How do pilots know when airplane engines aren’t performing correctly?

The engines are one of the most important parts of a modern airliner. Without them, the wings would not be able to generate lift to get airborne, the cabin and flight deck would have no electricity and, in most aircraft, you wouldn’t have suitable air to breathe.

As a result, knowing how the engines are performing is key information for the pilots to know at all times. These days, extensive engine performance data is sent not only to the screens in the flight deck but automatically broadcast to the airline’s and engine manufacturer’s operations centers. If an engine has a problem, the teams on the ground will know about it almost as soon as the pilots.

So how is this data displayed to the pilots, what does it mean and how do we deal with situations when the engines suffer a malfunction?

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In This Post

How jet engines work

Before diving into the technicalities of the engine, it’s useful to have an understanding of how it works. A turbofan engine, such as the Rolls-Royce Trent 1000 found on the Boeing 787 Dreamliner, has four key stages, colloquially known as “suck, squeeze, bang and blow.”

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

The purpose of the massive fan is to suck air into the engine. From here, you may be surprised to know that up to 90% of this air doesn’t actually go through the core of the engine. Instead, it bypasses the core and exits out the back of the engine. I’ll explain why in a bit.

The other 10% of air then passes through more sets of rotating blades which compress (squeeze) the air, increasing its pressure. From here, it passes into the combustion chamber of the engine, where it’s 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 rotating turbine blades.

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

Here, it meets with the cold bypass air, causing the thrust to increase further, which drives the aircraft forward.

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

Once the engine is running and self-sustaining, the extra assistance is no longer needed.

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

Related: ‘The most fascinating machines ever made’: How jet engines work

Engine parameters

With such a complex machine, there is a lot going on while an engine is in motion. As a result, the most important parameters of how the engine is performing are fed to the flight deck and displayed on the screens for the pilots to monitor.

Turbine pressure ratio

As mentioned, the turbines use the hot air created in the combustion chamber to turn the fan and compressors via the engine shaft. So, as this energy is taken out of the air to drive the turbines, the air temperature and pressure also drop. The ratio of the air pressure exiting the turbine versus the air pressure entering the turbine just after the combustion chamber is called the turbine pressure ratio.

The TPR — called “teeper” by pilots— is how much thrust the engine is producing. 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, giving a low TPR value. However, if lots of fuel is sprayed into the combustion chamber, the pressure of the air coming out is much higher than that entering, resulting in high thrust and a high TPR.

Pilots use the TPR gauges in the flight deck quite simply to see how much thrust each engine is producing.


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

The N1 stage of the engine includes the front fan and both the low-pressure compressor and the low-pressure turbine, which are all connected by the drive shaft. Not all aircraft have TPR gauges, so operators use N1 as the primary indication of the engine speed and thrust being produced. If the TPR sensing and indicating system were to fail, the N1 readings provide accurate engine speed indications.

The engine indications on a 787 midflight. CHARLIE PAGE/THE POINTS GUY

Exhaust gas temperature

The exhaust gas temperature provides an indication of how hot the air is exiting the rear of the engines just after the turbines. Quite often we’ll notice that the EGT on one engine is somewhat hotter than the other. This is normally due to the fact that one engine is a little older than the other.

When an engine reaches a certain number of operational hours, it must be removed from the wing and given a serious overhaul. During routine maintenance, engineers will 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 somewhat newer than the engine that remained on the aircraft. When pilots notice that the EGT on one engine is a little 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, a failure or a tailpipe fire.


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


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

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

Fuel flow

The fuel flow indication shows us how much fuel the engine is using per hour in thousands of kilograms. I know from experience that the aircraft uses roughly 5 tons an hour, so the fuel flow gauges should show around 2.5 on each side. If one flow rate is slightly higher, it is normally an indication that the engine is a little older than the other. A higher EGT value will back up this theory.

However, if the flow rate is much higher, it could be a sign of something much worse — a fuel leak. If this is the case, we would pay extra attention to regular fuel checks to see how our estimated fuel on arrival is doing. If it continues to indicate at or above our planned remaining fuel, we are fine.

However, if it looks like there is a significant leak that will result in us landing with less than our planned fuel, we may have to shut the engine down and divert the aircraft to a nearby airport.

Oil pressure

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

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

If the message still remains, they must shut the engine down before the lack of pressure causes severe damage to the engine.

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

Oil temperature

The oil temperature is key to ensuring it is at its prime viscosity or fluidity. On cold days when the outside temperature can be well below freezing, it can often take a little while for the oil temperature to reach the correct operating level after starting the engine.

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

Oil quantity

Finally, the amount of oil is also important. Over time, oil gets used up and the quantity levels will fall. Before each flight, engineers check the oil levels and top them up as required before the aircraft departs. However, if there has been a structural failure in the engine — like an oil pipe becomes loose — the oil quantity may decrease 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 accuracy. The blades that make up the fan, compressors and turbines need to balance each other out as they spin around thousands of times a second. As a result, they have to be weighed with an accuracy of 0.003%. If one of the blades becomes damaged, it can cause an unbalancing of the disc which causes vibration.

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

The vibration display shows the correct level of vibration detected by the engine. If this value reaches four units, the crew is alerted. In its own right, high engine vibration is nothing to worry about. However, it could indicate that something else is wrong with the engine.

Therefore, it acts as the pilot’s cue to examine the other engine parameters to find a source of the problem and think about a plan of what to do should they have to shut down the engine.

Bottom line

Keeping a close eye on how our engines are performing is key to completing a safe flight. By knowing what is going on at each stage of the engine, we are able to monitor exactly how the engine is performing and take proactive steps should things start to go awry.