Ranges of Injury Risk Associated with Impact from Unmanned Aircraft Systems
Ranges of Injury Risk Associated with Impact from Unmanned Aircraft Systems
EAMON T. CAMPOLETTANO 0 1
MEGAN L. BLAND 0 1
RYAN A. GELLNER 0 1
DAVID W. SPROULE 0 1
BETHANY ROWSON 0 1
ABIGAIL M. TYSON 0 1
STEFAN M. DUMA 0 1
STEVEN ROWSON 0 1
0 Institute and State University , Blacksburg, VA , USA. Electronic mail:
1 Virginia Polytechnic Institute and State University , Blacksburg, VA , USA
-Regulations have allowed for increased unmanned aircraft systems (UAS) operations over the last decade, yet operations over people are still not permitted. The objective of this study was to estimate the range of injury risks to humans due to UAS impact. Three commercially-available UAS models that varied in mass (1.2-11 kg) were evaluated to estimate the range of risk associated with UAS-human interaction. Live flight and falling impact tests were conducted using an instrumented Hybrid III test dummy. On average, live flight tests were observed to be less severe than falling impact tests. The maximum risk of AIS 3+ injury associated with live flight tests was 11.6%, while several falling impact tests estimated risks exceeding 50%. Risk of injury was observed to increase with increasing UAS mass, and the larger models tested are not safe for operations over people in their current form. However, there is likely a subset of smaller UAS models that are safe to operate over people. Further, designs which redirect the UAS away from the head or deform upon impact transfer less energy and generate lower risk. These data represent a necessary impact testing foundation for future UAS regulations on operations over people.
Drone; Skull; Brain; Concussion; Cervical spine; Neck
Small unmanned aircraft systems (UAS) represent a
potentially substantial market as their use becomes
more commonplace. It has been estimated that the
economic benefit from UAS operations may exceed
$82.1 billion by 2025.2,26 Since 2008, the Federal
Aviation Administration (FAA) has been attempting to
incorporate the use of UAS within the national
airspace system (NAS).1,6,9,16,21,25 The FAA
Modernization and Reform Act of 2012 set forth directives
towards assessing the risks associated with operational
UAS.26 Part 107 of Title 14 Code of Federal
Regulations, which stipulates the regulations regarding UAS
flight, was signed into effect in 2016. Operational
specifications limit the mass of any UAS to 55 lbs.
(25 kg), maximum speed to 100 mph (45 m/s), and
maximum altitude to 400 ft. (122 m) above ground
level. The rule further states that all UAS must be
operated within visual line-of-sight of the pilot and
may not operate over persons.9
Unmanned aircraft systems applications are
currently limited to monitoring and inspection for
agriculture, power lines, and bridges, educational pursuits,
research and development, aerial photography, and
rescue operations.9 Two applications considered to be
among the largest potential markets for UAS, freight
transport and public safety applications by police
officers or firefighters, are not included in this list.2,5
These operations would require flight over people,
which the FAA has yet to allow for two major reasons:
a paucity of safety data available for risk to humans
and that no other country with UAS regulations allows
for operation over people.8,9,16
Safety standards exist in most industries to regulate
the potential for catastrophic injury and death. Of
note, current safety standards in the automotive and
sport industries have been very effective in limiting
catastrophic and fatal events. In the automotive
industry, Federal Motor Vehicle Safety Standards
(FMVSS) 208 and 214 specify minimum occupant
protection requirements for frontal and side impact
motor vehicle crashes. These two standards, combined
with the New Car Assessment Program (NCAP),
which provides consumers with ratings of occupant
2017 The Author(s). This article is an open access publication
protection by vehicle model beyond the standards,
have reduced the fatality rate associated with motor
vehicle crashes by 80% over the last 50 years.15 In the
sport industry, the National Operating Committee on
Standards for Athletic Equipment (NOCSAE) governs
standards that specify minimum performance
requirements for protective headgear. When the NOCSAE
standard for football helmets was first implemented,
the number of fatal head injuries in football was
reduced by 74%.13 Safety standards such as these have
been so effective because they limit loads transferred to
the body during impact events.
