Originally Posted by
JW411
The height at which it is safe to change to 1013.25/29.92" is defined as the Transition Altitude.
How do we arrive at this figure?
It all has to do with the terrain around the airfield in question and the avoidance of collision with said terrain that would be possible before setting altimeters to the new "artificial setting". At the risk of being shot down in flames by the experts (I only went solo 51 years ago) the calculated Transition Altitude would be something like the sum of the highest ground within 25 nms plus 1500 ft plus 10% so that the usual UK airfield would come out to 3000 ft (or more in the frozen north).
The TA in significant parts of the UK provides less than 1500 feet of ground clearance and in 'mountains' it can provide negative ground clearance.
This is based on my understanding that the TA in the UK is 3000 feet unless it is specified as something else. The only specifications in the AIP are under a number of CTA/CTZ/TMAs - none of which are in the Welsh Mountains, the Lake District, the Highlands (the places where there is minimal or negative ground clearance at 3000ft.