read
https://en.wikipedia.org/wiki/Joint_...on_Authorities
https://web.archive.org/web/20110629...roduction.html
The JAA started as
Joint Airworthiness Authorities in 1970. Original objectives were only to produce common certification codes for large aeroplanes and for engines in order to meet the needs of European industry and international consortia (e.g.,
Airbus)
Basically every country had different rules, France would not except say a brake assembly made by one company, Netherlands would not accept say an autopilot system made by another company on their aircraft and the list went on, this meant Airbus would have to produce several versions of the same aircraft, different brakes for France, different Autopilot for the Netherlands.. none of them standardised and hence costing Airbus in their partners a fortune in producing different variants of the same aircraft, JAA standardised that so each country accepted a common standard of aircraft.. later as more parameters were added as to what JAA did they formed EASA to take over.