Airlines Trim Emissions with APU High Efficiency Mode

The world’s airlines are fully onboard when it comes to reducing the environmental impact of air travel per passenger mile. Many airlines have made public commitments to sustainability and the 300 airlines that are members of the International Air Transport Association (IATA) have set their sights on achieving net-zero carbon emissions by 2050.

Technology companies like Honeywell support the airlines in achieving their ambitious goals, complying with more stringent regulatory guidelines, and meeting the growing expectations of stakeholders, including the flying public.

One breakthrough development from Honeywell is already helping Airbus A320 series operators save fuel and reduce carbon dioxide emissions produced by the aircraft’s auxiliary power unit (APU). It’s the High Efficiency Mode (HEM) upgrade for the Honeywell 131-9A APU. Honeywell’s 131-9B APU HEM upgrade will be introduced later this year for the Boeing 737 family of single-aisle aircraft.

HEM Upgrade Delivers Immediate Results

Technologies in various stages of development and implementation will help airlines reduce fuel consumption and trim their carbon dioxide emissions in the years to come. The HEM upgrade is different because operators can realize results starting today.

“The HEM upgrade was introduced in July 2020 and has gained wide acceptance especially by A320 operators enrolled in Honeywell’s industry-leading Maintenance Service Plan (MSP) for APUs,” said Dave Shilliday, Vice President and General Manager for Power Systems at Honeywell.

“By the end of 2022, we had 27 Airbus operators flying HEM-enabled APUs on around 700 aircraft. Thanks to the HEM, the APUs on those aircraft are using less fuel and generating a smaller carbon footprint per hour of operation,” he added.

HEM Upgrade Reduces Fuel Burn and CO₂ Emissions

According to Honeywell analysis, those 700 aircraft would save more than 630,000 gallons of fuel and avoid releasing 6,300 metric tons of carbon dioxide into the atmosphere during a typical year (about 1,800 APU operating hours). That is equivalent to removing about 1,350 cars from the roadway for a year, according to the Environmental Protection Agency (EPA) carbon calculator.

“Those figures are pretty impressive,” Shilliday pointed out. “But imagine the impact HEM would have if it were installed more broadly across the global Airbus A320 fleet of 5,700-plus aircraft. We’re talking about the opportunity to save more than 5 million gallons of fuel and reduce emissions by 50,000 tons every year. That’s just from the existing Airbus A320 fleet. If we also add in potential results from 737 HEM implementation, those numbers more than double.”   

“With addition of the Boeing 737 family to the mix, HEM capabilities will be available for APUs on the world’s two most popular aircraft,” he added. “We’re very excited about the opportunity to help our customers reduce costs, improve operations and achieve their sustainability goals with HEM-equipped APUs.” 

How HEM Shrinks APUs’ Carbon Footprint

Honeywell invented the APU and has produced more units than anyone else – well over 100,000 since the 1950s. APUs generate power to run air conditioning, lights, and all things electrical while an aircraft is on the ground and can even restart an engine or provide power in flight if necessary.

The HEM upgrade takes APU performance and efficiency to a new level. It’s the product of a continuous seven-decade quest by Honeywell engineers to improve the company’s industry-leading family of APUs. HEM is essentially an innovative and highly efficient diffuser, controlled by advanced software, which autonomously regulates airflow to the APUs compressor section.

As a result, the engine burns fuel more efficiently at lower temperatures – delivering the highest-ever levels of performance and efficiency for the airline operator. Other benefits include easy installation, lower maintenance requirements and longer service intervals.

New Honeywell 131-9A APUs (Series 25 and above) are delivered with the new engine compressor diffuser already installed, which enables an on-wing upgrade to HEM.  Pre-Series 25 APU models can be easily upgraded during their next major service event, without additional APU downtime.

Sustainable Fuel Raises the Bar on CO₂ Reduction

Operators could significantly boost the environmental performance of the APU by transitioning to a fuel mixture that includes conventional jet fuel and sustainable aviation fuel (SAF), which is becoming more available all the time.

“Honeywell APUs run great on a combination of SAF and fossil fuel,” Shilliday said. “We were the first to certify our APUs to use 50-50 SAF blends and we’re getting ready to test other blends and 100% SAF soon. The great news is that SAF is a drop-in replacement for the fuels we use today, so there is no need to modify the APU or its fuel system to move to this environmentally friendly option.”

IATA expects 65% of the required carbon reductions to come as the airlines transition to SAF. Honeywell pioneered processes to produce next-generation, planet-friendly SAF, which are widely used by refiners.  

“The broader application of HEM-equipped APUs burning SAF or SAF blends will be a significant contributor as airlines look for opportunities to improve fuel economy and environmental performance,” Shilliday said. “The good news is that the HEM upgrade is proven technology that airlines can install and start using right away.”