There are also diffferences in the way some countries operate the same class of airspace. In the USA an a/c may penetrate a class D CTR without a positive clearance, providing two way communication has been established between the a/c and the controller (eg this is sufficient: a/c: 'Naples Tower, Cessna-12345.' Tower: 'Cessna-345 stand by.'). In the UK, of course, a positive clearance is needed to enter. Why is there this difference? What is the ICAO standard? It seems to be to be a potential source of danger for a US pilot flying in the UK for the first time.