Turbine engines are used widely across the aviation sector. They come in all shapes and sizes: Turbojets, Turboprops, Turboshafts, and Turbofans. While they all have different characteristics and use cases, they share a lot of similarities at the same time. So what are the most common turbine engine starting failures, and how can you recognise them as a pilot?
While we won’t dive into the mechanics of each engine here (we will in the future), there are all kinds of failures and pilot headaches we can expect during a turbine engine start.
We don’t see ground crew having to manually yank propellers anymore to start engines in modern airliners like with older piston aircraft, and that’s a good thing! Let’s first look at the basic function of a turbine engine, followed by the different ways we can start a them nowadays.
The turbine engine principle
So back to basic first: let’s quickly revise the basic function of a turbine engine. This way we can understand the ways things can go wrong during the starting process. There are 4 main stages.
- Air Intake (Suck)
- Compression (Squeeze)
- Combustion (Bang)
- Exhaust (Blow)
Air enters the engine inlet. The main job of the inlet is to provide a stable supply of smooth air to the compressor. The compressor then compresses this air as much as possible, until it is ready to be mixed with fuel and ignited. This ignition creates a massive increase in pressure.
This high pressure results in a fast flow with lots of kinetic energy going through the turbines, which can absorb the energy from the airflow and convert it into rotation.
This rotation is fed back to the the compressor, as it’s connected to at least one of the turbine stages. In the case of helicopters, the other turbine (power turbine) drives the main rotor.
How to start a turbine engine?
There’s a few ways! Let’s have a look:
A Turbine Engine is using something called a continuous combustion, rather than an intermittent combustion like in a piston engine.
This simply means that once there’s ignition, there’s a constant combustion going on inside the engine, unlike the stroke cycle process within a piston engine.
A Turbine Engine is self sufficient, meaning that once it gets going, it can keep itself going without the need for extra sparks. While that sounds all well and good, we still need to get it going first!
The goal is to get the compressor turning at a speed where a sufficient amount of compressed air is entering the combustion chamber so it can speed up the turbines, which drive the compressor. To accomplish this we need to overcome the compressor inertia as well as all the friction inside the engine.
Depending on the aircraft type and engines, as soon as the engine mode is set from OFF to IDLE by the pilots, the compressor will begin to accelerate by the starter system, no fuel or ignition is supplied yet. Then, once the compressor reached the required speed, fuel and ignition is supplied.
This initial supply of fuel and ignition (also called light up) will cause as sharp rise in the Exhaust Gas Temperature (EGT) usually within 20 seconds. This sharp rise is caused by the excess amount of fuel in the combustion chamber.
When that’s ignited though, it will go through the turbine section and drive the compressor, reducing the load on the starter, this increases the compressor speed as well (usually called N2 for conventional jet engines, unlike for helicopters). Have a look at the graph that illustrates the typical process for both N2 and EGT:
There are few ways to accomplish this self sustaining point, but we are not going to get there without a starter, which can come in a lot of different shapes, let’s have a look.
Types of turbine engine starters
Firstly we have Electric Starters, which use electricity from either the aircraft batteries or external ground power units to start the turbine. These come in 2 forms: the direct cranking starter (usually used on smaller engines like Auxially Power Units (APU’s), and Starter Generators.
Then there’s the Hydraulic Starters, which essentially uses a geared hydraulic motor and hydraulic oil to give the initial burst of energy.
Next up is the Air Starter, which uses a highly condensed flow of air going through the compressor or gas generator turbine, speeding up the compressor. Usually this supply of air comes from either the APU or a gas generator.
Finally there’s the Combustion Starter, which has a lot of different shapes, but military pilots might recognise this one as an explosive cartridge starter (like in the Sea Hawk). They all come down to the principle of using the kinetic energy that is released during combustion to start the compressor and turbines.
But all of these have the same mission: get the compressor to spin fast enough so it can become self sustaining. Most modern airplanes and helicopters are equipped with Full Authority Digital Engine Control (FADEC), making this process automated and a lot safer.
However, there’s still a lot that can go wrong to get to a self sustaining turbine engine, let’s have a look at the most common failures.
The Hot Start
Of course, we can’t compile a list of start failures and not include the infamous hot start. What is it and how does it happen?
A hot start is defined as a situation where the EGT (or turbine inlet temperature depending on the engine manufacturer) limit get exceeded.
The most obvious symptom is the EGT accelerating ’more than usual’. The problem is that the EGT could initially climb as expected, and then suddenly rise very quickly, making it tricky in certain circumstances to detect it in time.
The airflow through the engine does not just act as a driving force for the turbine / compressor. It also functions as a cooling mechanisms, there’s too much fuel and not enough air.
If the starter does not accelerate the compressor enough when the fuel and igntition get supplied, the temperature in the combustion engine (and therefore in the turbines and exhaust) can rise to level that could even melt certain metals.
The only way of stopping this sharp increase is by cutting off the fuel supply in time. Waiting for instructions or discussing what to do will result in an unserviceable engine.
Sounds pretty awful, doesn’t it? It’s usually caused by a weak starter, lower electric currents from batteries or APU’s, throttles not being properly set to idle, or anything else that contributes to a low N2 speed during start.
The Hung Start
Then there’s the hung start, a lesser known failure, but can damage the engine as well. A hung start means a situation where the engine rpm does not accelerate properly to idle rpm, and instead is stuck, or ’hung’.
Usually the EGT is higher than normal, as there is a lack of airflow which means there is inefficient combustion.
This inefficient combustion then provides less power to the gas generating turbine and therefore does not help speeding up the compressor. If the starter motor then cuts out after it thinks it did its job, the rpm will still not be at proper idle rpm.
From this position, the pilots will not be able to go from IDLE to FLIGHT due to insufficient power. EGT often won’t exceed the limit like with the hot start, although it is still possible.
The Wet Start
The wet start is a situation where the fuel is being pumped into the combustion chamber, but is not properly ignited, resulting in inefficient combustion. It is shown by the fact that the EGT won’t increase and the rpm stays below what is normally required for ’IDLE’, it instead stabilises at the maximum rpm the starter can supply.
The biggest threat during a wet start is the excessive amount of fuel suddenly igniting, creating a mini explosion in the combustion chamber which can damage the engine and the turbines.
Some wet start SOP’s are to just crank / motor the engine without ignition, to clear the engine before attempting a restart.
An Exhaust or Tailpipe Fire
A fire in the tailpipe (or exhaust) is caused by excessive amount of fuel compared to air in the combustion chamber. The mixture is too rich. A rich mixture often delays ignition, but when it does eventually happen, it tends to be more violent that normal and can create problems in and behind the combustion chamber.
It can be hard to detect from within the cockpit, other than an increase in EGT. A marshaller is usually the first one to detect it by seeing the flames or smoke coming out of the exhaust, or even from the engine inlet.
Because the engine materials are built to withstand very hight temperatures, tailpipe fires usually do not cause any major damage, but it is possible.
If it happens, the correct course of action after detection is to close the fuel valves, abort the start, and crank the engine without ignition to clean it.
Turbine engines come in a lot of different shapes, but they mostly all rely on the same principles. Getting them to the point where they become self sustaining is critical, and things can go wrong in different ways before even getting to that stage.
The Hot Start is considered to be the most crucial to detect in time, as it can turn an engine completely unserviceable if left too long.
We will cover the different types of engines and their characteristics in a future post. Keep sending in your topic requests and see you in the next article! For more information on the exact sequence during engine start, have a look at this Flight Mechanic article.
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