Our Methodology

We gather aircraft and flight data from multiple reliable sources to ensure accuracy.

Airport Database

Airports mapped to ICAO codes, used to calculate great-circle distance

Emissions Methodology

CO₂ estimates calculated using the EUROCONTROL EMEP/EEA Guidebook 2023

Engine & Aircraft Specs

Per-aircraft engine specifications used to calculate LTO emissions

ADS-B Flight History

Real-time flight tracking data from unfiltered ADS-B signals

A raw ADS-B signal contains a Mode S ICAO 24-bit address which is a hexadecimal identifier, not a tail number. Resolving that hex code to a confirmed registration, aircraft type, and engine specification requires four steps:

1. Hex to registration: Mode S code matched against our registration database to retrieve the civil tail number
2. Registration to aircraft type: confirmed registration matched to manufacturer, model, and variant
3. Type to engine specification: aircraft type matched to engine type and LTO fuel flow rates from the ICAO Engine Emissions Databank
4. Engine count confirmation: engine count confirmed from type specification and applied to the LTO calculation

Raw ADS-B data is a continuous stream of position messages, not a list of flights. A confirmed flight event requires:

• Position messages showing transition from ground to airborne state at a known aerodrome
• Continuous signal confirming sustained climb
• A return to ground state at a destination aerodrome
• Flight meeting minimum distance and duration thresholds. Events below these thresholds are classified as ground movements or signal anomalies and excluded

Where origin or destination cannot be confirmed from ADS-B signal, the endpoint is recorded as unknown (UNK) rather than excluded. These flights appear in the database with partial routing information.

Signal gaps and coverage limitations

Where an aircraft's ADS-B signal drops mid-flight and is never recovered, for example, a transponder signal lost at cruise altitude with no subsequent detection at a destination, the flight is excluded from the published record. It cannot be confirmed as a completed flight.

Carbon Sky Index applies a conservative approach throughout flight detection: where there is ambiguity, we err toward undercounting rather than overcounting. This means some legitimate flights may be absent from our database due to coverage limitations, particularly in regions with lower receiver density.

We calculate CO₂ emissions for each flight using a combination of aircraft-specific data and flight parameters:

LTO Cycle

Takeoff and landing emissions calculated using the ICAO default LTO cycle, scaled by number of engines and mode-specific fuel flow rates

Cruise Phase

Cruise emissions calculated as hours flown multiplied by per-aircraft CO₂ rate, derived from manufacturer fuel burn data

Great-Circle Distance

Flight distance approximated as straight-line airport-to-airport routing, representing minimum plausible distance flown

Tank-to-Wake Only

Output is mission CO₂ only. No reserve fuel, APU, ground power, NOx, contrails, or lifecycle emissions are included

The emission factor

All fuel burn figures are converted to CO₂ using a fixed emission factor of 3.16 kg CO₂ per kg of jet fuel burned. This figure is derived from the stoichiometry of Jet-A1 combustion — jet fuel is approximately 86% carbon by mass. This factor is used by IPCC, ICAO, EUROCONTROL, and the UK Government's Conversion Factors for Company Reporting.

Our Formula

Scope of our emissions figure

We prioritize transparency about what we know and what we estimate:

The data presented on Carbon Sky Index is inferential and should be understood as directional, not definitive. Emissions figures are modelled estimates based on ADS-B flight detection and aircraft specifications — they do not represent observed fuel burn or verified operational data.

Known limitations include:

• Great-circle distance underestimates actual flown distance
• Flight duration based on ADS-B detection may include taxi time or signal gaps
• LTO fuel flow uses ICAO default cycle values, not airport-specific conditions
• Cruise burn is modelled linearly and does not account for altitude, payload, or wind

We welcome corrections and additional information from aircraft operators or data providers.