It is hard to think of an airline that has not mentioned the word ‘sustainability’ during recent years, with mounting pressure being placed on the aviation industry to reduce emissions.
Whether it’s via Sustainable Aviation Fuel (SAF), engine performance improvements, or operational changes, such as reducing engine burn while taxiing and/or while on the ground, industry stakeholders have sought a variety of ways to control emissions.
One motivator for these controls came following the Paris Agreement, which was negotiated by 196 parties at the 2015 United Nations (UN) Climate Change Conference (COP21) in Paris, France. The agreement aims to manage the increase in global average temperatures to below 2°C compared with pre-industrial levels, while also pursuing efforts “to limit the temperature increase to 1.5°C above pre-industrial levels,” according to the United Nations Framework Convention on Climate Change (UNFCCC) secretariat.
There has been some discussion about whether the agreement is legally binding since there are no fines or legal consequences for not meeting goals. However, each country has its own Nationally Determined Contribution (NDC) which determines the ways that they will reduce emissions to reach the goals set under the Paris Agreement. For example, the European Union’s (EU) latest NDC issued in December 2020 includes a paragraph stating that “emissions from aviation are currently addressed by EU legislation and will be partially addressed by international measures under ICAO [International Civil Aviation Organization – ed. note]”.
Most recently, the European Commission (EC) announced an EU-wide mandate of SAF across airports in the bloc, with the minimum usage of SAF growing every several years starting from 2025. As such, aviation stakeholders will have little choice but to adhere to the mandate which will see the minimum SAF use at airports rising to 70% by 2050.
That regulation is just one example of how the EU, alongside other governments across the globe, has enforced climate-related regulations on the aviation industry.
However, rising temperatures could also result in an additional problem for airlines, aircraft, and engine manufacturers, as well as airports – degrading aircraft and aircraft engine performance.
Hot weather operations
Both hot and cold weather operations have their challenges. The air is less dense when it is hot, resulting in reduced engine performance, while ice forming on control surfaces during colder months can cause the loss of aerodynamic performance.
These are just a few factors that airlines must consider when operating flights during more adverse weather.
“Air is less dense when it is hot, which reduces both lift and engine efficiency. Even worse, high temperatures are often found in combination with dry conditions, which means blowing sand and dust: bad news for anything mechanical, and especially bad for a modern aircraft,” read a Lockheed service news publication.
In an informational page about Exhaust Gas Temperature (EGT) overlimit events Airbus highlighted that “every increase of 1°C in OAT [Outside Air Temperature – ed. note] can lead to an EGT increase of approximately 3°C to produce the same thrust at takeoff, depending on the engine type”.
Subsequently, reduced engine performance results in reduced fuel efficiency and, in turn, greater emissions during certain flight profiles. In the most extreme cases, airlines are forced to cancel flights, which is what happened with some American Airlines flights from Phoenix Sky Harbor International Airport (PHX) in June 2017.
“It’s so hot in Phoenix that airplanes can’t fly,” read a headline by the Washington Post, referring to June 20, 2017, when temperatures hit 118 °F (47.7 °C) in Phoenix, Arizona.
At the time, American Airlines canceled 50 flights to and from PHX on the Mitsubishi CRJ (then known as Bombardier CRJ) as the aircraft is not certified to operate under temperatures above 118 °F (47.7 °C).
While larger jets with bigger engines have a greater tolerance for hot weather, they are still impacted by adverse weather conditions.
For example, in its “Getting to grips with aircraft performance” manual, Airbus highlighted that with increasing temperatures and a subsequent decrease in air density, flight crews must increase the true air speed (TAS) “to compensate for the air density reduction”. Furthermore, “when the Outside Air Temperature [OAT] increases, the takeoff distances are longer, and takeoff climb gradients are reduced”.
The manual also offered an example of a Regulatory Takeoff Weight Chart (RTOW) of an Airbus A319, that is departing from Paris-Orly Airport (ORY) from runway 08 at a wind of 0 knots and an OAT of 24 °C, with the Maximum Takeoff Weight (MTOW) being limited to 73.6 tons. Under the same conditions, but with an OAT of 4 °C, the MTOW rises to 74.4 tons. In contrast, the same conditions with an OAT of 44 °C reduces the MTOW to 72.7 tons.
Changes to the overall tonnage of the aircraft are even more extreme with faster headwinds or tailwinds: with an OAT of 24 °C, but with a tailwind of -10 knots, the MTOW is 71 tons. At a headwind of 20 knots and the same OAT, the MTOW is 75.2 tons.
The A319 was certified with an MTOW of 75.5 tons, with this figure including everything from onboard fuel to food items and even passengers.
According to SKYbrary, a repository of safety knowledge and a project by EUROCONTROL, the organization that looks to optimize the air traffic management network performance in the EU, OAT is defined as “the ambient temperature measured outside an aircraft”.
Engines experiencing issues
There have been some cases where engines have responded poorly to their operating environment, namely in India.
Pratt & Whitney, which supplies the PW1100G engine for the Airbus A320neo aircraft family, experienced operational issues between 2017 and 2019, specifically in India. The harsh weather environment, including high levels of pollution and high temperatures, contributed to the degradation of the combustion chamber linings.
In a report from October 2017 citing a person familiar with the matter, Reuters said that Pratt & Whitney was working on a new design for the combustion chamber lining, specifically addressing India’s adverse weather conditions. Following regulatory approval, all PW1100G engines would come with the new engine combustion chamber linings, the source added.
