Direct CO₂ production is the CO₂ output from an aicraft's engines as a product of the combustion of aviation fuel.

Aircraft and their engines are subject to comprehensive testing prior to their rollout as airline stock. From this performance research it is possible to make reasonably reliable predictions about an aircraft's direct CO₂ output for a given distance.

While there are come complicating factors such as holding pattern times, flight corridors and varying performance at different alitudes, a performance curve can be inferred over a range of distances. Using the same rules for all aircraft, we can at least make reliable comparisons of flight options based on the various equipment scheduled for a given route.

Nitrogen Oxides (NOx) are a further byproduct of global air traffic at cruising altitudes. NOx contributes to ozone formation, most significantly in the upper troposphere and lower stratosphere. Therefore in our calculations NOx account for a further ~80% environmental impact on top of that of direct CO₂ output.

This is based on exhaustive research from a number of institutions over the last few decades. It is largely accepted that the impact of NOx has a rough RFI value of 0.8.

Vapour trails (aka contrails) form above 25,000 feet, provided that air is moist and colder than -40 degrees Celsius. Like clouds, they are the result of water vapor that forms into droplets by condensing into particles in the air, such as soot from aircraft engines. Depending on the conditions (humidity and temperature), they may dissipate immediately or endure for long periods.

Bernd Kärcher, also of the Institute of Atmospheric Physics in Oberpfaffenhofen, reckons contrail cirrus clouds cover around 0.6 percent of the global skies at any one time — nine times the amount covered by contrails themselves. In areas with high amounts of air traffic, they can merge to cover as much as 38,000 square miles, roughly the size of Indiana, and last for many hours or even days. How Airplane Contrails Are Helping Make the Planet Warmer - Fred Pearce 2019

Like regular cirrus clouds, contrail cirrus clouds have two competing effects on climate - they shade us slightly by reflecting incoming sunlight back into space - but they also trap heat radiating from the earth’s surface, so causing warming in the air below.

Unlike other effects from aircraft engines, this effect is not cumulative, so if air traffic were to suddenly stop then the environmental effects from contrails would be over in a matter of hours. However, the overall impact from constant contrail cover is significant enough to have a pronounced effect on the resultant environmental calculation.

Contrail formation is the strongest contributor to the RFI performance curve for a given aircraft. Some fuel-efficient engine designs lead to an increase in contrail formation probability, significantly reducing the environmental performance of the aircraft - often in contrast to the improvement stated by aircraft manufacturers and airlines.

Since these effects are dependent on conditions, as well as the quantity and type of contributing soot produced by different engines, this is the hardest calculation to make. It is generally agreed that the RFI impact value for an airliner will be between 0.5 and 3.