Shock Waves (2003) 12: 267 275
Digital Object Identifier (DOI) 10.1007/s00193-002-0162-1
Shock waves in aviation security and safety
G.S. Settles1, B.T. Keane1, B.W. Anderson1, J.A. Gatto2
1
Gas Dynamics Laboratory, Mechanical and Nuclear Engineering Department, 301D Reber Bldg., Penn State University,
University Park, PA 16802, USA
2
W.J. Hughes Technical Center, Federal Aviation Administration, Atlantic City, NJ 08405, USA
Received 22 July 2001 / Accepted 19 July 2002
Published online 4 November 2002 © Springer-Verlag 2002
Abstract. Accident investigations such as of Pan Am 103 and TWA 800 reveal the key role of shock-wave
propagation in destroying the aircraft when an on-board explosion occurs. This paper surveys shock wave
propagation inside an aircraft fuselage, caused either by a terrorist device or by accident, and provides
some new experimental results. While aircraft-hardening research has been under way for more than a
decade, no such experiments to date have used the crucial tool of high-speed optical imaging to visualize
shock motion. Here, Penn State s Full-Scale Schlieren flow visualization facility yields the first shock-motion
images in aviation security scenarios: 1) Explosions beneath full-size aircraft seats occupied by mannequins,
2) Explosions inside partially-filled luggage containers, and 3) Luggage-container explosions resulting in
hull-holing. Both single-frame images and drum-camera movies are obtained. The implications of these
results are discussed, though the overall topic must still be considered in its infancy.
Key words: Shock waves, Aviation security, Terrorism, Schlieren, High-speed cinematography, Explosions
1 Introduction due to shock overpressure (hull-holing), followed by exten-
sive fuselage cracking driven by the hull service pressur-
1.1 Aviation security
ization. The report also describes shock-wave propagation
through internal ducts and channels within the aircraft,
The rise in worldwide terrorism, especially directed at
causing damage at locations remote from the primary ex-
commercial air transportation, has required that measures
plosion due to pseudo-explosive sources. However, the
be taken to harden aircraft against catastrophic in-flight
shock wave patterns and gas dynamics described in this re-
failure due to concealed explosives. The Pan Am 103 disas-
port are based mostly on conjecture; no attempt was made
ter (Anon. 1989) and several other incidents in the 1980 s
at the time to predict such shock motion numerically or
and 90 s costing many hundreds of lives (Gatto 1992)
to reproduce it experimentally. Since that disaster, sev-
showed that the modern terrorist is sophisticated enough
eral large-scale experiments have examined the damaging
to obtain and conceal modest amounts of explosive ca-
effects of blast waves due to explosions onboard aircraft.
pable of destroying an entire aircraft. Most recently, the
Unfortunately, despite these experiments, the actual gas-
September 11, 2001 terrorist attacks in the United States
dynamic phenomena of such an onboard explosion remain
involved at least one case (United Flight 93) where terror-
poorly understood.
ists claimed to have smuggled a bomb on board. Accord-
ingly, research to make commercial airframes less vulner-
able to internal explosions has been conducted for several
1.2 Literature review
years under the FAA s Aircraft Hardening Program.