Impact safety standards employ pass-fail thresholds
for biomechanical parameters experienced by a human
surrogate. In order to be certified as safe, meaning use
of the product is unlikely to result in catastrophic or
fatal injury, impact tests of products must produce
biomechanical parameters below the threshold. It is
important to note that these thresholds represent a
specified risk of catastrophic or fatal outcome that is
considered acceptable. Passing the standards does not
imply that products are injury-proof. People still die in
car crashes and football players still occasionally die
due to head injury. Rather, the likelihood of these
outcomes are minimized by regulating impact
With the economic and public benefits spurring
UAS regulations towards more applications and
eventually flight over people, there is a need to
understand and limit the risk to human life in the event
of UAS failure in the air. The objective of this study
was to estimate the range of head and neck injury risk
to humans due to UAS malfunction by conducting live
flight and falling impact tests with a range of
commercially-available UAS. This testing represents a first
step towards developing a UAS safety standard that
minimizes threat to human life.
MATERIALS AND METHODS
Three commercially-available UAS were tested in
this study. The UAS varied in mass and maximum
speed in order to assess a range of potential energy
inputs (Table 1). It should be noted that all of these
UAS fall within the mass and speed limits regulated by
the FAA.9 The UAS were tested by performing two
kind of tests: flight tests and falling impact tests. In the
flight tests, an operational UAS was flown into the
head of the Hybrid III test dummy while a
non-operational UAS was dropped from a height of 5.5 m onto
the head of the Hybrid III test dummy for the falling
impact tests. Flight tests were conducted in an indoor
testing facility that measured 120 9 55 9 18 m
(Fig. 1). This large space represented an open and
controlled testing environment to conduct these live
flight scenarios. Each UAS was accelerated over a
distance of about 40 m in an effort to impact the
Hybrid III head at full speed. Falling impact tests were
conducted in a dedicated drop space with a high ceiling
and an upper level from which to safely drop the UAS
and achieve an impact velocity of 10 m/s. Batteries
were removed from the UAS for these falling impact
tests and replaced with an equivalent mass in order to
minimize risk of fire during the testing process. The
destruction of the UAS upon impacting the Hybrid III
or ground during testing limited the number of tests
that could be completed. Between 1 and 3 flight impact
tests were conducted for each UAS by a certified pilot
(Table 1). Falling impact tests were repeated between 5
and 7 times for each UAS model since safe operation
was no longer a limitation. A variety of impact
orientations were able to be tested to assess the spectrum
of potential impact configurations and their
All tests were conducted with an instrumented 50th
percentile Hybrid III test dummy. The Hybrid III head
was instrumented with a nine accelerometer array
(7264-2000b, Endevco, San Juan Capistrano, CA).17
Three groups of 2 single-axis accelerometers were
orthogonally mounted to the skull and 3 single-axis
accelerometers were positioned at the center of gravity
(CG) of the head. This orientation allowed for
determination of linear and rotational accelerations about
the CG of the head. The neck of the Hybrid III dummy
was instrumented with a six-axis upper neck load cell
(Denton 1716, Rochester, MI) to measure forces and
moments about the x, y, and z axes. All data were
sampled at 20 kHz (TDAS SLICE PRO SIM, DTS,
Seal Beach, CA) with a 5 g level trigger along the axis
of impact. A high speed video camera (Phantom v9,
Vision Research, Wayne, NJ) sampling at 500 frames
per second was used to determine the UAS orientation
A four-pole phaseless Butterworth low-pass filter
was applied with a channel frequency class (CFC) 1000
for force data and linear acceleration data taken from
the three accelerometers located at the CG of the head
and a CFC 600 for moment data. Acceleration data
were filtered at CFC 180 to compute rotational
The measured resultant head accelerations and the
neck forces and moments were used to assess risk of
catastrophic or fatal injury to a human. Risk of skull
fracture was determined by using the head injury
criterion (HIC) value from each impact (Eq. 1).10,11,20
HIC is calculated for time durations lasting a
maximum of 15 ms. A HIC value of less than 700 is
currently required by FMVSS and NCAP during
automotive impact tests to pass the safety standard.
All three models were well below the maximum mass and speed thresholds currently allowed by the FAA. The overall design also differed
between models, allowing for evaluation of impact energy transfer for different UAS components. More falling impact tests were conducted
than live flight tests. Unmanned aircraft systems were tested until the structure was compromised beyond repair. A variety of impact
orientations were evaluated during the falling impact phase of testing.
