Strength -5
The Upper Medium category only applies to arriving aircraft, so a departing 757 is considered a Medium by ATC and so can be followed 1 minute later by any aircraft that is either another Medium or a Small. OK?
It is in our instructions that some operators may require more than this "standard" and advises ATCOs to ask before line up that they will accept it. More and more carriers will not but many still do. This is where the confusion arises..
What is needed is a standard that everyone is happy to use. If that turns out to be 2 minutes then so be it, but until then we are still permitted to use 1 minute as I said above.
Categorising a 757 as a Heavy when its in front and a Medium when its behind (as in Oz) seems like a good compromise. It will of course mean a slight reduction in landing rates, but what price safety?
What is needed is harmonisation and standardisation, and soon.