Lowest point before starting G/A? I agree with previous poster - prior to transitioning to flare. Approx 20'.
IMHO wrong answer
The
latest point to initiate a G/A I have been repeatedly taught is on selection of Reverse Thrust i.e. at that point you are committed to land. A bounced / deep landing is an example...
If we repharse the question as to "latest point to GA without landing clearance" I believe it is not defined? (well, it is for us in LVPs). 20' might be an answer, but it depends on what is going to cause NATS to run to the press
Are they going to be upset if you briefly touch down on the GA? If so, then as much as height being important, it is the point at which you reduce thrust - since this determines the spool up time making a touchdown more likely.
If NATS are going to get upset by a brief touchdown, then what is the minimum height for them not to be upset? 1'? 5'? 10'? As the BA 744 v bmi 321 @ LHR showed, it might be an aircraft fin in the way.
There is value in learning from this, and really questioning what is being achieved by LHR's (at least) "late landing clearance" sometimes given past the threshold and into the flare. By implication in proceeding to that point, both I and the ATCO have somehow agreed the runway threshold area, at least, is clear - yet no specific R/T calls have established that
Interested in ATCOs' viewpoint on that?