The most appropriate place to start with is the Machynlleth Loop which is usually referred to by aircrews as ‘The Loop’ although the USAF crews refer to it as ‘The Roundabout’. It is literally a roundabout of flowed valleys running counter-clockwise following the A470 north eastwards from Machynlleth in the south to Dinas Mawddwy then heading north west to join the A487 at the Cross Foxes Inn. From here it follows the A487 southwards through Corris to end back at Machynlleth. Ordnance Survey Explorer Map OL23 is recommended for anyone planning a visit.
It is arguably the busiest part of the UK low fly system and although the cold war days of up to 80 plus movements in a day are long gone it is still sometimes possible to see 30 plus aircraft in one day. The usual daily total is usually between 10 and 20 aircraft mainly made up of Hawks with the odd Tornado, Harrier or Hercules thrown in. It is certainly the place to go to practice your panning technique.
It takes about 3 minutes for a jet aircraft to do a circuit of the Loop and multiple passes by aircraft is not an uncommon sight, especially by Hawks. So whenever you see an aircraft it is worth checking to see if it looks like doing a circuit as you may be lucky enough to see it again in 3 minutes.