1.2.1 Full-scale airframe and luggage-container tests
The Pan Am 103 investigation revealed the key role
of blast overpressure and shock wave propagation in the
A primary goal of full-scale airframe experiments has been
disintegration of the aircraft within seconds after the ex-
to realistically simulate scenarios of terrorist bombings. To
plosion. The detailed accident report (Anon. 1989) sug-
this end, several explosive tests have been performed on
gests that the aircraft hull was shattered near the blast
actual aircraft structures to analyze damage and to ac-
Correspondence to: G.S. Settles
quire data on blast loading in such large explosions. For
(e-mail: gss2@psu.edu)
example, in May 1997, a retired Boeing 747 was sacri-
An abridged version of this paper was presented at the 23rd In- ficed at Bruntingthorpe, UK, for the purpose of validating
the most recent aircraft hardeningtechniques employed by
ternational Symposium on Shock Waves at Fort Worth, Texas,
from July 22 to 27, 2001. the FAA and DERA (Flitcroft and Harris 1998; Morrocco
268 G.S. Settles et al.: Shock waves in aviation security and safety
1997). Aging military aircraft were also used to study blast energy, thus amplifying pressure levels and increasing the
damage in terms of shock holing of the hull, quasi-static risk of damage to surrounding structures (Gvozdeva et
pressure loading, and cabin pressurization (Gatto 1992; al. 1999; Rough and Skews 1999). Several materials and
Ashley 1992; Gatto et al. 1995). High-speed direct cinema liners have been tested for use in HULDs, the most suc-
observations, post-blast inspection, and pressure trans- cessful of which tend to be composite materials with high
ducers were used in these tests, but no optical imaging strength-to-weight ratios. Despite some success, research
of shock wave motion was attempted. Pointwise pressure on blast-mitigating materials continues due to the exces-
data are sometimes lost due to the harsh test environ- sive weight and cost of present HULDs.
ment, and are often not very revealing of the key flowfield
phenomena. Furthermore, such full-scale tests are very ex-
pensive, difficult to repeat, and specific to the aircraft type 1.2.3 Numerical simulations
and explosion scenario employed.
Further full-scale research has examined the hardening Numerical modeling of aircraft explosions is important
of aircraft luggage containers. Pan Am 103 and similar in aircraft hardening research. While some computational
aviation terrorism incidents show the need for a luggage fluid dynamic (CFD) simulations have concentrated only
container capable of withstanding a significant internal on flowfield modeling (Mundy et al. 1995) or structural
explosion. Thus, subsequent research focused on meth- damage (Sanai et al. 1995), several others have attempted
ods to vent explosive energy from these containers and to simultaneously predict both interior blast wave behav-
to contain explosive overpressure using blast-mitigating ior and related structural damage (Ashley 1992; Bhara-
materials (Morrocco 1997). Experiments also examined tram et al. 1995; Chen 1997). These computations typi-
the effects of luggage and charge location on the pres- cally involve the use of codes that link together the fluid
sure loading inside luggage containers, as well as damage and structural mechanics. Some numerical simulations
due to luggage fragmentation (Gatto, 1996; Anon. 1993). also attempted to model blast wave patterns in an aircraft
Under the FAA Aircraft Hardening Program, Jaycor Inc. interior (Baum et al. 1993). However, it is very challenging
and Galaxy Scientific Inc. developed prototype HULDs to model 3-D fluid and structural response simultaneously,
(Hardened Unit Load Devices) capable of containing the and to provide sufficient resolution to see shock wave re-
blast of a luggage bomb having a certain size and config- flection patterns. Furthermore, such numerical simulations
uration (Anon. 1999). Initial test results were promising may involve simplifying assumptions that detract from
and some airlines agreed to conduct operational trials us- the accuracy of the result. Thus, in order to predict ex-
ing these units (Anon. 1999; Weinstein 2000). However, plosive damage and to understand blast wave behavior
these HULDs are heavier and more costly than standard thoroughly, experimental validation of such computations
luggage containers, and the FAA has not yet developed a is necessary. Only a few experimental datasets are avail-
deployment plan for them in airline service. Further effort able for this purpose, though they are much needed. Like-
appears necessary before hardened luggage containers are wise new computational methods (e.g. Long and Anderson
accepted by the aviation industry. 2000) show promise.