where a is acceleration and t is time. The potential for
risk of concussion was estimated using a risk function
that considers both linear and rotational resultant
FMVSS and NCAP consider peak axial force and
the Neck Injury Criteria (Nij) when assessing neck
injury risk.14,19 Nij considers 4 different loading modes:
compression-extension, and compression-flexion. A Nij value less than 1 is
required by FMVSS and NCAP to pass the safety
Nij ¼ Fint þ Mint
where Fz is the axial neck force, Fint is the critical neck
load for the loading condition (i.e., compression or
tension), My is the neck bending moment, and Mint is
the critical bending moment for the loading condition
(i.e., flexion or extension). The critical neck load does
not vary between tension and compression and is
4500 N. The critical bending moment is dependent on
the loading condition and is 310 N-m in flexion and
125 N-m in extension.
The head and neck injury risk functions employed in
this study consider the risk of Abbreviated Injury Scale
(AIS) 3+ injuries.3 The AIS is an injury coding system
used to classify injury severity. The AIS classifies
injuries from 1 to 6, where AIS 1 is minor, AIS 2 is
moderate, AIS 3 is serious, AIS 4 is severe, AIS 5 is
critical, and AIS 6 is maximum (not survivable).
Three successful flight impact tests were conducted
with the DJI Phantom 3. In the first flight, the
leftfront propeller blade struck the face first, causing the
UAS to rotate (Fig. 2a). Then, the mass of the UAS
itself struck the Hybrid III head above the left eye.
Linear peak resultant acceleration measured 52 g and
rotational peak resultant acceleration measured
4701 rad/s2. In the second flight, one of the UAS legs
struck the top of the head of the Hybrid III and then
deformed, causing the UAS to rotate away from the
head, end-over-end (Fig. 2b). This limited energy
transfer resulted in a peak linear acceleration of only
7.2 g and a peak rotational acceleration of 878 rad/s2.
In the last flight, the center of mass of the UAS
impacted the Hybrid III directly in the face, with
components breaking off upon impact (Fig. 2c). Peak
accelerations of 72 g and 4689 rad/s2 were measured.
These three tests represented distinct impact
orientations that led to different biomechanical outcomes,
albeit with relatively low risk overall (Table 2). The
low mass of the DJI Phantom 3 resulted in low levels
of neck loading during these live flight tests.
No live flight tests with the DJI Inspire 1 proved to
be successful. One attempt struck the Hybrid III in the
abdomen before flying off, while the final attempt
struck the shoulder and led to damage that could not
be repaired conveniently. A single successful flight test
was conducted with the DJI S1000+. One of the eight
propeller arms struck the Hybrid III in the face before
breaking off while the rest of the UAS continued its
flight away from the test dummy (Fig. 3). Peak
accelerations were measured to be 43 g and 2097 rad/s2.
Though the mass of the DJI S1000+ was the greatest
among all those tested, the center of mass rotated
around the head and was not aligned during impact,
leading to lower measured biomechanical values
(Table 2). A portion of a propeller blade from the arm
that struck the face broke off and became lodged in the
Hybrid III’s skin.
Falling Impact Tests
A total of 18 falling impact tests were conducted, 7
with DJI Phantom 3 (Figs. 4a and 4b), 6 with DJI
Inspire 1 (Figs. 4c and 4d), and 5 with DJI S1000+
(Figs. 4e and 4f). In general, impacts from falling
impact tests were of a higher severity than the live flight
test impacts (Table 3). Impact orientations varied
between the falling impact tests, resulting in a wide
range of biomechanical parameters for each UAS
model. The biomechanical parameters measured in this
study increased with increasing mass. Further, the
impact orientation with the Hybrid III varied from test
to test. By testing several possible ways in which the
UAS may fall and then impact a person’s head, a
better estimate for the range injury risk could be
Since biomechanical measurements were higher for
falling impact tests than live flight tests, estimates for
injury risk were also increased for falling impact tests.
DJI Phantom 3 was still not associated with
catastrophic head injury greater than 5%, while impacts
from DJI S1000+ were estimated to result in wider
ranges of injury, with some tests estimating 100%
injury risk (Fig. 5). Impacts from DJI Inspire 1 were not
likely to result in skull fracture. For all of the live flight
impact tests, the highest risk of injury was AIS 3+
neck injury as estimated by Nij. For the four successful
tests, these risk values ranged from 3.9 to 11.6%, with
the 11.6% risk stemming from the 3rd impact with the
DJI Phantom 3. Risk of neck injury due to impact
from DJI Phantom 3 was below 10% during falling
impact tests, while injury risks of 70% represented the
median due to falling impacts from DJI S1000+.
Impacts from the DJI Inspire 1 resulted in appreciable
neck injury risk estimations (Fig. 5).
