Lortie Aviation owns and operates a unique fleet of special British fighter jets known as Hawker Hunters, which the Quebec-based company maintains and leases to Canadian, American, and European armed forces for tactical combat training. Whether used for high-speed attacks and missile simulations, towing targets, or replicating adversary roles for ships and aircraft, the Hawker Hunter offers the capability and flexibility to satisfy the training requirements of all branches of the armed forces.
Lortie Aviation’s delicate task each day is to adjust, maintain, and repair this complex transonic aircraft without relying on the support of the original equipment manufacturer (OEM). This means that a lot of old components and broken parts must be inspected and replaced, extending the working life of the aircraft through failure diagnoses and root cause analysis.
To ensure maximum aircraft lifespan, Lortie Aviation counts on Creaform Engineering, whose team of multi-disciplined experts, state-of-the-art tools, and innovative solutions to complex challenges have met Lortie Aviation’s advanced and specialized engineering needs and supported their technical activities since 2017.
Faulty Landing Gear Causing Incidents on the Airstrip
Breakdowns and incidents—severe and otherwise—occurring both on landing and takeoff have forced Lortie Aviation to question the reliability of the Hawker Hunter’s main landing gear.
Our aircraft’s main landing gear had a weakness we weren’t aware of. And we couldn’t find any solution for repairing or correcting it. This situation led us to seek out Creaform Engineering’s expertise for diagnosing the root cause of the failure.
-John Lusa, General Manager, Lortie Aviation
Creaform Engineering’s initial responsibilities were to identify the limitations of the original design. The investigation and analysis activities then evolved into a development project. After reviewing the original engineering, the team designed and validated new replacement parts with improved durability and resistance capacity, increasing the useful life of the main landing gear.
To fulfill its mission successfully, the engineers used a combination of various services, including 3D metrology (for the reverse engineering of current parts and their environment and for the inspection of new parts), design, engineering, computer modeling, and geometric dimensioning and tolerancing (GD&T). Moreover, numerical simulation—more precisely, finite element analysis—and physical tests helped solve the problem at hand.
The Contribution of Numerical Simulation
Creaform Engineering’s expertise in numerical simulation was called upon to identify why the main landing gear was not performing as expected. After acquiring data from the aircraft, finite element analysis (FEA) was used for stress and structural analysis.
Numerical modeling and computer simulation —often referred to as virtual prototyping—were also used to predict the lifespan and behavior of the mechanical system. This allowed the team to reproduce testing without the need for excessive and destructive physical testing.
Technical Challenges at Stake
Although the aircraft is aging, it is still considered robust and viable. The original landing gear was made of forged aluminum, built with a 7014 aluminum alloy that was known at the time of its design (1954) for its excellent mechanical properties. Nevertheless, the fragile fractures recorded over the past few years have generated catastrophic failures, causing airstrip exits and subsequent defects to other aircraft parts.
Experts conducted mechanical and material failure analysis in laboratory for each incident to establish the root cause of the failure. Their conclusion was that there were no apparent signs of fatigue. However, the analysis highlighted the presence of many intergranular cracks—mainly caused by stress corrosion cracking (SCC)—and some surface corrosion punctures with traces of sulfur and chlorine. The severe overload at touchdown demonstrated that the landing gear was being pushed to its limit.
Stress corrosion cracking, which can generate catastrophic failures, occurs due to the combined effects of three factors: a sensitive material, the presence of tension stresses in the material, and an environment conducive to corrosion.
Meticulous Research and Modeling Work
The team set out to find all the information available on the landing gear. Research uncovered maintenance and pilot manuals, original engineering documents from the 1950s, and incident reports, all of which were wisely sorted and analyzed. The engineers also modeled hundreds of parts from recovered 2D drawings or used 3D scanning to obtain a representative sample. This allowed them to avoid relying on anecdotal data, leading to better decision-making. Creaform’s 3D scanners—the company is also a designer and manufacturer of 3D scanning solutions—were used to scan the landing gear’s surrounding environment in the wing of the aircraft, ensuring that no interference would occur between new and existing parts.
Scrutinizing 300 Load Cases
After collecting all the required data, the team of experts had to identify the various loads applied to the landing gear. To do so, they performed static and dynamic analyses while considering the aircraft parameters, such as the speed of descent, the speed at the impact of landing (related to lift), the aircraft’s position during landing, and its weight. In total, 300 load cases were analyzed.
Numerical simulation enabled the simulation experts to obtain performance results on all of these load cases without having to physically test the landing gear in each scenario to know which were the most problematic. Thus, the team could focus on the cases that accentuate part limitations.
