The only time the difference in procedure will make a difference is if you have an 'interaction' with ATC or another aircraft.
If it is aircraft-aircraft and identified by TCAS, these are always generated vs 1013.2
If it is aircraft-ground and identified by EGPWS, this has nothing to do with the pressure setting.
If it is an interaction with ATC, it will either be
a) and instruction to expedite (nothing to do with setting) or
b) a request to report passing level/altitude (which clearly needs the right setting)
c) an unplanned instruction to stop climb/descent and an altitude/level (same issue)
These last two are the only cases where the setting matters and I would be interested in the ATC view.
If you have cleared someone from an altitude to a FL and need to ask them their current level or to stop climb, do you expect the pilot to be climbing with reference to a QNH or SAS?
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note - I have ignored the somewhat special case of UK ATSOCAS calling traffic
Last edited by mm_flynn; 10th Sep 2009 at 17:03.