UK ATC will, in the climb and after you have been cleared to a FL, reference any subsequent requests to FLs and conversely, in the descent and after you have been cleared to an altitude, reference any subsequent requests to altitude (i.e. QNH). I imagine most other countries' ATC would adopt similar procedures.
Therefore, it is sensible to set 1013mb when cleared to a FL in the climb and to set QNH when cleared down to an altitude in the descent. If you have an autopilot engaged and you are selecting a cleared altitude/FL, then it becomes essential to set the appropriate subscale at the same time to avoid a potential "level bust". As has been said, many (most?) IFR equipped aircraft will have a third altimeter set to QNH during CLB/DES anyway for reference if needed - unless, of course it is metric!
In any event, I think
763 jock has it right - if you are operating under an AOC, stick to your SOPs.
I have respect for
BelArgUSA's postings but I must correct him on one point.
Contrary to common belief, QNE is
not a subscale setting (1013mb). QNE is an altimeter reading (i.e. height) and is defined as what the altimeter will indicate at the surface (usually at an aerodrome) when 1013mb (STD) is set. It is little used now but it had its uses at high altitude airfields for operators who used QFE, when QFE was very low. It was an alternative used either to avoid a very long wind-down of the subscale to QFE or if the QFE was less than the lowest subscale setting of the altimeter.
To use the term QNE in discussion as if it equated to "setting 1013" is therefore wrong.
JD
