Aircraft Testing and Certification: The Engine

A few months ago I had the privilege of attending a fireside-chat hosted in Nairobi. The main guest was a friend from high school who was, at the time, a senior director at Tesla Motors. I was curious how much Tesla spends on research and development so I asked him as much. It was fascinating to learn that Tesla doesn’t have a dedicated R&D department. Instead, all R&D is integrated into the engineering functions at the points of execution. So, changes to the design can be quickly conceptualized, actualised and implemented right on the floor – a very practical and time-efficient way of working.

Since that session, I’ve been thinking about working that way and how practical it would be to implement this in aircraft manufacturing. The sad reality is that it’s unlikely to happen for a long time – if ever – or at least not on a large scale. Our industry is heavily regulated and changes to approved designs need to be thoroughly assessed, reviewed and validated before they can be implemented in a production aircraft – usually with at least one redundancy incase there’s a failure in flight. And even after the design concept has been validated, it will have to go through a rigorous test schedule as part of the aircraft’s overall certification testing.  This is not a bad thing – the rigour is the reason flying is the safest mode of transport – but it isthe reason the development of a new aircraft model is so time consuming and costly. Design and build of a new aircraft from scratch – a clean sheet design like the B787 or the A350 – can cost a manufacturer upwards of $15B and take over 5 years from start to first customer delivery.

Today I want to outline part of that certification-testing schedule to demonstrate the level to which aircraft are tested before they’re allowed to carry passengers. For that, I will start with the largest component of an aircraft – the Engine.

The Engine is arguably the most important component of an aircraft. (In fact, some engine manufacturers half-jokingly say that the aircraft is attached to their engines, instead of vice versa ;-)). The Engine propels the aircraft forward (which actually keeps it in the air) , generates electricity for aircraft systems and even drives the air-conditioning systems that provide that all-important breathable air in the cabin at altitude.

As part of the design process, the first tests an engine goes through are pretty much what you’d expect. They test the systems to make sure they all work. They test to make sure there are no leaks and that the vibrations are within acceptable limits. They then test for specification to make sure the fuel consumption (and oil consumption) meets what is required and that the engine generates the required power. All of this is done as an iterative process where they go back to the drawing board to tweak whatever they need to, based on the test results. Once the engine design is firm and final, they need to go through a process to certify it as safe to fly.

Safety is paramount in aviation certification. A lot of time is spent imagining real life situations that could occur in flight. Thereafter the engine is tested to make sure it will be able to handle those situations. Some of those tests are outlined below:

Water Ingestion

Since planes will often fly in rainy conditions, the engine has to be capable of operating in such an environment without flaming out or operating under distress. So to test for this, they turn the engine on and spray a whole load of water into the engine – quite literally tons of it! On some modern engine this can be in excess of 3 tons of water per minute.  They spray far more water than would normally be encountered in operations to be sure that there’s a healthy margin of operations in real flight. 

Icing and Ice Ingestion

In the same way the engine must operate in rainy conditions, it needs to do the same for ice, snow and hail. To ensure this, they again chuck buckets of ice at the engine and test how it performs. This is everything from the texture of snow to fist sized hailstones. Again, the engine is expected to continue in full operation until at least the next landing (at which point, in commercial service, it would be inspected and repaired as required before it’ll be allowed to fly again).

Here’s a video of some of the weather proof testing that GE takes its engine through (apologies in advance for the corny commentary ;-))

Bird ingestion

Bird strikes are among the biggest causes of damage to aircrafts and engines in flight. I know it sounds strange to think of a small bird being a threat to such a massive, metal machine. But think of the bird as being just a weight – let’s say 1kg. Now factor in how fast the plane could be moving when it hits a bird or sucks it into the engine (maybe 300-400Km/h at the lower altitudes where you’d find birds). In essence, from a relative motion perspective, it’s the same as the plane or engine being still and getting hit by a 1kg mass moving at over 400km/h. If you’ve ever seen what happens to a car windscreen when it’s doing 100Km/h and gets hit by a small stone, imagine the damage caused if it’s hit by something 100 times heavier, moving 4 times as fast. The damage is substantial – even if it’s a metal structure.

Since this is such a threat, the engines are tested to make sure they continue to operate to the next landing following ingestion of different sizes of birds. They run the engines to a pre-determined power setting (to simulate the relevant flight segment) then throw the bird in (it’s already dead when they do). The engine needs to continue running for a number of minutes –maybe 5 or 6 – before they let it cycle down to stop. Thereafter, once the engine has stopped, they inspect it thoroughly to confirm the level of internal damage caused.

In addition to tests with single birds, some tests are also carried out to simulate flocking situations where several birds are ingested at the same time.

Blade Separation

This is the most violent and damaging of the certification tests. The intention is not to ensure the engine is ok after – it never is! It is intended to ensure any shrapnel is contained within the engine in the event a fan blade separates while the engine is running. They simulate this by attaching a small explosive charge to the base of one of the fan blades. Once the engine has be been run up to maximum speed – at which point the blades are moving fastest – the charge is detonated. This video shows just how spectacular the damage is  – and how well the structure contains the shrapnel and debris.

These are just some of the tests carried out on an engine. Others are more mundane and less noteworthy but they are all meant to ensure the engines will support safe operations of the aircraft in commercial service.

All this testing doesn’t mean the engines never fail or that they are foolproof. However, it does mean that the likelihood of failure is increasingly remote. And in the rare event that an unforeseen failure occurs, the tests are revised and improved to ensure it doesn’t happen again in the future.