This is the second of two initial reports exploring airport sustainable finance trends and indirect emissions KPIs. For a look at recent transactions, see: Will aircraft emissions KPIs gain traction in airport sustainable finance?
With input from Rob Neale, Chief Product Officer at PACE.
Sustainability-linked financing has become increasingly popular among airports over the past three years, and several transactions have begun incorporating indirect aircraft emissions targets. This report digs further into the most common aircraft emissions metrics for airports and why, unlike for airlines, these KPIs are rarely cut from the same cloth.
Aircraft indirect emissions: LTO and CCD
Indirect (Scope 3) emissions from aircraft are usually measured until 3,000 feet (the ‘LTO’ phases the airport can, to varying degrees, influence). A broader scope than Landing and Take-Off (LTO) is Climb, Cruise, and Descent (CCD) or ‘cruise’ (the entire emissions from a departing flight):
- LTO GHG emissions metrics, like those used in the Heathrow and Gatwick sustainability-linked transactions, are derived from the ICAO Doc 9889 definition of an LTO cycle for aircraft engine emissions certification purposes. The LTO cycle consists of four modal phases chosen to represent approach, taxi/idle, take-off and climb. It assumes that operation at take-off power abruptly changes to climb power at the end of the take-off roll and that this is maintained unchanged up to 3 000 ft. The ICAO definition also provides reference emissions (thrust setting as a percentage of rated thrust) for each of the four phases (see page 44).
- CCD emissions, on the other hand, go beyond LTO phase to also account for cruise, or the full duration of the flight. This is the metric used by Heathrow Airport in its pioneering sustainability-linked bond. In the case of Heathrow, CCD has a unique relevance. Since at least 2014, and amid climate scrutiny for the airport’s proposed third runway, CCD estimates have been central to the discussion of whether Heathrow expansion (already Europe’s busiest) could compromise the UK’s carbon budget.
LTO and CCD calculations
Using average taxi-in and taxi-out times, location, and altitude data, LTO emissions can be calculated for a given aircraft type and airport. “We take the typical periods that the engine is in each of these [four LTO] states to determine the fuel flow and therefore then apply the emissions factor,” explains Rob Neale, Chief Product Officer at PACE, which in addition to its flagship PACE Platform, now also offers a PACE Airports calculator.
An increasing number of airports are including aircraft LTO emissions in their sustainability disclosures, but as it is a once-in-a-year data gathering exercise, most work with limited data granularity. “What we are hoping to do here is provide this [PACE Airports] on a near real-time basis, but also uncover the underlying drivers behind it,” Neale points out.
No set metric, and limited basis for comparison
The Airport Carbon Accreditation (ACA) programme, a seven-level carbon management certification covering 618 airports across 94 countries, recommends airports at Level 3 or above to track at least LTO as part of their minimum Scope 3 emissions footprint monitoring. Airports seeking higher accreditation levels (particularly 4+ and 5) also need to include Scope 3 emissions in their targets. However, unlike lessors or airlines, there is no consensus among airports on whether absolute or intensity metrics should be followed.
Emissions per aircraft movement (an aircraft landing or taking off), emissions per passenger, or absolute emissions are just some of the ways airports examine their Scope 3 aircraft emissions. Different geographies, resulting route lengths and networks or infrastructure design are some of the features that make like-for-like airport comparisons difficult. To give one example, according to PACE data, LTO emissions for an Airbus A320 during the Summer of 2024 were approximately 24% higher in London Heathrow than in Brussels, largely due to longer taxiing times at Heathrow. Both airports have a comparable size of 12 square kilometres, but taxiway distances differ, Heathrow is a substantially busier airport, and Brussels has one more runway – just to name a few likely factors.
“There is no universally recognised intensity metric to compare airports to airports. What we can see is there is an appetite to compare operators at the same airport and league tables,” explains Neale. Last October, Heathrow relaunched its airline sustainability performance table, offering data on airlines that use the airport as part of the airport’s effort to incentivise deployment of quieter and more fuel-efficient aircraft.
Taxiing times are taxying times
While comparisons may be difficult to draw, there is a universal consensus that reducing taxi times is a key priority for lowering fuel burn, noise, and emissions.
Much like cars in bumper-to-bumper traffic, long taxi times for aircraft are a drag on fuel efficiency. Idle thrust (typically a 7% power setting) is usually enough to move an aircraft on the ground, but breakaway thrust (30% to 45% power setting) is required to overcome stationary friction after every stop, increasing fuel burn. The longer the taxi time and the more frequent the stops along that journey, the higher the emissions. In addition to noise impacts and CO2, at idle thrust NOX is emitted in higher quantities, and soot particles are more prone to react with their surroundings, impacting air quality at ground level.
This makes reducing taxi times a core priority for any airport. “Out of [all] variables influencing LTO emissions, primarily the biggest lever is taxi times, and that is directly controllable,” comments Neale. Optimising aircraft movements to reduce taxi and idle times, or encouraging airlines to use single-engine taxi, are some strategies to reduce taxi times and emissions. Eventually, solutions like in-wheel electric taxiing system WheelTug, which is aiming to be operational by 2026, will be able to further reduce taxiing emissions.
The Ishka View
A growing interest by certain airports to reduce their Scope 3 aircraft emissions – whether on an LTO or CCD basis – could set the scene for incentives (or penalties) to favour the least polluting aircraft types. This makes it important for airlines and aircraft financiers to understand airport decarbonisation strategies. Modulated airport charges to encourage less environmentally impactful aircraft types are already a reality in some major European airports. Airport initiatives to reduce fuel burn, particularly during the taxi phase, are also set to take on greater importance. In addition to the expected entry into service of autonomous taxiing solutions like WheelTug, which is initially targeting narrowbody aircraft, there is also considerable R&D investment into non-autonomous taxiing (TaxiBots) and single-engine taxiing practices.
