There was a document from sept 2001,(Noise From Arriving Aircraft- An Industry Code Of Practice)which stated:
For practical purposes a working definition of CDA is Below 6000ft,
*No level flight or
*One phase of level flight not longer than 2.5nm
1)Due to constraints of the GEMS system and the different elevations of airports ,Cda achievement will be monitored from a height of 5,500ft AAL
2)'Level flight' is interpreted as any segment of flight having a height change of not more than 50ft over a track distance of 2nm or more, as recorded in the airport NIK system