The EU has recently updated their regulataions on social insurance as per below.
EU Regulation 883/2004 is soon to incorporate an amendment to Article 11.5
the concept of ‘home base’ for flight crew and cabin crew members is defined as the location, assigned by the operator to the crew member, from where the crew member normally starts and ends a duty period or a series of duty periods and where, under normal circumstances, the operator is not responsible for the accommodation of the crew member concerned.
Therefore even whilst training for example, you must register and pay social security in the country where you activity it. There are some special circumstances where by with the agreement of the countries concerned, you can pay social security in the country of your residency. However, this is not your accountants decision to make, and the various countries must be approached and decide between them. Sometimes this takes several months to conclude.