9 takeoffs,
13 landings, and approximately 69 approaches to
about 150 ft above the prepared crash site under
remote control. On the final flight (15) with no
crew onboard on December 1, 1984, all fuel tanks
were filled with the special antimisting fuel,
and all engines ran from start-up to impact (flight
time was 9 min). The aircraft was to have a sink
rate of 17 ft/sec and a longitudinal velocity of
approximately 150 knots. After the primary impact,
the airplane fuselage was to slide between a corridor
of wing openers designed to cut the wing tanks
and ensure spillage of the special fuel. The structural
crashworthy experiment would be completed before
the airplane contacted the wing openers. In the
actual impact, however, the approach to crash was
not controlled as precisely as desired and the
outboard engine of the left wing contacted the
ground first as a result of a 13∞ roll and
yaw attitude. The controlled impact was spectacular,
with a large fireball enveloping and burning the
720 aircraft. From the standpoint of the antimisting
fuel, the test was a major setback; but for Langley,
the data collected on crashworthiness was deemed
successful and extremely significant. Ninety-seven
percent of the channels were active at impact,
and the interior photography was also very successful.
One hundred percent of the cameras functioned.
The film contained unique information on the development
of fire and smoke in the interior of the aircraft.
From a human tolerance point of view, the CID test
was the simulation of a survivable crash.
Impact of Boeing 720 during
NASA and FAA Controlled Impact Dynamics Program
in 1984.
Edwin L. Fasanella and Martha
P. Robinson of Langley and E. Widmayer of Boeing
led the acquisition and analysis of data from the
CID structural objectives. Their use of DYCAST
in a series of progressively more difficult modeling
tasks was extremely successful. Following the modeling
of isolated aircraft frames and fuselage section
vertical drop tests, modifications to DYCAST were
made in predictions correlated with the results
of the CID crash. Predictions of crush and acceleration
levels agreed well with the flight data; this indicated
the validity of the building-block analysis approach
of using results from detailed models of the substructure
to form hybrid elements for inputs to more complex
structures (thereby limiting the size of the model)
and provided a useful prediction to the crash assessments.
Further analyses of transport
aircraft crash characteristics were contributed
by Edwin L. Fasanella, Karen E. Jackson, Yvonne
T. Jones, and Gary Frings of Langley and Tong Vu
of the FAA during a 30-ft/sec vertical drop test
of a fuselage section of a Boeing 737 aircraft
conducted in October 1999, at the FAA Technical
Center in Atlantic City, New Jersey. This test
was performed to evaluate the structural integrity
of a conformable auxiliary fuel tank mounted beneath
the floor and to determine its effect on the impact
response of the airframe structure and the occupants.
The test data were used to compare with those from
a finite-element simulation of the fuselage structure
and to gain a better understanding of the impact
physics through analytical-experimental correlation.
To perform this simulation, a full-scale three-dimensional
finite-element model of the fuselage section was
developed. The emphasis of the simulation was to
predict the structural deformation and floor-level
acceleration responses obtained from the drop test
of the Boeing 737 fuselage section with the auxiliary
fuel tank.
Instrumented dummies in
Boeing 720 for CID test.
Applications
Crash safety systems, devices,
and concepts that have been and continue to be
the focus of research at the Langley IDRF are capable
of moving accidents categorized as “potentially
survivable” (having serious injury and fatalities)
into the “survivable” category (minor
or no injury). The database, analytical methods,
and design guidelines provided by the extensive
Langley studies have been incorporated into design
considerations for virtually all general aviation
aircraft.
A striking example of the
application of this technology is the experience
of the Jungle Aviation and Radio Service (JAARS)
Organization, a mission service group that supports
mission aviation aircraft that operate all over
the world. Langley had conducted tests on an energy-absorbing
seat being qualified for JAARS in support of efforts
to retrofit many of the mission aviation aircraft
with the improved seat concept. Personnel from
JAARS subsequently shared feedback relative to
two real-world crash events. The information regarded
two mission aviation aircraft that were involved
in accidents in different parts of the world. Investigators
were able to determine that the attitude and impact
parameters were quite similar for the two accidents.
In one accident the aircraft had been retrofitted
in all positions with the energy-absorbing seats
and restraint systems that had been tested at the
IDRF. All the crew and passengers of this aircraft
walked away with just minor scrapes and bruises.
In the other accident, the aircraft was equipped
with standard seating, and the occupants suffered
serious back injuries and fatalities.
|