Originally Posted by
Smithy02
Outside Europe where transition altitudes/levels are as high as FL180 you MUST set STD or QNH at the transition altitude/level and not before
You should not set it when cleared, as in the UK. At our airline we call passing transition both up and down and carry out an altimeter check.
Operating into UK airports we set it when cleared, as described above.
That's quite correct. ATC can be using your Mode C or ADS-B level for vertical separation with other traffic. (Disclaimer - apparently not so in Nudging's unit) If you have 1013.2 set below the transition level, or QNH set above it, then your reported level/altitude will be incorrect, and increasingly incorrect as QNH varies more from 1013.2. ATC only applies a 200ft buffer to these reported levels for separation purposes. Obviously this buffer is negated if using the wrong pressure setting and QNH is more than 6 hPa different from 1013.2.