Beating the bird strikes

Cyient's vice-president and global industry lead for aerospace and defence, Chris Jones, looks at the work done on analysing the impact of aircraft bird strikes and company's work with a leading aerostructure tier 1 supplier. The first recorded incident of a bird strike on an aircraft occurred on September 7th 1908 during a test flight piloted by Orville Wright, one of the fathers of modern aviation. The Wright brothers had been showcasing their flying machine near Dayton, Ohio, when Orville collided with a flock of birds following the same flightpath, killing one of the birds in the process.

While this bird strike episode was relatively inconsequential – for the pilot, if not for the bird – it was merely the first of many, a somewhat unforeseen consequence of air travel, subsequently becoming a real issue for the industry.

According to the Federal Aviation Administration (FAA), more than 255 people have been killed and over 243 aircrafts destroyed over the last 27 years due to wildlife strikes, and the number seems to be on the rise: annual incidents rose from 1,851 in 1990 to 11,313 in 2013, in part due to increasing populations of large birds as well as more air traffic from quieter, turbo fan-powered aircraft. In all, there were over 142,000 recorded strikes in this time period.

Thankfully, although not all strikes have a fatal human consequence there is still a cost associated with every incident. The aftermath of a bird strike means expensive aircraft repairs as well as a loss of flying hours and revenue, the potential reputational damage and the consequential costs of putting passengers in hotel or re-scheduling aircrafts and crews. Overall, the total economic loss is estimated at around $1.28 billion every year.

According to the statistics, the take-off and landing phases of any flight are the most vulnerable – nearly three quarters of bird strikes occur at just 500ft above ground while nearly all incidents occur below 3,500ft.

We also know that the most vulnerable points on the aircraft are the wings and engine with 53% of all strikes affecting these areas (see Fig 1.). Memorably, back in 2009 a US Airways flight was forced to make an emergency landing on the Hudson River after a flock of Canada geese knocked out its engines. Hailed as the ‘Miracle on the Hudson', only the calm, quick-thinking of Captain Sullenberger ensured that the incident incurred no fatalities and all 155 occupants could be successfully evacuated.

Challenges in strike modelling

Given the scale of the problem, the physics of bird strikes need to be accurately modelled in order to make aircraft structures safe after a collision and reduce the damage incurred. Aircraft manufacturers, therefore, are increasingly resorting to simulation techniques during product development to inform design and production process.

There are a number of technical challenges associated with the current strike modelling process. Firstly, certification testing for securing aircraft structures against bird strikes is a complex, expensive and time-consuming process. It requires huge investment and manufactures simply can't afford for the process to be derailed for any reason.

Additionally, while there are many analysis approaches to model the physics of a bird strike, each have their own strengths and drawbacks. Constraints over resource, time and level of accuracy mean that selecting the right method requires significant insight into all three – a costly undertaking for manufacturers.

Working out a single, overarching approach to impact simulation first requires a comparative assessment of the three most common current methods of modelling: Pure Lagrangian formulation (PL), Couple Eulerian Lagrangian formulation (CEL) and Smooth Particle Hydrodynamics formulation (SPH). Each method has specific areas of merit: PL is best used in preliminary checks for FEM formulation though has a lower level of accuracy than other methods. Meanwhile, CEL is primarily used where a detailed view of fluid response is required, and SPH is ideal for cases in which severe deformations are expected, though for both, the computation time required is often prohibitive.

Choosing the right approach is based on three parameters: FEM set up time, computational time and required accuracy of results. To address this, a Bird Strike Simulation Index (BSSI) has been proposed. Offering a holistic approach, BSSI guides the industrial user in choosing the right approach for the right purpose through analysis.

From simulation to reality

The BSSI methodology can then be applied practically. One such example is Cyient's work with a leading aerostructure tier 1 supplier in developing a new inboard flap for the next generation of business aircraft.

During the design phase, Cyient worked to create an experimentation methodology which could calculate the most critical location for a bird hit while avoiding the cost of multiple tests in a bid to save time, money and the number of tests that had to be undertaken. Using the BSSI, it was decided that the SPH method of analysis would offer the most representative case.

While the aerostructure supplier built the physical test model and subjected it to bird-strike test, Cyient was able to construct a simulation model, using the SPH technique. Keeping the bird's size, weight and initial velocity consistent, the model was run for 70ms, with a computation time of around three hours.

The model simulated videos of bird strike to match the supplier's physical experiment, these were then placed side-by-side for comparison. An analysis of both results showed similar splash patterns for the real and virtual birds, while localised damage was also consistent. The accuracy rate was 98% – demonstrating the value of the method and removing the need for further physical experimentation.

Overall, BSSI has allowed manufacturers to not only save cost and resource involved from multiple tests, but it does so whilst improving the analysis process – providing more valuable information to help secure FAA certification.

Ultimately, bird strikes present a very real danger to air travel. While the loss of human life may only be a rare outcome, the loss of revenue is a certainty. Delays and aircraft damage resulting from these incidents can have a serious impact on the ability of airlines to do business, costing billions each year. Anything that can be done to minimise the disruption is a valuable exercise.

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