Thank you, Roffa, I'd love to come to see your TC TRUCE radar replays ... but wouldn't they just be proving that the current system we are debating doesn't prevent altitude busts ... ?
Actually, no they wouldn't. They'd be proving the fact that some crew are still stupid enough to get airborne on the wrong QNH, despite reading back the correct one.
Reading back the QNH instead of just acknowledging the ATIS letter is a way of cross checking that the crew have copied the QNH correctly as it is a safety critical piece of info in a very congested TMA.
I don't think Heathrow have ever done a scratchpad survey to show how many times they have had occasion to correct crews who have written down the wrong QNH by mistake but the fact of the matter is they do so on a fairly regular basis.
The fact that some crews
still manage to get airborne on the wrong setting despite correction, or even
despite reading back the correct QNH in the first instance is
the only proof that you need that this check, far from being redundant, is actually a good thing.
Because mistakes do happen; how many more instances of climbing out on the wrong QNH would happen if this readback was not required?
That's why it gets done, there is no logical argument against the above facts!