1.2.2 Blast mitigation and shock propagation 1.2.4 Scale-model tests and interior blasts
in porous media
Following the decades-old international ban on above-
In efforts to develop HULDs, researchers have studied the ground nuclear testing, numerous scale-model studies have
blast-mitigating effects of luggage and of liner materials to used ordinary chemical explosives to simulate nuclear
protect luggage containers against explosive damage. Ex- blast wave effects according to established scaling laws
periments show that luggage plays a key role in absorbing (Glasstone 1962; Witt 1979; Naz and Parmentier 1991).
blast energy (Anon. 1993; Wilson 2001). However, the de- These experiments illustrate the usefulness of scale-models
gree to which luggage mitigates the blast wave depends to simulate up to a point full-scale blasts. However,
on the material and contents of the luggage. Moreover, simulating blast effects and structural response simulta-
it is unknown whether the explosive shock overpressure neously at model scale is seldom possible. Further, the
or the hurling of hard luggage fragments at high speed detailed geometry of an aircraft cabin or luggage compart-
is the primary damage-inducing mechanism in the case ment is difficult to simulate except at or near full-scale.
of a luggage-container explosion, though one study favors The emphasis also shifts dramatically from open-air
the latter (Wilson 2001). In this complicated scenario, the nuclear-weapons tests to interior shock wave propagation,
presence of luggage may even have a detrimental effect on such as inside an aircraft fuselage. Depending on Mach
airframe integrity. number, the peak overpressure behind a shock wave re-
Related basic studies have considered shock wave prop- flected from an interior wall can be 2 8 times the incident
agation in porous media. Experiments show that most overpressure. Even-higher overpressures occur where the
porous materials attenuate shock waves and thus reduce wall geometry produces shock focusing (Loftis 1993). Vari-
the potential for damage (Anon. 1993; Lind et al. 1999). ous recent studies (Phan and Stollery 1985; Marconi 1994;
However, some tests reveal an adverse effect: certain foams Neuwald et al. 1997; Reichenbach and Neuwald 2001;
and textiles may act to reflect shock waves or store blast Heilig and Igra 2001) amply illustrate that shock motion
G.S. Settles et al.: Shock waves in aviation security and safety 269
in interior spaces quickly becomes very complicated, and 2) to work at or near full-scale in order to avoid scaling
is poorly understood compared to simpler outdoor blast problems and to be applicable to full-scale field tests, 3)
effects. For example, Heilig and Igra (2001) demonstrate to provide experimental validation for CFD studies, and
how interior baffles and wall perforations can attenuate 4) to gain a better understanding of blast wave behavior
shock wave strength, draining the shock energy by vortex in aircraft environments. In that present space does not
formation. Similarly, the details of blasts in multi-room allow a complete presentation of these experiments, the
interiors (Reichenbach and Neuwald 2001) are muted by reader is referred to Settles et al. (1998); Keane (2001);
distance from the blast center. Settles et al. (July 2001, Nov. 2001); Anderson (2002) for
more detail.
1.2.5 Aviation safety
2 Experimental methods
Finally, we note that explosions onboard aircraft are not
always deliberate terrorist acts, but also can be acciden- 2.1 Full-Scale Schlieren System
tal. TWA Flight 800 was lost, with all 230 persons aboard,
due to a fuel-air explosion in its center fuel tank (Anon. The Penn State Full-Scale Schlieren System is based on
2000). The overpressure ruptured the tank and damaged the lens-and-grid principle first developed by Schardin
the structure sufficiently to cause in-flight breakup of (Settles 2001). Filling a university warehouse building, it
the aircraft. A 1990 Philippine Airlines incident and the is the largest indoor schlieren system in the world. As
March 2001 explosion of a Thai Airways Boeing 737 on shown in Fig. 1, a large front-lit retroreflective source grid
the ground are thought to have been of similar origin. covers one wall of the building. Vertical black gridlines
Likewise the July 2000 Concorde accident resulted from of width 5.08 mm are silk-screened 5.08 mm apart onto
the high-speed impact of an exploded tire fragment upon the retroreflective material. Opposite this source grid, a
a wing fuel tank, causing water-hammer shock waves in large-format camera lens produces a cutoff grid size of 20
the fuel that led to a tank rupture and subsequent catas- × 25 cm, and an image plane slightly larger. The cutoff
trophic fire (Morrocco and Sparaco 2001). grid is a precise negative image of the source grid that,
when properly aligned, provides multiple schlieren cutoffs
for the many source-grid lines that make up this multiple-
1.3 Objectives of present research source schlieren system. The test area, located roughly
halfway between the source grid and the lens, is centered
Our principal observation from the above literature re- 2.4 m above the floor. Its nominal field-of-view is a rect-
view is that traditional optical methods are conspicuously angle 2.3 m high by 2.9 m wide.
