London FIR
Minimum Stack Level
The London TMA operates something called "Minimum Stack Level". MSL is the lowest useable Flight Level. The MSL is dependant on the QNH pressure at London Heathrow.
QNH |
Minimum Stack Level |
|---|---|
< 978 |
090 |
978 >= < 1013 |
080 |
>= 1013 |
070 |
Why do we use MSL?
MSL is used because it adds flexibility when controlling and because many real world procedures are built around the concept of MSL. Don't be confused by the word "stack". MSL isn't just used in holding stacks, it is used for enroute flying within the London TMA.
MSL also ensures separation between arriving flights descending to Flight levels, and departing aircraft climbing to an altitude. For example, if we had a QNH pressure of 980, outbounds climbing to 6000ft, and inbounds descending to FL70:
1013 (standard pressure) - 980 = 33
33 * 30 = 990
6000 + 990 = 6990ft
The aircraft flying at FL70, is only vertically separated from the aircraft at 6000ft by 990ft.The lowest useable Flight level which would ensure vertical separation is therefore FL80
TMA controllers should ensure that they only clear aircraft down to MSL +1 when handing aircraft to Approach Control.