Crash survivability is one of the key features to be attended during the design of an airworthy aircraft. Belly/crash landing is the most common phenomenon to be considered in developing a crashworthy product. That makes it essential to have redundant structure to enhance the safety of occupants and also limit the damage to easily repairable state in case of such event. Even from the certification point of view, it is necessary to investigate this event by test/analysis. Recent development of advance computing and their capability to simulate such phenomenon to acceptable accuracy under given conditions conveniently replace the need for test which is otherwise costly. At the same time, one has to be cautious while selecting the modeling parameters to simulate the condition near to reality. Taking advantage of this feature an effort is made to simulate the belly landing and its consequences on the structure complying to the guidelines of the federal aviation regulations. This paper presents the methodology adopted to successfully simulate the belly landing phenomenon for a light transport aircraft flying prototype.

1. International Journal of Research and Scientific Innovation (IJRSI) | Volume IV, Issue VIS, June 2017 | ISSN 2321–2705
www.rsisinternational.org Page 81
FE Based Crash Simulation of Belly Landing of a
Light Transport Aircraft
Pratheeksh M 1*
, Dr. D L Prabhakara1
, Akshatha H T 2
1
Sahyadri College of Engineering and Management, Adyar Mangaluru, Karnataka, India, 575007
2
Centre for Civil Aircraft Design and Development, CSIR-National Aerospace Laboratories, Bengaluru, Karnataka. India
Abstract: Crash survivability is one of the key features to be
attended during the design of an airworthy aircraft. Belly/crash
landing is the most common phenomenon to be considered in
developing a crashworthy product. That makes it essential to
have redundant structure to enhance the safety of occupants and
also limit the damage to easily repairable state in case of such
event. Even from the certification point of view, it is necessary to
investigate this event by test/analysis. Recent development of
advance computing and their capability to simulate such
phenomenon to acceptable accuracy under given conditions
conveniently replace the need for test which is otherwise costly.
At the same time, one has to be cautious while selecting the
modeling parameters to simulate the condition near to reality.
Taking advantage of this feature an effort is made to simulate the
belly landing and its consequences on the structure complying to
the guidelines of the federal aviation regulations. This paper
presents the methodology adopted to successfully simulate the
belly landing phenomenon for a light transport aircraft flying
prototype.
Keywords: crash landing, belly landing, FE simulation, Impact
energy
I. INTRODUCTION
elly landing is a most common type of crash landing
encountered in a life time of an aircraft. This may occur
due to two practical reasons: either an emergency case where
the landing gear does not extend while landing due to
mechanical malfunction or a case where the pilot forgets to
engage the landing gear before landing. In both cases the
landing would be performed at an approach landing speed
under controlled conditions. As per the certification
requirement for a light transport aircraft(LTA), the
crashworthiness of the aircraft during belly landing needs to
be shown either by test or analysis corresponding to aircraft
descend velocity. To test/ simulate the condition one needs to
take extra care in idealizing the situation in
practical/numerical environment. Few noteworthy works have
been carried out in this area.
A Adams and H M Lankarani [1] have evaluated the fuselage
crashworthiness by conducting impact test to determine the
stability of fuel tank which is mounted beneath the floor and
to access the damage of the aircraft structure. To understand
the impact behavior of the structure better, the test data were
compared with finite element (FE) simulation results. For this,
a 3d FE model of the fuselage section was built and
simulation was observed to access the structural deformation
and acceleration levels in floors. Mou Haolei et. al [2] have
studied crashworthiness of fuselage section with composite
skin. Fuselage section was subjected to a dynamic analysis
using LS-DYNA under different ply numbers and ply angles.
By the simulation they could successfully come out with an
improvement in crashworthiness by selecting appropriate ply
numbers and angles. Karen E. Jackson and Edwin L.
Fasanella [3] evaluated crashworthy behavior of fuselage
section through impact testing and finite element simulation.
The model was divided into 5 parts and one fifth of the model
was taken for analysis. The main aim of the analysis was to
design the fuselage section according to impact design
requirements by maintaining ground acceleration and impact
velocity.
Literatures [4] [5] [6] in the end also refer to some noteworthy
work in this area and helps in giving a better insight to the
concepts like crashworthiness and belly landing.
To simulate the actual condition by test or analysis software
are equally challenging process. One needs to understand the
kinematics of the phenomenon to perfectly simulate the
landing phenomenon. The methodology differs from aircraft
to aircraft depending on their geometrical, functional and
operational requirements. In this work an effort has been
made to simulate the belly landing for a 14 seater transport
aircraft. Various parametric studies have been carried out to
come out with the methodology to effectively simulate the
crash landing. This paper also highlights in detail the stepwise
procedure and assumptions adopted. The work can be
considered as the first step towards building the aircraft
numerical model and simulation procedure to assess the
occupant and system safety for future analysis.
II. DESIGN FEATURES OF BELLY
The present LTA is a single cell fuselage construction with an
external fairing attached at the bottom called “belly fairing”. It
extends from front of the fuselage near cockpit to rear of the
aircraft with varying cross section (figure 1). The fairing is a
simple frame- skin construction with stiffened access panels.
It is mainly made up of aluminum alloy. Apart from housing
the system components like fuel, electrical, landing gear
wheel well etc., it also serves as an external shielding to
B

