The aeronautical industry is often referred to as one of the safest in the world. Could it perhaps be 99% safe? If there was no such thing as chance and all events occurred according to their probability, a 99% safe aircraft would malfunction for 7 hours every month, which is clearly too risky. If we increase the safety level to 99.99%, the chance of a malfunction is reduced to 5 minutes per month, which is starting to approach a more reasonable value. But how do we achieve these safety levels?
Aeroplanes are, statistically speaking, the safest form of passenger transport there is. However, there is a long process that has to be followed in order to arrive at that fact.
Many factors contribute to making the risk so small. However, a good example of the thought that goes into safety is the large number of tests an aircraft is subjected to before being put into flight.
Perhaps the most fundamental step can be found in the materials aircraft are made of.
If we take a titanium rivet as an example, the base material must be supplied to the rivet manufacturer by a supplier that has been approved by the aircraft manufacturer. Subsequently, once a batch of rivets has been manufactured, some of its units will be tested by the manufacturer itself to ensure that they meet the required standards.
But that’s not all. Whoever receives the rivets must also test some units from the batch before assembling them, in order to make sure that the product has maintained its properties following transportation, and has not been manipulated or replaced by a different, poorer quality product.
All these steps are of course recorded, with the aim of ensuring that there is proper traceability for every single element involved in the manufacture of an aircraft.
This will be the case for all materials that end up being part of the finished aircraft. In addition, workers, designs, and manufacturing and testing machines must be approved for the functions they perform, which further raises the quality standards of the industry.
At this point, the properties of each element can be regarded as verified – but what about the assemblies? Here, the chain of tests continues, and there is the possibility of testing the different assemblies that are manufactured. Not only are their strength and integrity assessed, but also their resistance to corrosion and their behaviour as electrical insulators in the case of outer surface parts.
A striking case is the wing flex test. This involves placing cables along the entire top and bottom of the wing, and checking the maximum flex achieved by the wing in each direction. If the rivet of the previous example ends up in a wing, its strength will be tested again, along with the quality of its placement and tightening.
Another option is to carry out tests on parts of the aircraft that are exposed to highly demanding conditions due to their function; for example, the engines. Some of the most curious tests carried out on engines include water ingestion, bird strike and “blade-off” tests.
In the first case, the engine’s operation is tested while a large amount of water is projected into it. This test is simulating cases of heavy rain during the take-off and landing phases, when the engine absorbs a large amount of water. This can also happen during the flight.
The second test evaluates the damage caused by the impact of a projectile similar in size and weight to a large bird (for example, a goose) on the blades of the engine. This situation will likely occur at some point in the life of the aircraft, mainly during the take-off or landing phases.
The third test is a bit different. It does not introduce anything new into the engine, but instead involves breaking one of the blades to check whether the engine can resist the passage of the blade without losing its integrity or starting a fire in the process.
When all the components of the aircraft are considered satisfactory, a key moment arrives: the first flight. During the first flight, the aircraft will leave the ground by its own means for the first time, and its flight performance will be tested. There will be multiple test flights in which the aircraft’s response to certain manoeuvres will be assessed, along with the operation of the electronic systems installed and the behaviour of these systems and the engines in the event of a malfunction.
A large amount of data is taken from these flights, such as the loads on the fuselage, wings, wing sockets and landing gear. When the manufacturer is satisfied with all the corrections made to any defects found, the aircraft shall be made available to the competent authority to complete its certification.
But there is still one last step to complete before passengers can be carried on board: the evacuation drill. According to European regulations, aircraft with more than 44 passengers must be able to evacuate completely in a maximum of 90 seconds.
What’s more, to make the situation even more difficult, the test must be carried out in the dark and with half of the emergency exits blocked. Participants acting as passengers must represent a heterogeneous sample of the population, and they must not have received any training recently. To use an example from a large aircraft, the A380 (with its 873 passengers) managed to pass this test in just 78 seconds.
As one might expect, all these tests involve a considerable added cost to the manufacturing and certification processes for any aircraft. However, nobody is likely to question their necessity. Otherwise, how could aeroplanes be the safest means of transport in the world?