A tip that I found useful: Your transponder will always show the flight levels, even below the transition layer. So you can use your transponder for two things:
- As your secondary altimeter when flying above the TL: It should give the same indication as an altimeter set to 1013.2.
- As a gross error check when flying below the TL: If the QNH is lower than 1013.2, then the transponder should indicate higher than the altimeter set to the proper QNH. If the QNH is high then the transponder should indicate lower than the altimeter. How much higher or lower? About 30 feet per hPa of course.
(I always get a brainfreeze when trying to remember whether the transponder should indicate high or low. The way I remember it is as follows: If the QNH is high, then the transition layer (from 3000' to FL35, say) is thicker (more than 500'). So if I'm climbing from 3000' to FL35, I need to climb more than 500'. This means if I arrive at 3000' and reset the altimeter to 1013.2, it needs to indicate 3500 minus a bit more than 500', so less than 3000'. For example, 3000' then becomes FL29. So with a high QNH the altimeter at QNH indicates more than the transponder (which is always at 1013.2).
This is useful when we fly aerobatics: Due to airspace restrictions we typically get a block between 3000' (below the TL) and FL55 (above the TL). Which is a block height of 2500' with a standard QNH, but less than 2500' if the QNH is low, and more than 2500' if the QNH is high.)