Concussion risk during live flight tests was
estimated to be less than 5%, while the falling impact tests
resulted in a wider range of concussion risks. Several
impacts from the DJI S1000+ estimated 100% risk of
concussion (Fig. 6). The higher levels of concussive
risk relative to AIS 3+ head injury risk stem from
concussion being a lower severity injury.
UAS regulations have progressed over the last
several years to allow for increased operations; however,
flight over people is still not allowed given the lack of
knowledge regarding potential risk to humans from
UAS impacts. This research represents a first step
towards estimating risk of head and neck injury by
conducting impact tests into a Hybrid III dummy
through live flight and falling impact test
configurations. The risk functions employed in this study come
from motor vehicle occupant and football impact data
but represent a good initial estimate to inform research
geared towards future regulations.
For the DJI Phantom 3, impact severity varied with UAS orientation and was greatest (Flight C) when the center of mass striking the Hybrid III
head was aligned during impact. Despite the greater mass of the S1000+ relative to the Phantom 3, impact severity was lower for this flight
A greater number of falling impact tests than live
flight impact tests were conducted in this study.
Impacting the head of the Hybrid III test dummy with
an operational UAS proved to be a challenging task,
which is why no successful flights were conducted with
DJI Inspire 1. Further, the center of mass during these
Increasing UAS mass was associated with higher severity impacts. Variation in impact orientation for each UAS resulted in a wide range of
biomechanical values. Results are presented as Median (Interquartile Range).
live flight tests was not always aligned with the Hybrid
III head during impact. UAS leg or arm impacts
transfer less energy from the UAS to the Hybrid III
upon impact, leading to lower overall risk estimates
(Table 2). In these impacts, the center of mass of the
UAS is either redirected away from the headform or is
not aligned with the headform during impact. Beyond
this, the UAS models tested deformed upon impact,
dissipating some of the overall impact energy towards
deformation rather than transfer to the Hybrid III.
The arm of the DJI S1000+ that struck the head broke
off upon impact, while the leg of the DJI Phantom 3
deformed before deflecting away from the Hybrid III
head (Figs. 2 and 3). The initial non-centric impact,
coupled with the deformation of the UAS, likely
contributed towards the overall lower risk values observed
in live flight testing. Redirection away from the head
and deformation of the UAS after impact limit the
energy transfer to the head and are key to the safe
design of an UAS suitable for flight over people. The
variation in impact orientation and mass distribution
demonstrates the need for comprehensive testing to
fully characterize the risks associated with UAS
During falling impact tests, a variety of UAS impact
orientations were investigated. Direct impacts from the
base of the UAS, as well as indirect impacts from arms
or legs of the UAS, were tested. For the indirect
impacts, similar deformation patterns to live flight tests
were noted on high speed video, as the impacting arm
broke off or the impacting leg deformed before
redirecting the UAS away from the head. Falling impact
tests generally resulted in higher energy impacts than
live flight tests despite lower impact velocities. Though
these were free fall tests from a height of 5.5 m, the
falling impact tests were more controlled than the live
flight tests. Direct contact between the base of the
UAS, where much of the overall mass is concentrated,
and the Hybrid III head was attained in several tests.
For these tests, overall risk estimates were increased
(Figs. 5 and 6), with some of the tests estimating a high
likelihood of injury.
While redirection away from the head was observed
in most tests, it is still possible that the center of mass
of the UAS could align with the head during impact.
The 3rd live flight test with the DJI Phantom 3
represented an impact in which the center of mass was
aligned with the head during impact and resulted in
higher biomechanical parameters than other live flight
tests (Table 2). While this test likely represented the
worst-case scenario for a live flight impact from the
DJI Phantom 3, a similar test was not able to be
conducted with the DJI Inspire 1 or S1000+. The
successful impact with the DJI S1000+, in which an
arm struck the Hybrid III head before breaking off,
likely represented the best case scenario for this UAS
model. The challenges in controlling impact conditions
for these live flight impacts highlight the need for a
modified experimental approach, such as a guided
flight test rig to test UAS impact configurations.
For the DJI Inspire 1 and S1000+ , wide ranges in
injury risk were observed for the falling impact tests
(Figs. 5 and 6). The levels of risk generally varied
based on the orientation of the UAS. Impacts in which
the center of mass of the UAS, which is
non-deformable in these models, was aligned with the head
during impact were associated with higher risk than
impacts with the deformable arms or legs of the UAS.