2. International Journal of Research and Scientific Innovation (IJRSI) | Volume IV, Issue VIS, June 2017 | ISSN 2321–2705
www.rsisinternational.org Page 82
protect the cabin occupants in the event of crash landing as
can be seen from figure 1.
Figure 1: LTA with a typical cross sectional view.
III. BELLY LANDING PHENOMENON
As belly is integral with the aircraft and is the primary landing
device responsible for absorbing the impact in case of crash
landing, the airworthiness requirement to be demonstrated as
per the regulatory requirements. The present LTA has an
approach velocity of 180 ft/s [7] with a descend angle of 3
degree which results in a descent velocity of 9.83ft/s. The
landing can be visualized as a vertical drop low velocity
impact phenomenon at a tail down inclination of 6 degrees as
depicted in figure 2.
Figure 2: Belly landing phenomenon.
IV. IMPACT ANALYSIS METHODOLOGY
The impact being a transient phenomenon necessitates the
analysis to be carried out in the dynamic environment. The
impact classification depends on the impact velocity. The
velocity below 10 m/sec can be categorized as low velocity
impact assuming the force equilibrium as quasi static [8] as
depicted in figure 3. In this purview current problem falls into
low impact study category meaning the impact response and
energy absorbing capability of the structure depends upon the
area and size of the impacting surface [9]
Figure 3: Impact velocities for sample events [10].
The impact between the rigid surface and elastic aircraft belly
is investigated with the more emphasis on modeling and
simulation issues. The commercially available FE solver
abaqus [11] with non-linear analysis capability is used as a
solver.
4.1. FINITE ELEMENT MODELING
The belly model is built using Altair Hypermesh. The aircraft
being thin walled structure 2-D QUAD elements were used to
simulate the skin as well as frames. Several approximations
were followed while modeling the structure. Sensitivity of the
mesh density is checked with an initial iterative analysis.
Accordingly finer mesh density is adopted only in and around
the impact zone which is the rear side of the belly. The other
parameters such as height of the impact, time interval were
also assessed independently. Initially the belly was simulated
along with the access panels. It was found that the presence or
absence of these panels did not alter the global results
appreciatively, also the micro level details were not of
immediate concern and hence to reduce the computational
complexity, the panels were merged with the skin of belly.
Another important approximation was the representation of
whole aircraft which makes the analysis computationally
expensive and time consuming. Hence the fuselage portion of
the aircraft was omitted and was represented in terms of
mass/inertia and Center of gravity.
4.2. FAILURE MODEL
It is essential to incorporate the appropriate constitutive model
for failure of the material. As the impact loads are material
strain rate dependent, different material modules were
incorporated. Initially, an elastic perfectly plastic material was
chosen, which resulted in fairly higher stress as the material
reaches failure upon crossing the yield. As the simulation is
significant with respect to the material failure, to utilize the
strain absorbing capability of the material, Johnson-cook
constitutive failure model equation was chosen. Here the yield
stress becomes the function of the strain rate and hence yields
a reasonable result. However, the temperature effect was
ignored. The equivalent material parameters used in the
analysis are also indicated.
   
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pn
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eq
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
Table 1: Johnson cook parameters [12]
Description
Values used
in analysis
Abbreviation
Johnson cook failure parameters
Johnson cook stress - σ

3. International Journal of Research and Scientific Innovation (IJRSI) | Volume IV, Issue VIS, June 2017 | ISSN 2321–2705
www.rsisinternational.org Page 83
Plastic strain - εp
Temperature - T
Melt temperature - Tmelt
Room temperature - Troom
Johnson cook damage - D
Hydrostatic pressure - p
Equivalent stress -
σeq
Yield stress 369 MPa A
Hardening constant 684 MPa B
Hardening constant 0.73 n
Thermal softening 1.7 m
Rate dependence 0.0083 C
Johnson cook fracture parameters
Rate dependence 1 €0
Strain parameter 0.035 D1
Strain parameter 0.035 D2
Strain parameter -1.5 D3
Strain parameter 0.011 D4
Strain parameter 0 D5
4.3. DYNAMIC CONTACT ANALYSIS
The analysis is based on the impact between 2 bodies. The
impactor (aircraft belly) as an elastic body with an ability to
absorb kinetic impact energy and impacting surface is a rigid
body which is a presentation of hard soil/concrete/ground. It is
shown in figure 4. The impact study is based on the contact
mechanics approach where the contact stresses develop
between the colliding bodies and dynamic response of the
system is involved in assessing the impact duration. General
contact between the surfaces is chosen.
Figure 4: Typical model of the belly.
V. SIMULATION
The model was initially simulated for static and normal mode
analysis approach to check the stiffness and strength
characteristics of the model. With this confidence and
appropriate approximations the model was analyzed using
Abaqus with explicit method. From the analysis, it was seen
that the impacting region would certainly fail at the instance
of impact, but the impact loads get diffused to adjacent frames
and hence there is a smooth transition of the loads with local
dents and buckling of the frame at selected instance of time
are shown in the figure 5. The deformations shown are scaled
up to bring in some clarity. Hence the actual deformations are
relatively low compared to what is seen in the figures. The
deformed belly with stress distribution is shown in figure 6.
.
Figure 5: Deformation stress pattern at t=0s,0.015s,0.05s
Figure 6: Displacement contour of belly in deformed state with formation of dents.

