CDAs and speed control enable me to stick aeroplanes down the approach 2.5nm apart
Did you mean that? Speed control is absolutely essential to 2.5nm spacing, but why is CDA?
CDA is about the vertical profile of the arrival stream; spacing is about the relative lateral positions in the arrival stream - the latter is not dependent on the former. In fact, when aiming for tight spacing it is safer to suspend the CDA, establish vertical separation, turn in, and then resume descent. Having spent a number of months reviewing the Heathrow final approach incidents of the last 18 months, it is clear to me that if separation was given priority over CDA the serious encounters on final approach would be largly eradicated.
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