Background Notes

Introduction

Statistics on Aviation are collected under Statutory Instrument (S.I.) No. 10 of 2020 - Statistics (carriage of passengers, freight and mail by air) Order 2020.

The data in this release provides details on the number of passengers classified by arrivals and departures, and by reference to national and international traffic; the number of flights by arrivals and departures and information on air freight and mail (in tonnes).  The release also highlights the top routes for each of the five main airports: Dublin, Cork, Shannon, Knock and Kerry.

Data sources

Aviation statistics are compiled from data supplied by all Irish airports. The following Irish airports provide data to the Central Statistics Office: Dublin, Cork, Shannon, Kerry, Knock, Waterford, Connemara, Donegal and Inishmore. Galway and Sligo airports ceased operations in 2011. There have been no commercial flights in Waterford Airport since June 2016. Data for the five main airports is supplied on a monthly basis. Data for regional airports is supplied annually to the Central Statistics Office.

Coherence 

The statistics in this release are not fully coherent with the Air and Sea Travel statistics. The differences in passenger numbers which are typically less than 0.5%, arise because the Air and Sea Travel statistics exclude transit passengers, domestic air travel and flights to Northern Ireland. It should be noted that passenger numbers for Shannon and Dublin airports exclude transit passengers on technical transit flights as these are fuel stop only flights.

Seasonal adjustment

Seasonal adjustment for passenger numbers is conducted using a direct seasonal adjustment approach. However, the overall passenger totals are estimated using the indirect approach, i.e. the ‘Total passengers’ is derived by adding the monthly seasonally adjusted passenger numbers for each airport. The main reason for adopting the indirect approach is to attribute the monthly and annual changes in passenger numbers to each airport. Seasonal adjustment models are developed for each airport based on unadjusted data spanning from January 2012 to the current period. These models are reviewed on an annual basis; however seasonal factors are updated each quarter.

The adjustments are completed by applying the X-13-ARIMA model, developed by the U.S. Census Bureau to the unadjusted data.  This methodology estimates seasonal factors while also taking into consideration factors that impact on the quality of the seasonal adjustment such as:

• Calendar effects, e.g. the timing of Easter,

• Outliers, temporary changes and level shifts in the series. 

Seasonally adjusting the Aviation data series during the COVID-19 crisis period will be challenging until the scale and shape of its impact on the time series is better understood. The initial seasonally adjusted results might be revised for some months ahead as future observations become available. Users should be aware that there is increased uncertainty around the seasonally adjusted figures during this period. 

For additional information on the use of X-13-ARIMA see https://www.census.gov/srd/www/winx13/

Definitions

Main airport: A main airport is defined as an airport through which in excess of 150,000 passengers fly per annum. The five main airports in Ireland are Dublin, Cork, Shannon, Knock and Kerry.

Regional airport: A regional airport is defined as an airport through which less than 150,000 passengers fly per annum. The three regional airports in Ireland are Donegal, Connemara and Inishmore.

Scheduled flight: The term 'scheduled' in this release, refers to a commercial air service operated according to a published timetable, or with such a regular frequency that it constitutes an easily recognisable systematic series of flights.

Non-scheduled flight: The term 'non-scheduled' in this release, refers to a commercial air service other than scheduled air service e.g. chartered or private aircraft flights.

Direct transit passengers: These are passengers who, following a short stop, continue their journey on the same aircraft with the same flight number as the flight on which they arrived. These passengers are en route to another destination and generally do not disembark from their aircraft.

Transfer passengers: Transfer passengers or ‘connecting passengers’ are defined as passengers who make a stop at an airport without any purpose other than to board another aircraft to reach another destination. Unlike transit passengers, transfer passengers disembark the aircraft and pass through the airport en route to their connecting flight.

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