absent from all experiments to date on aircraft harden- For present purposes, the source grid is illuminated by
ing. Optical flow visualization and measurement are the a Viewstar, Inc. Model 955-5 high-speed-flash light source
most valuable experimental tools in the general study of for single-frame schlieren imaging. A xenon flash tube
blast wave propagation (Dewey 1989 and 1997). Their discharges 5 10 Joules in about 5 µs when a microphone
application to aircraft hardening research does not nec- placed an arbitrary distance from the center of the sub-
essarily require full scale, but a reasonably-large scale is ject explosion provides the electronic trigger signal. While
needed if the complicated geometry of the problem is to three time-delayed, colored flashes are available from the
be well-represented. This rules out table-top optics, but in light source, only a single white or blue flash was used in
fact large-scale outdoor optics for blast studies, based on most of the experiments reported here. Front-lighting of
shadowgraphy or background-distortion schlieren princi- objects in the test area is also provided by two additional
ples, have been known since at least the 1950 s (Edgerton studio flash-heads located off-axis.
1958; Dewey 1989 and 1997; Settles 2001) For the single-frame photographs shown here, a Fresnel
Optical imaging of blast waves can thus be useful in lens near the schlieren image plane allows re-focusing the
aviation security studies for several reasons. First, it pro- test area into a Pentax 67 medium-format camera. Pho-
vides a key physical picture of the phenomena at hand. tography is done using 120-size ISO 800 color negative film
Second, high-speed images can yield quantitative data push-processed to approximately ISO 3200. Photographs
such as shock speed and Mach number, from which flow are taken in the darkened laboratory with the camera
properties behind the shock wave can be found. Finally, focal-plane shutter open during the explosive event.
such optical data from aircraft hardeningexperiments may Shock waves were generated by the detonation of an
be used to support quantitative data from other (e.g. oxygen-acetylene gas mixture in a small (about 10 cm di-
pointwise) instruments, and to check the accuracy of nu- ameter) toy balloon ignited by a nichrome-wire glow-plug.
merical predictions. This technique is advantageous in the present case for sev-
Accordingly, the remainder of this paper describes ex- eral reasons. First, it provides a virtually smoke-free blast
periments at Penn State involving full-scale optical imag- that does not contaminate the facility. Second, it gener-
ing of shock motion in aircraft interior scenarios. The ob- ates no hazardous shrapnel. Third, it is easily-repeatable
jectives of this research are 1) to demonstrate the impor- in terms of blast strength. Fourth, storage and handling
tance of optical methods especially schlieren and shad- of high explosives is not required. Finally, the detonation
owgraph techniques - to observe shock motion in this field, of even small amounts of oxygen-acetylene gas mixture,
270 G.S. Settles et al.: Shock waves in aviation security and safety
Fig. 1. Schematic of the Penn State Full-Scale Schlieren System
requiring ear protection only, produces shock waves that
are clearly visible to the Full-Scale Schlieren System.
Experiments were performed to determine the explo-
sive yield of these small gas detonations in terms of a
TNT-equivalent mass. It was found (Keane, 2001) that
the range of charges tested produced yields equivalent to
approximately 1 10 grams of TNT. This knowledge is sig-
nificant because it provides a means of scaling the blast
waves produced by these gas detonations, which may be
important in comparing the present experiments with the
results of other aircraft-hardening studies.
