You can cruise at the transition altitude if you want. The quadrantal or semicircular rules only apply
above the transition altitude.
The transition level in the UK is the first flight level
above the transition altitude.
Note that while the transition level is above the transition altitude it is not separated from it i.e. the transition level may be 1ft above the transition altitude.
If the QNH is 1013.2 then the flight levels will co-incide with altitudes i.e. an aircraft cruising at FL30 will be 3000ft AMSL.
In this case since FL30 is not above the transition altitude then of course the transition level will be FL35 and you can see that by chance an aircraft at FL35 will be 500ft above an aircraft at 3000ft QNH.
As the pressure reduces below 1013.2, the flight levels move closer to the surface. Thus you can see that with reducing pressure FL35 will get closer to 3000 AMSL and eventually be under 3000ft AMSL.
Similarly, as pressure increases above 1013.2 the flight levels move up. Thus with a QNH of 1014, FL30 is actually 24ft above 3000ft AMSL and is therefore the transition level.
You can in this case cruise at FL30 in the appropriate direction however you need to be aware that while you will have 500ft separaton from the closest traffic above cruising at FL35, you will only have 24ft separation from traffic cruising at the transition altitude.
If the pressure increases enough, FL25 and then FL20 will eventually be above 3000ft AMSL and thus they will in turn become the transition level. Get your calculator out to work out when
Since the quadrantal rule applies to flight above 3000ft AMSL (or the transition altitude if higher), you can never fly at a flight level that causes you to be below the transition altitude.
Thus in the example given, FL50 is the first available level for that direction which is
above the transition altitude.
Note that in other countries, the transition level may be the lowest flight level that provides a minimum of 1000ft separation from the transition altitude.
From a practical point of view you will not care much about the transition level, you will be more interested in a flight level that provides a minimum of 500ft separation from the transition altitude.
Regards,
DFC