Eject, eject, eject! When these words begin repeating urgently, a complex technological process is set in motion to safeguard aviators’ lives. To accomplish this, every facet of the safety system must work perfectly, from initiation, escape path clearance, ejection sequencing, stabilization, life support, and parachute descent to the final rescue. After all, an ejection seat may represent a TOPGUN pilot’s last chance of survival.
Introduced by the Germans during WWII, the technology behind the ejection seat has never stopped advancing, especially since aircraft speeds have broken the sound barrier. For the past 70 years, the UK-based company Martin-Baker has been the most prominent provider of ejection seats. With over 17,000 systems currently in service, the company has contributed to saving over 7,600 lives.
The Hawker Hunter fighter, which was designed in the 1950s and used in combat until 2014, is equipped with Martin-Baker’s ejection seats. Now mainly used in tactical training programs and in support of air-to-air missions, the transonic aircraft must be meticulously maintained and repaired. The Quebec-based company Lortie Aviation aims to extend the working life of the Hawker Hunter fighter and provide its customers with safe flight environments. To complement its quality task force, the aircraft maintenance team relies on Creaform Engineering’s group of experts, state-of-the-art tools, and innovative solutions.
Reduce Risk of Injury and Improve Pilot Safety
Lortie Aviation entrusted Creaform Engineering with the tasks of identifying the current risk of severe back injuries during pilot ejection and then proposing design corrections to minimize their severity and occurrence.
To fulfill its mission successfully, Creaform Engineering first focused on modeling the impact of ejection on the pilot’s body. After analyzing the results, the team was able to identify possible mitigation measures to reduce back injuries and improve pilot safety.
During the project, the engineers relied on a combination of various techniques, including 3D scanning, 3D modeling (for reverse engineering the pilot seat), physical tests for material characterization, and numerical simulation (more precisely, explicit dynamics finite element analysis using LS-Dyna).
Survival with Spinal Injuries
When the ejection is activated, the seat is propelled out of the aircraft with strong kinetic energy, carrying the pilot with it. As the impulse originates from under the seat, an intense load is transferred from the seat to the pelvis and through the spine.
The pelvic acceleration and lumbar forces are so strong during the ejection that the body struggles to absorb all the energy, leading to high risks of vertebral fractures on specific lower sections of the spine. With multiple recent cases of ejection leading to severe injuries, Lortie Aviation mandated Creaform Engineering to study the seat design and propose mitigation strategies for reducing impulse acceleration and peak magnitude transferred to the pilot body.
Numerical Simulation to the Rescue
Creaform Engineering used explicit dynamics numerical simulation using LS-Dyna software to reproduce the dynamics of pilot ejection with a Martin-Baker MK3 seat on board a Hawker Hunter fighter.
The initial scope of work was to evaluate the impact of adding highly viscoelastic foam to the seat and its potential in injury mitigation at ejection. First, numerical simulation experts represented the seat’s structural components with a detailed finite element model. The structure was discretized (i.e., the continuous structure was subdivided into small elements) to allow mathematical modeling.
Less Energy Transferred, but Very Intense Load
Since the Hawker Hunter uses an earlier version of the Martin-Barker seats (MK3), Creaform Engineering contacted the suppliers of the components and foams used in the newest versions of seats. This enabled them to start by updating the current foam with an improved foam amalgam that would help absorb energy more efficiently.
The finite element model enabled the engineers to conclude that the new foam did, indeed, lower the total impulse acceleration absorbed by the human body, but it was also raising the peak load. Since these viscoelastic foams harden with the speed of the applied force and the force is induced very quickly in ejections, it creates a higher peak load. So, we transfer less total energy, but we have a very intense load for a fraction of a second, which can be dangerous for the human body.
More Innovative Solutions to Help Instrumented Numerical Dummy
The numerical simulation experts also used a simplified anthropomorphic test device (ATD) model—an instrumented numerical dummy common in the automotive industry for crash tests and injury prediction—with specific kinematics and kinetics outputs to establish the risk of injury. They monitored the acceleration and forces transmitted from the seat to the pelvis and lower spine. The scientific literature then established the limits that should not be exceeded to prevent the risk of fractures. This simplified modeling approach to predict injury enabled the engineers to iterate quickly and test multiple improvements that could be implemented in the actual seat.
Creaform Engineering also carried out a complete scan of the seat and the foam cushions. They even scanned a person seated with the belts fastened to understand the position of the harness in physical space. They could then use the 3D scan data to position the numerical dummy.
