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The arrangement under which later derivatives of an initial aircraft type design can be manufactured under variations to the original Type Certificate thereby avoiding the more complex procedures involved in gaining approval under a completely new Type Certificate. The effect of this is that although the standards applied to enable the issue of as Type Certificate have always progressively increased over time in the light of experience and general technological progress, these benefits are not reflected in the certification of ‘similar’ aircraft by means of variations. There is no general time limit to these grandfather rights and they can remain effective over a long period, for example the sequence which began with the Airbus A320 subsequently continued thought the A321, A319 and finally, the A318 and a similar sequence led from the Doublas DC9 through to the much later MacDonnell Douglas MD-80 and MD-90 series and eventually to the Boeing 717.