At the time, Reuters also reported that IndiGo and GoAir (now known as Go First) had grounded a dozen aircraft for a prolonged period due to a lack of spare engines.
Between 2018 and 2019, the United States (US) Federal Aviation Administration (FAA), as the State of Design, which issued the Type Certificate (TC) under the jurisdiction of the US, released several Airworthiness Directives (AD) addressing various issues with the PW1100G. For example, an AD from November 2018 was issued following reports “of in-flight engine shutdowns and aborted take-offs as the result of certain parts affecting the durability of the rear high-pressure compressor (HPC) rotor hub knife-edge seal”. Subsequently, the FAA ordered operators to replace “the diffuser case air seal assembly, the high-pressure turbine (HPT) 2nd-stage vane assembly, and the HPT 2nd-stage borescope stator vane assembly with parts eligible for installation”.
Another directive issued by the FAA in November 2019 was due to “multiple reports of LPT [Low-pressure turbine – ed. note] 3rd-stage blade failures causing a reduction of engine thrust”. Furthermore, the agency received information about additional failures between July 2019 and issuing the Notice of Proposed Rulemaking (NPRM), with these engine issues occurring “primarily on engines operated by certain airlines”. As such, the AD required operators, none of which were in the US, to replace certain part number (P/N) LPT 3rd-stage blades within the specified timeframes.
However, upgrades to an engine throughout its lifespan are nothing out of the ordinary. Aircraft engine manufacturers routinely issue improvements to reliability and performance following feedback from customers, as well as data fed directly from an airline to the Original Equipment Manufacturer (OEM). Likewise, ADs are nothing unusual, with regulators addressing problems that are seemingly small to ensure that the broader picture of safety remains intact.
But the PW1100G was one of the most high-profile cases that experienced issues related to the operating conditions of a specific region. By February 2018 this had spiraled into Airbus suspending deliveries of all PW1100G-powered A320neo family aircraft, until such time as Pratt & Whitney modified its design. The engine OEM did so at the end of the same month, resuming deliveries of the PW1100G following the release of “a revised configuration as a solution to the issue relating to a knife edge seal on the High Pressure Compressor (HPC) aft hub which affected a limited subpopulation of the PW1100G-JM engine that powers the Airbus A320neo aircraft,” according to a statement from Pratt & Whitney on February 21, 2018.
Could climate change increasingly affect engine design?
One of the consequences of an ever-changing climate will be more extreme weather events, meaning more days when the temperature will rise well above the norm.
“The warming resulting from current climate change (around 1 °C) has raised the mean airport density altitude (i.e., the altitude associated with air at a given pressure at standard atmospheric conditions) by approximately 100 ft, and expected additional warming of 1–3 °C by the end of the century will result in further increases of additional hundreds of feet,” reads one study conducted by three scholars and published by Climatic Change, an academic journal focused on climatic variations, in April 2017. “Prior work has shown that the frequency of days on which a Boeing 737-800 requires weight restriction is likely to increase by 100–300% at several airports in the USA in the coming decades,” the article continues.
The question remains whether this will affect engine design in the near-term future. Designing, building, and establishing a supply chain for an aircraft engine takes years: Rolls-Royce flew its UltraFan engine on a Boeing 747 testbed for the first time in 2014, having unveiled the concept the same year. However, only in December 2022 did the company finish building a technology demonstrator, before going on to successfully test the engine in May 2023.
“[…] We are witnessing history in the making; a step-change in engine efficiency improvement. When combined with Sustainable Aviation Fuels, more efficient gas turbine engines will be key to hitting the industry’s target of Net Zero flight by 2050. Today we are closer to achieving this ambition,” said Tufan Erginbilgic, the Chief Executive Officer (CEO) of Rolls-Royce at the time.
Shortfalls or rather slow advancements in engine technology have resulted in aircraft programs being scrapped, too. Embraer put off its new turboprop when it was not satisfied with the proposals submitted by engine OEMs. Notoriously, Boeing shelved the New Midsize Airplane (NMA), with Stanley Deal, the Chef Executive Officer (CEO) of Boeing Commercial Airplanes (BCA) saying during the company’s Investor Conference in November 2022 that new aircraft have “to be 20% better than the other airplanes that they [airlines] can buy in the existing portfolio”.
“There is nothing proven in the propulsion side of the house, nothing that’s going to deliver that in this decade. There are concepts, but there is nothing that’s going to do it,” continued Deal. Similarly, when David Calhoun, the CEO of Boeing, spoke during the Qatar Economic Forum organized by Bloomberg in May 2023, he opined that every new aircraft family, which requires billions of investments, needs to be “at least 20% if not 30% better than the last airplane”.
“We will [design a new aircraft], but we have to prove out that the technology suite that will deliver that before we design the airplane,” Calhoun continued. He added that, because of constraints related to propulsion and wing design, “it’s going to be at least until the mid-2030s” when Boeing and Calhoun assumes, Airbus, will be in a position to announce a new clean-sheet aircraft.
Perhaps, much like today, engine manufacturers will investigate upgrading current aircraft engines in response to the fact that a warming planet could mean less revenue for their customers. A different study by three scholars, also published by Climactic Change in February 2020, analyzed meteorological parameters at multiple airports in Greece and concluded that “results show that for an Airbus A320, operating from the relatively short 1511m runway at Chios Airport [JKH], the required reduction in payload would be equivalent to 38 passengers with their luggage, or fuel for 700 nautical miles (1300 km) per flight, for the period between the A320’s entry to service in 1988 and 2017”.