How do pilots know when airplane engines aren’t working properly?

Engines are one of the most important parts of a modern airliner. Without them, the wings couldn’t generate lift to take off, the cabin and cockpit wouldn’t have power, and in most airplanes you wouldn’t have proper air to breathe.

Therefore, knowing how the engines work is key information that pilots need to know at all times. Today, detailed engine performance data is not only sent to cockpit displays, but also automatically broadcast to airline and engine manufacturer operations centres. If an engine has a problem, the ground crews will know about it almost as soon as the pilots.

So how is this data presented to pilots, what does it mean, and how do we handle situations where engines malfunction?

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How Jet Engines Work

Before diving into the technical details of the engine, it is useful to understand how it works. A turbofan engine, such as the Rolls-Royce Trent 1000 in the Boeing 787 Dreamliner, has four key stages, colloquially referred to as “suck in, squeeze, knock and blow out”.

The front stage of the engine, the huge set of blades you’ll see when boarding an aircraft via steps, is known as the fan. This is just the first of many sets of rotating blades through the engine.

The purpose of the massive fan is to draw air into the engine. From there, you may be surprised to learn that up to 90% of this air does not pass through the heart of the engine. Instead, it bypasses the core and exits out the rear of the engine. I’ll explain why in a moment.

The other 10% of air then passes through several sets of rotating blades which compress (squeeze) the air, increasing its pressure. From there it passes into the engine’s combustion chamber, where it is mixed with fuel and ignited, causing an explosion (the bang) and an increase in heat and energy. This hot air then passes (blows) out the back of the engine through spinning turbine blades.

The stages of a high bypass ratio jet engine. BOEING.COM

Here it encounters bypass cold air, which further increases thrust, which drives the aircraft forward.

It may sound strange, but the compressors and fan at the front of the engine are actually powered by the turbines downstream. Therefore, to get them moving during the engine starting process, they need to be powered by either high pressure air from the auxiliary power unit (a small engine in the tail of the aircraft)​ ​or, in the case of the 787, an electrically powered motor.

Once the motor is running and self-contained, additional assistance is no longer required.

The thrust generated by the engine is proportional to the amount of fuel injected into the combustion chamber. Thus, to increase engine power, pilots push the thrust levers forward. This sends an electrical signal to the engines to increase the amount of fuel flowing through the combustion chamber. This increases the speed of the turbines, which also increases the speed of the compressors and the fan.

Related: “The Most Fascinating Machines Ever Made”: How Jet Engines Work

Engine Parameters

With such a complex machine, a lot happens while an engine is in motion. As a result, the most important parameters of engine operation are transmitted to the cockpit and displayed on screens for pilots to monitor.

Turbine pressure ratio

As mentioned, turbines use the hot air created in the combustion chamber to spin the fan and compressors via the motor shaft. So, as this energy is extracted from the air to drive the turbines, the air temperature and pressure also drop. The ratio between the pressure of the air leaving the turbine and the pressure of the air entering the turbine just after the combustion chamber is called the turbine pressure ratio.

TPR – called “teeper” by pilots – is the thrust that the engine produces. If there is little fuel flowing into the combustion chamber, the air pressure entering the turbine is not too different from the air pressure leaving the turbine, so the thrust produced is low, which gives a low TPR value. However, if a lot of fuel is sprayed into the combustion chamber, the pressure of the outgoing air is much higher than that entering, resulting in high thrust and high TPR.

Pilots use the TPR gauges in the cockpit simply to see the thrust produced by each engine.


Unlike the TPR gauge, which shows us a pressure difference between two different parts of the engine, N1 values ​​are measurements of the speed of one part of the engine, displayed as a percentage of maximum.

The N1 stage of the engine includes the front fan as well as the low pressure compressor and the low pressure turbine, all connected by the drive shaft. Not all aircraft have TPR gauges, so operators use N1 as their primary indication of engine speed and thrust being produced. In the event of a fault with the TPR sensing and indicating system, the N1 readings provide accurate indications of engine speed.

Engine indications on a 787 in mid-flight. CHARLIE PAGE/THE DOT GUY

Exhaust gas temperature

The exhaust gas temperature gives an indication of the temperature of the air exiting the rear of the engines just after the turbines. Quite often we will notice that one engine’s EGT is a bit hotter than the other. This is normally due to one motor being a bit older than the other.

