On 09/11/2010 07:18, Phil Layton wrote:
On 08/11/2010 23:01, Richard Dixon wrote:
Phil wrote in news:8jrb1cF3e5U1
@mid.individual.net:

The issue is MSL (Minimum Stack Level) goes up to FL90 below 976mbs to
provide 1000ft separation from 6000ft QNH - which has capacity impacts
in the LTMA.

Translation for a numpty here please Phil !

Richard



Richard,

Aircraft that fly in the UK do so based on the airfield pressure reduced
to MSL (QNH) if they are at or below the Transition altitude which is
set at 6000ft. If they fly above the Transition altitude, they do so set
to the Standard Atmospheric Pressure (1013.2mbs). This is so that
flights on long haul do not have to set different pressures over various
parts of the world.

The SIDs from Heathrow (Standard Instrument Departures) generally all
climb to 6000ft (QNH). The arrivals come into the holding stacks at the
lowest Flight levels that provide 1000ft separation from the SIDs. So
you see we have two sets of ac flying on separate pressure settings. We
need to always provide 1,000ft separation between them. Now 6000ft on a
QNH of 1013mbs is the same as FL60 (on 1013). Therefore when pressure is
1013mbs or more FL70 is separated from 6000ft (providing the 1000ft
separation). When the pressure drops below 1013mb, we lose 32ft for
every millibar, then we have to hold at FL80. When the pressure drops to
976mbs or below, we have to hold at FL90.....all of this takes
'capacity' out of the Terminal airspace as the levels that a lot of the
aircraft fly in SE England is 'fixed' i.e they have to cross certain
points at fixed levels.

Probably not done a very good job at explaining it - the old pressure
question is a good one to get controllers getting pens and paper out...

Phil