120 secs is the standard vortex wake separation for aircraft following the same route. However 2mins is the standard for aircraft of the same category and this is designed to achieve 5nm separation for the first 25nm of flight. The A340 is just about the slowest jet off the blocks and the B757 about the fastest. If the tower controller applies 120 sec vortex wake separation they are achieving their immediate requirement but that 5nm separation will be out the window by Burnham. If any tower controller puts a B757 only 120secs behind a A340 they need their brains testing. Hopefully there are none of those reading this, but there are a few.
Question. Why isn't the A340 in a lower speed group? Sensible answers only.