Resulting shock Mach numbers were also calculated
from multi-flash image data. This revealed that, for the
range of balloon sizes tested here, the blast waves had av-
erage Mach numbers between 1.0 and 1.2 at a distance
of approximately 1 meter from the blast center. Although
these are weak shock waves far weaker than those pro-
duced by an actual terrorist bomb their clear visibility in
the Full-Scale Schlieren System justifies their use to study
Fig. 2. Side view of a full-scale under-seat blast in a mockup
shock wave motion in an aircraft interior environment.
aircraft cabin (original in color)
2.2 Drum camera
In our most recent experiments, high-speed schlieren
imaging was done using a Cordin Model 377 rotating-
drum framing camera. In this case, continuous illumina-
tion was triggered by the explosion and was provided by a
Cordin xenon flashlamp of variable preset duration. The
drum camera uses twin strips of 35 mm ISO 3200 black-
and-white film to produce up to 500 frames of a high-speed
event at frame rates up to 200,000 fr/s. Here, however,
150 frames at 30,000 fr/sec proved sufficient. The result-
ing frames are individually digitized and assembled using
Adobe Premiere software in order to yield a digital mpeg
movie of the imaged event.
3 Results and discussion
Fig. 3. Front view of a full-scale under-seat blast in the Several mockup aircraft interior settings have been de-
mockup aircraft cabin (original in color) signed and fabricated to represent actual aircraft envi-
G.S. Settles et al.: Shock waves in aviation security and safety 271
Fig. 4a d. Blasts in mockup luggage container show the progression of shock reflections (originals in color)
ronments for the purposes of simulating terrorist bomb- an acetylene/oxygen detonation was produced under the
ings onboard commercial aircraft. These settings provide front seat row.
a clear optical path allowing shock wave motion to be
studied on a realistic scale.
3.2 Shock wave generation
3.1 Aircraft passenger cabin
Figure 2 reveals several aspects of shock wave motion in
this scenario. First, the strong initial shock front above
Our first experiments consider the scenario of an explo- the chest of the male mannequin indicates that much of
sion under an aircraft seat in a side-view of a mockup the blast energy is channeled upward between the two seat
aircraft-cabin environment. The motivation for this comes rows. Second, despite the complicated geometry, a near-
from actual terrorist attempts at aircraft sabotage involv- spherical wavefront is produced (it can be seen diffracting
ing bombs placed in the passenger cabin, especially Ramzi over the head of the female mannequin). Third, multiple
Yousef s failed Dec. 1994 attempt to bring down a Philip- shock reflections are observed between the two seat rows,
pine Airlines flight with a nitroglycerin bomb under a seat. indicating shock reverberation effects. Finally, the direct
Baum et al. (1993) numerically simulated such an explo- light from the explosion is confined to the fireball itself
sion in the first class section of a Boeing 747. However, and does not fogor otherwise obstruct the schlieren image.
prior the present work, no experiment comparable to this Of course, a terrorist-scale bomb would likely destroy the
computation had ever been performed. seat under which it was placed, but in our simulation only
To simulate the aircraft cabin environment, two rows minor scorching damage occurs.
of actual jetliner seats were positioned in the test area A second set of tests involved a front view of the same
of the Full-Scale Schlieren System (Fig. 2). Two full-sized passenger-cabin scenario, including a mockup of a full-
department-store mannequins were placed in these seats scale aircraft bulkhead to study the shock wave reflection
to represent passengers. By the method described earlier, that would likely occur at the cabin sidewall (Fig. 3). The
272 G.S. Settles et al.: Shock waves in aviation security and safety
Fig. 5a d. Blast in a mockup luggage container demonstrating holing of the aircraft hull (originals in color)
Pan Am 103 accident report (Anon. 1989) suggested that delay time following the blast is about 1.5 ms in Fig, 4a,
a shock reflection from the inner hull propagated back and 2 ms in Fig. 4b. Figure 4c shows the shock front just
into the fuselage, reverberated, and led to extensive dam- after reflection from the container ceiling, with reflections
age at remote sites. Multiple shock reflections are indeed also occurring at the walls of the container (2.6 ms delay).
observed here, accompanying the spherical primary wave Finally, Fig. 4d reveals the wave pattern at 3.2 ms delay,
from the blast. with multiple wave reflections leading to a complicated
The observation of near-spherical blast propagation, shock reverberation pattern. This complication continues
despite the complicated geometry in Figs. 2 and 3, is im- to increase with time, though the remaining images in this
portant in that it suggests the utility of a simplified view sequence are omitted for brevity; see Keane (2001).
of fuselage interior shock wave propagation. Computations
(Baum et al. 1993) also appear to indicate this, though at
much lower resolution.