When an engine reaches a certain number of running hours, it must be taken off the wing and given a serious overhaul. During routine maintenance, engineers often remove one engine for an overhaul, keeping the other engine on the wing if its remaining hours are sufficient.

This means that the replacement engine will be a bit newer than the engine that remained on the aircraft. When pilots notice that one engine’s EGT is a bit hotter than the other, it’s normally a good indication that that engine is the older of the two.

A higher than normal EGT can also indicate an engine surge or stall, exhaust pipe failure or fire.


The N2 indication refers to the speed of the high pressure compressor and the high pressure turbine. Although not particularly useful once the engine is running, pilots monitor it during engine start-up. As mentioned, at this point the engine is driven by air from the APU or via an electric motor. Keeping an eye on the N2 rotation gives us an indication of the quality of the boot process and, more specifically, if a problem is about to occur.


The N3 only appears on engines that have a third stage coil, such as the Rolls-Royce Trent 1000 engine on the 787, and is an indicator of the high-speed section of the engine. Like N2, it is normally only used during engine cranking, but a rapidly fluctuating N3 can be a sign of an engine surge or stall.

In the event of an engine problem, zero rotation of the N3 section is a good indication of severe engine damage, leading us to safely shut down the engine.

fuel flow

The fuel flow indication shows us the amount of fuel the engine uses per hour in thousands of kilograms. I know from experience that the plane uses about 5 tons per hour, so the fuel flow gauges should read about 2.5 on each side. If one flow is slightly higher, it is normally an indication that the motor is a little older than the other. A higher EGT value will confirm this theory.

However, if the flow rate is much higher, it could be a sign of something much worse – a fuel leak. If so, we will pay close attention to regular fuel checks to see how our estimated fuel is doing on arrival. If it continues to indicate at or above our expected remaining fuel, all is well.

However, if there appears to be a major leak that will require us to land with less fuel than expected, we may need to shut down the engine and divert the plane to a nearby airport.

oil pressure

As with any engine, oil is important to keep parts lubricated and moving. It also acts as a coolant and cleaner. During normal operation, oil pressure must remain within certain limits to ensure that it continues to circulate around the engine.

If the pressure starts to drop, the engine could fail if the pilots don’t react quickly. The aircraft alert system will display the “ENG OIL PRESS” checklist. This instructs the crew to reduce power on one of the engines until the oil pressure message disappears.

If the message persists, they must shut off the engine before the lack of pressure causes serious engine damage.

In the video below, you can see the oil pressure rising as the engines start.

Oil temperature

Oil temperature is essential to ensure it is at its optimum viscosity or fluidity. In cold weather, when the outside temperature can be well below freezing, it often takes some time for the oil temperature to reach the correct operating level after the engine is started.

As with pressure, if the oil temperature gets too high, the “ENG OIL TEMP” checklist is performed by the crew, leading them to shut down the engine if they are unable to maintain temperature. within certain limits.

Oil quantity

Finally, the amount of oil is also important. Over time, the oil will deplete and the quantity levels will drop. Before each flight, engineers check oil levels and top up as needed before the plane departs. However, if there has been a structural failure in the engine – such as an oil hose that is coming loose – the amount of oil may run low in flight.

Related: What Is Jet Fuel And How Does It Work?


A modern jet engine is made up of thousands of parts, all expertly assembled with supreme precision. The blades that make up the fan, compressors and turbines must balance each other as they spin thousands of times per second. Therefore, they must be weighed with an accuracy of 0.003%. If one of the blades is damaged, it can cause an imbalance in the disc which causes vibrations.

Such damage is normally caused by bird strike or ingestion of FOD (foreign object debris), failure of another blade or icing.

The vibration display shows the correct level of vibration detected by the motor. If this value reaches four units, the crew is alerted. In itself, high engine vibrations are not to be feared. However, it could indicate that something else is wrong with the engine.

Therefore, it acts as a cue for the pilot to look at other engine parameters to find the source of the problem and come up with a plan of what to do if they need to shut down the engine.

At the end of the line

Keeping a close eye on the performance of our engines is essential for safe flight. By knowing what is happening at every stage of the engine, we are able to monitor engine performance exactly and take proactive action if things start to go wrong.

About Theresa Burton

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