Sorry Shaz, didn't wish to lose or confuse!
TMA (Terminal Monouvring Area) is the area surrounding the London Airports of which Heathrow (LHR) is obviously one. Its basically a big mass of airspace where all the inbounds and outbounds from the London Airfields can be kept safe and adequately separated. Its upper limit is about 17,000 feet. the four main stacks for Heathrow are known as the inner stacks, but when these get full we start holding aircraft further out, for instance as WHBM out by Clacton VOR at a point called BRASO. All the surrounding ATC sectors have holding points for many of their routings just in case they cannot get into the TMA.
Hope that helps and explains a bit better
5mb