Most of the DJI Phantom 3 is deformable, which
explains the low levels of risk estimated for those
impacts. The data from the tests in this study suggest that
UAS deformation and deflection away from the head
result in less severe impacts with lower estimates of
The current UAS regulations restrict the max speed
of UAS to about 45 m/s and the max mass to 25 kg.
These values are more than twice as large as either the
top speed or mass of the three UAS models used in this
study. Given that many of the falling impact tests
resulted in estimated risk of injury over 50%, further
consideration to the maximum mass threshold should
be taken if UAS are to be permitted to operate over
people. While these tests only provide estimates of risk,
the variety of impact configurations tested in this study
and the appreciable risk values highlight the potential
for catastrophic injury from UAS-person interaction.
It has been reported widely that the 50th percentile
Hybrid III neck is longitudinally stiffer than the human
neck. Sances et al. reported that the Hybrid III
transmits about 75% of applied force to the lower neck,
compared to only about 25% for cadavers.24 The
Hybrid III neck has limited compliance in axial
loading, which would be expected to produce greater force
measurements and head accelerations than would be
obtained with humans. It is difficult to assess how
much lower the measured forces in this study should be
to be more biofidelic, as cadaveric studies rarely record
upper neck load measurements. Similarly, there is not
a way to relate head acceleration measurements to
human measurements. Because of these unknowns, the
measured values could not be adjusted based on a
transfer function between the Hybrid III response and
cadaveric response. Though the Hybrid III dummy is
imperfect as a human surrogate, it is the most widely
available anthropomorphic test device for estimating
risk of injury and has been used in a variety of
traumatic impact loading events. Currently, no
anthropomorphic test device exists specifically for assessing
injury risk due to impacts from UAS.
This study was limited in investigating only three
available UAS, but the different masses of the UAS
tested provided a continuum from which injury
estimates using intermediate masses may be extrapolated.
The number of live flight tests was also limited in this
study. Though the falling impact tests allowed for
some improvement in impact orientation over live
flight tests, none of the tests in this study were
controlled. We recommend the use of a guided rig in future
tests in order to accurately control the orientation of
the UAS upon impact. Nij likely overestimates low end
risk, but is currently used in safety standards for
automotive applications (FMVSS and NCAP). The
underlying data that generated the concussion risk
curve in this study comes from the Head Impact
Telemetry System (HITS), which has been associated
with errors for individual acceleration measurements.
Compared to reference values from a Hybrid III head,
acceleration measurements for individual impacts have
been observed to vary by as much as 40%, though
these large discrepancies have been found for impacts
to the facemask in which the accelerometers of HITS
may become decoupled from the headform.4,12 The
effect of these errors is minimized when looking at the
overall distribution of data, as the concussion risk
function does.7 For concussive impacts, accelerations
measured by HITS were similar to those reported in
the National Football League dataset, which did not
consider low magnitude, non-injurious head impacts.18
The HITS-based risk function represents the best risk
function currently available as it considered both
concussive and non-concussive impacts. Lastly, risk
functions do not currently exist for UAS-person
interactions, so the risk functions utilized in this study
only represent estimates for injury risk.
Three commercially-available UAS were tested in
two distinct testing environments to estimate the risk
of head and neck injury to humans. By testing a range
of masses and different impact configurations between
the UAS and the Hybrid III, a better estimate for the
levels of injury risk was determined. Designs that
deform upon impact or redirect the UAS away from the
head transfer less energy and resulted in lower severity
impacts. Impacts with the smallest UAS tested in this
study, the DJI Phantom 3, were associated with
catastrophic injury risks below 5%. Some impacts
from the two larger UAS investigated in this study
estimated a high likelihood of injury and design
alterations are likely necessary prior to these UAS
being permitted to operate over people. More
controlled and robust testing in the future will work to
completely capture the level of risk associated with
UAS-person interaction. These injury risk data
represent a necessary foundation for the development of
future UAS regulations on operations over people.
The authors would like to thank the Mid Atlantic
Aviation Partnership and Institute for Critical
Technology and Applied Science for providing the UAS
tested in this study. The authors also thank Mark
Blanks, John Coggin, and Andrew Kriz for
coordinating and assisting with the live flight testing.
CONFLICT OF INTEREST
The authors declare no conflicts of interest
regarding the methods used in this study or the findings
included within this manuscript.
This article is distributed under the terms of the
Creative Commons Attribution 4.0 International
which permits unrestricted use, distribution, and
reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons license, and
indicate if changes were made.
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