BOAC said -
-------------------------------------------------------------------------------------------------------------------------------------------------------------- My only query to your post is was separation degraded by 160 to 4 from 18.5 miles as opposed to 170 to 6 etc?
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Assuming a ground speed of 100kts I reckon the catch up would be 42 seconds or just over 1 mile.
If you were 3 miles behind the preceeding at 18 miles, by 6 dme the separation would be less than 2. (and the controller would be suspended).
That's not to say you wouldn't get landing clearance but SEPARATION would have been lost.
In my experience, 170 kts to 4 is only suitable to a small number of aircraft. Not every 737 wants 170 to 4.
747, 757, 767 and all the airbuses seem to manage 160 to 4 so why change the standard to suit a small minority ?
That's not to say I won't accomodate 170 to 4 on an individual basis if I can but you cannot wait till minimum spacing is established then say 'can we do 170 ?', it just won't work.
To further illustrate my point, one of your collegues in a -500 last night waited till on the tower frequency to say they had a particularly low approach speed and were slowing early. The company 737 three miles behind very quickly became 2 miles behind and only a rapid speed reduction avoided a go around.
It beggars belief that a small minority of pilots don't realise the impact a change in speed has on spacing and separation.
BOAC, if you want 170 to 4 ask downwind or even on base leg and if you do ask later than base, don't be surprised or offended when we say no.