In 2012, the EU-wide cap on aviation emissions is set at 97% of the average annual emissions for the years 2004-2006. In 2013-2023, the cap was lowered to 95%.
The first trading period is the year 2012, and the second is the 2013-2020 period (initially but now extended until 2023).
85% of the emissions cap is given to airlines free of charge based on reported payload for 2010.
15% of the emissions cap is available through auctioning while additional allowances to cover growth must be purchased from other sectors (open trading).
See more info here: EU ETS Aviation - Baseline and Trading Periods