VFR Airborne Implementation
All aircraft flying VFR in designated airspace are required to carry and operate Mode S
ELS airborne equipment by 31 March 2005 with the following Transition Period:
New production aircraft to be compliant by 31 March 2005, although there is now a general relaxation until 31 March 2008.
Completion of retrofits, irrespective of date of first CoA issue, by 31 March 2008.
http://www.eurocontrol.int/msa/public/standard_page/modes_homepage.html