3.4 Luggage container blowout and hull holing
In another set of experiments, a blowout of the diagonal
3.3 Luggage container
luggage-container wall and holing of the aircraft hull were
simulated. The motivation for this comes from the Pan
A 60%-scale model of a luggage-container and lower wide- Am 103 disaster, in which the terrorist bomb was located
body aircraft fuselage cross-section was constructed (Figs. inside a suitcase near the outer wall of the luggage con-
4 5) and partially filled with typical luggage bags. Explo- tainer, and the hull-holing from the explosion led to the
sions beneath the bags were imaged with different micro- loss of the aircraft. To simulate this, the present explosions
phone (trigger) locations in order to obtain a sequence were positioned along the diagonal luggage-container wall
of images demonstrating the interior shock wave motion. inside a pre-vented hard-shell suitcase. Given the small
Figure 4a shows a simple spherical shock front, with inte- yield of our explosions, the jagged hull hole also had to be
rior reflections from the bags beginning to intersect. The simulated. As in the previous experiment, a sequence of
G.S. Settles et al.: Shock waves in aviation security and safety 273
Fig. 6. Blast underneath passenger seat, showing five selected frames from drum-camera footage. Timing of the frames shown
here is at 0.2, 0.8, 2.47, 3.4, and 4.5 ms after blast initiation (left-to-right, top-to-bottom). 150 such frames were taken at a
frame rate of 30,000 fr/s
Fig. 7. Blast inside a partially-filled luggage container (60% scale model), showing six selected frames from drum-camera footage.
Timing of the frames shown here is at 0.1, 1.2, 1.6, 2.1, 3.4, and 3.8 ms after blast initiation (left-to-right, top-to-bottom). 150
such frames were taken at a frame rate of 30,000 fr/s
274 G.S. Settles et al.: Shock waves in aviation security and safety
single-frame images showing the progression of the blast a valuable tool that should not be neglected in the future.
wave was obtained (Keane 2001). However, the understanding of the complications inherent
Selected results of these hull-holing tests are shown in in internal blast effects is still rudimentary and requires
Fig. 5. Figure 5a demonstrates the initial release of energy additional effort, so the present paper should be consid-
from the explosion at a time delay after blast initiation ered a report of work in progress. In particular, more fun-
of about 1.3 ms. Figure 5b shows the shock patterns 0.5 damental study of interior explosions is needed.
ms later. Here, a series of shock reflections occurs at the Further work is also planned using the current appa-
upper-left wall of the container and the other interior re- ratus, including the extraction of quantitative shock-wave
flections are expanding. Figure 5c is at 2.6 ms delay, and strength data from drum-camera results like those shown
finally Fig. 5d, at 3.3 ms delay, reveals that the shocks in Figs. 6 and 7. Piezoelectric transducers in these ex-
have reflected from all container walls creating a compli- periments will also provide additional quantitative data,
cated internal wave pattern. Shock motion outside the hull and direct comparison with computational flowfield sim-
is also clearly seen in all 4 frames, but the high-speed jet ulations will be made. Finally, an outdoor optical demon-
of hot gases from the hull hole (Mundy et al. 1995) is not stration in an actual aircraft-hardening test is also con-
properly simulated in these experiments. templated, using simple, robust optics like those first pro-
posed by Edgerton (1958).
3.5 Drum-camera results
Acknowledgements. This research was supported by FAA
Grant 99-G-032. The authors thank J. D. Miller, L. J. Dodson-
The aviation security scenarios illustrated in Figs. 2 and
Dreibelbis, and Dr. H. Kleine for their assistance and advice.
4 were also imaged using the drum camera, which typi-
cally yielded 150 frames of a given event at 30,000 fr/s.
The many additional frames came at the price of reduced
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