The team also had access to some original blueprints of the Martin Baker MK3 ejection seat to study the original materials and the component dimensions, considering every detail to reproduce the seat’s 3D and finite element models accurately.
The simulation team applied the ejection impulse induced by the explosive cartridges on the seat. The load was previously determined based on the Martin-Baker MK3 ejection seat specifications and cartridge test results. Then, the engineers monitored the acceleration and loads applied to the different body segments, such as in the pelvis and lumbar regions. In this way, they could extract the kinematics and kinetics at these specific locations. They also varied the initial body positioning and harness setting to record their impact on the kinematics of the body segments. Thus, the team of experts could observe that, due to the ejection orientation and in order to reach the prescribed ejection velocity, a massive load was transferred from the seat to the pelvis, which was then transmitted upward to the spine.
After further calculations, Creaform Engineering could link potential injury severity to the acceleration and force magnitudes. Consequently, the team could discern mitigation strategies to reduce the load impulse and magnitude of pelvic acceleration, leading to a lower overall risk of injury.
More Detailed Representation of the Human Body
To better represent the dynamic response of the human body with regard to the injuries identified, the team decided to replace the simplified ATD model with a biofidelic dummy (THUMS 50th male model). This model of the human body could show the effect of ejection dynamics on each vertebra, each intervertebral disk, and other soft tissues. This gave the team a more precise representation of the strain and stress applied to the lower spine, indicating all potential injuries to the spine, bones, and tissues. This modification enabled the experts to be more accurate in assessing the risk of injury.
The objective was to study not only the seat design but also the operations and chain of events during ejections. This way, Creaform Engineering could provide Lortie Aviation with better practices in the form of improved positioning and operating parameters, such as seat adjustments, seatbelt tension, external acting forces, and more.
Seat Design Modifications to Mitigate Risk of Injury
As established with the finite element model, highly viscoelastic foam, when used alone, reduced the impulse (force × time) but resulted in a high initial peak load. Therefore, the engineers had to find a way to limit and absorb this initial peak load that could cause back injuries. The idea of inserting a material that could deform itself plastically, such as a crushable honeycomb pad, emerged as the best option to reduce both the impulse and the peak of pelvis acceleration and lumbar loads.
The team knew they could free up some space under the seat to add material without compromising Martin-Barker’s past certifications. Consequently, they included a crushable aluminum honeycomb layer to the current configuration. The material is pre-crushed during manufacturing to guarantee the crushing once the ejected pilot reaches a specific g-force. This way, the pilot sinks naturally into his seat when ejected from the cockpit, which helps to absorb enough energy to substantially reduce the risk of severe spinal injury.
Better Design, Safer Pilot
Creaform Engineering used advanced numerical simulation to reproduce the dynamics of pilot ejection on a Martin-Baker seat in a Hawker Hunter fighter. After simulation analyses, the team was able to propose and implement mitigation strategies to reduce the impulse and peak of pelvis acceleration and lumbar loads.
Armed with finite element models, the numerical simulation experts proposed improvements to the seat design that could reduce the energy transmitted to the pilot during seat firing. They were also able to demonstrate that these modifications offer better temporal distribution of forces and acceleration. Convinced that all these numerical simulations and analyses on models and biofidelic dummies could reduce the risk of spinal injuries, Lortie Aviation chose to proceed with the new seat configuration.
Numerical Simulation: An Accessible and Cost-Effective Technique
Creaform Engineering solved this real and concrete problem thanks to numerical simulation.
Ejecting a pilot from a Hawker Hunter fighter would require risky and expensive testing. Doing so numerically, however, is feasible at a low cost and is easily repeatable. In addition, this process can clearly be applied to other industries.
Simply put, rather than building costly prototypes and testing them physically, opting for numerical simulation offers all the advantages without the exorbitant costs. The investment is just building the numerical model. Once operational, the process is similar whether you are performing one or 300 analyses, resulting in massive gains and return on investment.
At Creaform Engineering, numerical simulation is a mature and mastered expertise, minimizing physical tests and maximizing knowledge of the issue at stake.
-Pier-Olivier Duval, Operations Manager in Numerical Simulation at Creaform Engineering
After all, if Creaform Engineering manages to solve complex problems, such as mitigating the risk of back injuries during the ejection of a pilot caught in the middle of a danger zone, its team of experts in numerical simulation can undoubtedly provide value to any product design and development process, regardless of industry.