Climate change has been a topic of significant interest in aviation. From OEMs to aircraft operators and material suppliers, all processes and products are optimized to minimize carbon footprint. Major contributors to aircraft performance are rising temperatures, fluctuating wind shear, and increased turbulence.
During the Sustainable Aviation Futures Congress in 2022, Professor of Atmospheric Science at the University of Reading in the UK, Paul Willams, stated that the aviation industry is going to become a strong victim of climate change. He highlighted the importance of having airports below sea level to combat the effects of the jet stream and clear air turbulence (CAT) that comes along with it. According to Professor Williams,
We have a lot of evidence that the jet stream is now 15% more strongly sheared since satellites began measuring it in the 1970s. And this is what causes a lot of turbulence, especially clear air turbulence. And the satellites show that it has become 15% stronger since the 1970s; that's a massive shift. And we understand why it's happening in terms of the physical mechanisms behind it. Our calculations indicate there is going to be twice or three times as much severe turbulence in the next few decades because of climate change.
Rising temperature is one of the most crucial aspects of climate change. The global increase in average temperatures can significantly affect aircraft performance. Higher temperatures mean less dense air, affecting aircraft performance, especially during takeoffs and landings. Commercial aircraft have a harder time generating the required lift to become airborne.
Longer runways, reduced mass takeoff weight (MTOW), and optimized density altitudes are some ways to combat reduced performance. Having said that, whether or not rising temperatures can adversely affect the performance of future engines can be debatable in some respects.
Thrust performance of jet engines
Jet engines work on the principles of compression, combustion, and expansion. The incoming air passes through a series of compressor stages (blades and vanes), where it is compressed. As the air velocity decreases, the pressure and temperature increase. The compressed air is mixed with pressurized fuel and ignited in the combustion chamber. The hot gasses expand and pass through a series of turbine stages (blades and vanes) before exiting through the exhaust. During the process, the exit velocity becomes greater than the free-stream velocity, which generates thrust and propels the aircraft forward.
Density is directly proportional to pressure and indirectly proportional to temperature. As air pressure increases, with constant temperature, air density increases. Conversely, when air temperature increases, with constant pressure, air density decreases. Air density decreases by about 1% for a decrease of 10 hPa in pressure or 3 degree Celsius increase in temperature.
Since the aircraft performance is based on density altitude, it is greatly affected at higher temperatures and elevations. This is because more work is required (by turbines) when the air is less dense. Conversely, the engine performance is optimized at lower altitudes with relatively colder temperatures. Are jet engines limited by the thrust performance at higher temperatures or greater elevations? The answer is no. Despite popular belief, if this was the case, no engines could function at or around hot deserted locations.
Effects of rising temperatures on the engine’s Exhaust Gas Temperature (EGT)
The temperature of the incoming air, also known as the outside air temperature (OAT), is one of the key parameters defining the engine's performance. Almost all commercial jet engines are limited by the maximum temperature they can achieve during operation, attributed to the EGT. It is a measure of the temperature leaving the exhaust of the turbine.
In order to determine the performance of the engine, the EGT Margin is calculated. It is the difference between the incurred takeoff EGT and the Redline (maximum limit) EGT. Newer engines have a higher EGT Margin compared to engines that have been on the wing for longer periods of time. As the EGT Margin decreases, the specific fuel consumption (SFC) of the engine increases.
As a rule of thumb for modern engines, every 1 degree Celsius increase in OAT correlates to approximately 8 degree Celsius increase in the EGT. The performance of the engine is, in fact, limited by the maximum EGT the turbines can withstand. Therefore, EGT limits differ for the same thrust engines operating in different geographical regions. For example, the CFM International LEAP 1B28 and 1B28B2, powering the Boeing 737 Max, are both rated at a maximum takeoff thrust of 29,320 lbf (130.41 kN). However, CFMI's Type Certificate data sheet indicates,
Engine models which have the same approved ratings in standard static conditions will provide different levels of thrust at altitude and/or high temperature conditions. This is controlled by the engine identification plug.
In this example, the 1B28B2 will provide much less thrust than the 1B28. This is because the B2s are subjected to hot and high locations. Similarly, the maximum take-off EGT of the LEAP 1B28 is 1060 degrees Celsius compared to 1038 degrees Celsius for the 1B28B2. It is noteworthy that while less dense air marginally reduces the output thrust, the engine performance is limited by the maximum EGT limit.
Some of the operators of the LEAP 1B28B2 are located in the Arabian Gulf region. Notably, the life span of such engines is shorter than the engines operating in near-sea-level altitudes and cold temperatures.
Summary
With average global temperatures rising, a further increase in the redline EGT is needed. Constant advancements in temperature-resistant materials are driving manufacturers to push the EGT limit of future engines. It is notable that, on average, the EGT limit of engines has increased by over 100 degrees Celsius in the last few decades.
Novel superalloys and composites have the potential to improve engine performance further. Moreover, increasing tropopause fluctuations due to climate change may drive future commercial aircraft to fly even higher (~50,000 ft) to achieve better fuel efficiency. So, would climate change adversely affect the performance of future engines? Perhaps not. The technology is going to catch up.
What are your thoughts on the effects of climate change on future jet engines? Have you noticed a difference in engine thrust departing from a hot and high airport? Tell us in the comments section.