Three Reasons Behind the Weakness
1. Overlooked Load Case
First, the original design did not adequately consider a specific load case. Counterintuitive, this load case was caused by the system configuration and could explain the vast majority of past failures. Once this load case was analyzed with advanced tools (e.g., numerical simulation), the team could see the excessive stresses generated at landing and the consequences on the aircraft’s structure. In a simplified way, this load case represents the spin-up, spring-back effect of the landing gear right at the impact of landing. Like a spring, once charged, the landing gear rebounds with a reverse force, making it vulnerable to breakage.
2. Conditions to Develop SCC
Second, the three conditions were met for the landing gear to develop SCC—a failure mechanism that reduces its resistance capacity to fatigue. Indeed, the landing gear was submitted to permanent and variable tension stresses, it was immersed in a corrosive environment, and its forged aluminum alloy was sensitive. There were constraints related to tolerancing in the landing gear assembly. Once positioned in the aircraft, the landing gear was constantly in tension at critical points, as pins were continuously forced into holes, creating a favorable environment for stress corrosion cracking.
3. Inadequate Coatings and Surface Finishes
Third, the main forged aluminum part was subjected to corrosion, and its coatings and surface finishes could not protect it against the environment. Furthermore, the different alloys and materials present in the assembly created galvanic corrosion.
Design Improvements for a More Resistant and Safer Landing Gear
After the engineers fully understood the weaknesses of the original design, they proposed redesigning the landing gear to improve each of the identified limitations. Although this was seen as a daunting task, the team was ready to take up the challenge while preserving the original system’s main operating components; after all, the original landing gear had been tested and certified. Nevertheless, the structural elements had to be redesigned and rebuilt, which meant changing how the landing gear was attached to the aircraft and seated in the wing.
The objective was to reduce—in all points and for all load cases—the observed constraints while limiting the addition of weight. Therefore, the team opted for a new, more resistant material (Aluminum 7050-T74) with superior mechanical properties. Adding material to specific points under high stress also resulted in enhanced performance. Better surface treatments and protection systems were prioritized. Moreover, dimensional links and critical tolerances were also modified since they could generate residual stresses.
Numerical simulation enabled the engineering team to quickly compare the original design to the proposed improved design.
The fact that we did use numerical simulation opened our eyes to new ways of doing stuff—better ways, faster ways, safer, more efficient.
-Steve Guillemette, Engine Shop Supervisor and R&D Director, Lortie Aviation
Manufacturing and Testing the New Landing Gear
At that point, the manufacturing and testing of the first units could begin. The engineers correlated physical tests with FEA models. The team submitted the new landing gear to physical forces in different directions, ensuring that observed behaviors and the simulated models matched. Not only would that allow them to fully trust their process but also to choose in advance the worst-case scenario to carry out a destructive test. Therefore, instead of having to perform several expensive and destructive tests, numerical simulation has limited this number to only one physical test.
Outcomes: Better Product Reliability and Durability
Creaform Engineering’s experts followed a tested and proven product development process. Less than a year passed between the agreement and the final design presentation. Then, six months were required for prototyping, mechanical testing (to validate the numerical models), and manufacturing. As a result, 50-60 copies of the new landing gear were produced to replace the existing parts of the aircraft fleet.
Creaform Engineering came in with a solution that was not only effective for the aircraft but also cost-effective for the company. Our customer was delighted by everything they saw, and it kept us in business.
-John Lusa, General Manager, Lortie Aviation
Creaform Engineering assisted Lortie Aviation in all design, manufacturing, testing, and implementation phases. In all, it took 18 months to develop a new product with better mechanical properties. Three months later, the entire aircraft fleet was equipped with the new landing gear. Without a doubt, numerical simulation contributed to developing the right product the first time while reducing costs and optimizing the choice of materials and coatings.
Thanks to numerical simulation, Lortie Aviation now has a new, safer landing gear with a proven mechanical resistance that is 50% higher than the original without adding any weight.
-Pier-Olivier Duval, Operations Manager – Numerical Simulation, Creaform Engineering
A Better Product Developed Thanks to Numerical Simulation
Numerical simulation enabled Creaform Engineering to begin the process by crystallizing the root cause and explaining with certainty why the landing gear had failed. Then, numerical simulation helped remove the original design’s weaknesses quickly. Following a few iterations, the new design was refined and optimized, pushing the development work of the engineering team a little further. The new product development process was accelerated once again thanks to numerical simulation, leading to a more resistant, safer landing gear without increased weight or cost.
The way I see the collaboration between Lortie Aviation and Creaform Engineering in the future is in their hands because the beauty of our relationship is that they will show us things we do not see ourselves.
-John Lusa, General Manager, Lortie Aviation