4. International Journal of Research and Scientific Innovation (IJRSI) | Volume IV, Issue VIS, June 2017 | ISSN 2321–2705
www.rsisinternational.org Page 84
Figure 7: Velocity plot at impact region. Figure 8: Plot for kinetic energy.
Figure 9: Acceleration v/s Time during contact.
The velocity and energy plots with respect to time (in sec)
clearly indicate the energy absorbing capability of the
structure thus creating minimum impact to the occupants
inside the fuselage (figure 7 and 8). The energy is absorbed by
larger deformation.
The acceleration was monitored at the fuselage contact points
(figure 9). They were found to be gradually decreased with the
time. However monitoring the acceleration at occupant seats
in future analysis would give an appropriate insight into the
crashworthiness of the whole aircraft. With the present results
it is clear that for the given descend velocity, the impact area
needs to be repaired locally in case of such an event.
However, it can also be inferred that the aircraft operation and
occupant safety is not hindered by belly deformation during
belly landing, the present analysis stage is premature to
quantify them.
VI. CONCLUSION
The present work is a clear demonstration of the procedure
and methodology that can be adopted for the belly landing of
the present Light transport aircraft flying prototype. Various
approximations and assumption made to achieve a reasonable
simulation for the performance parameters as per the
regulatory requirements are explained in detail. The main
motivation behind this work was to develop a valid model for
future assessment of occupant safety and hazard analysis
during unintentional belly landing for the purpose of
certification. In the process, the belly fairing design was also
found to be crashworthy without leading to catastrophic
failure. Through the present low velocity impact simulation, a
workable model platform has been successfully built for
further investigations thus helping the certification of the
aircraft for belly landing conditions.
REFERENCES
[1]. A Adams, H M Lankarani, “A modern aerospace modeling
approach for evaluation of aircraft fuselage crashworthiness”,
National Institute for Aviation Research, Wichita State University,
Wichita, Kansas, USA, 08 July 2010.
[2]. MOU Haolei, ZOU Tianchun, FENG Zhenyu, REN Jian,
“Crashworthiness Simulation Research of Fuselage Section with
Composite Skin”, Tianjin Key Laboratory of Civil Aircraft
Airworthiness and Maintaenance, Civil Aviation University of
China, Tianjin 300300, P.R. China.
[3]. Karen E. Jackson, Edwin L. Fasanella, Sortis Kellas, “Crash
simulation of a 1/5th-scale model composite fuselage concept”,
U.S Army Vehicle Technology Center, Hampton.
[4]. Akhilesh Kumar Jha, S. Sathyamoorthy, Bharath Kumar,
Laxminarayan. K, “Impact Analysis of Mini UAV during belly
landing”, HTC 2012.
[5]. Hans Magnus Thorsen, “Belly Landing of the Local Hawk UAV”,
Chalmers University of Technology, Sweden 2013.
[6]. Ramalingam, V. K., Lankarani, H. M., “Analysis of impact on
soft soil and its application to aircraft crashworthiness”,
International Journal of Crashworthiness. Vol. 7, No. 1, 2002, pp.
57-65.
[7]. Flight Aviations Regulations, FAR-23.
[8]. “Modeling of Impact Dynamics: A Literature Survey”, Salah Faik,
Ph.D., Senior Engineer and Holly Witteman, Engineer Schneider
Electric Toronto, Canada, 2000.
[9]. International ADAMS User Conference, “Comparison of the low
and high velocity impact response of CFRP”, W.J. Cantwell and J.
Morton, Composites Part A: Applied Science and Manufacturing;,
Volume 20, Issue 6, November 1989, Pages 545–551.
[10]. Serhan Yuksel, “Low Velocity Impact Analysis of a Composite
Mini Unmanned Air Vehicle during Belly Landing”, METU May
2009.
[11]. Abaqus documentation 6.13, Dassault Systemes, 2013.
[12]. “Failure Modeling of Titanium 6AI-4V and Aluminium 2024-T3
with the Johnson-Cook Material Model”, September 2003, U.S
FAA.