samotnik,
The setting of altimeters is a critical operation and when done correctly in accordance with SOP will take time. Here is an example of what the SOP could be;
PF = Pilot Flying and PM = Pilot Monitoring
PF - "Set QNH 996" sets altimeter to 996
PM - sets altimeter to 996 and reports "QNH 996 set passing 6 thousand 1 hundred feet
PF - checked.
Now in the UK, at somewhere like Heathrow the transition altitude is 6000ft. If the QNH is 996 the transition level will (under UK rules) be FL65
So let's look at where they really are. The TA is 6000ft AMSL and (using 28ft per milibar) the TL is 6024ft AMSL.
Do you think that if the aircraft is cleared to descend to 6000ft QNH 996, the crew can wait until the transition level and carry out a safe SOP for setting altimeters as well as avoiding a level bust?
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mcdhu,
correct and as I say above, waiting until the TL to change over could be very dangerous.