On several days in the summer, the height of cloudbase (or the tops of blue thermals) may well exceed 4,000m, but we must observe certain altitude limits in the Swiss Alps.  Throughout “MIL ON” times, the restriction is a ceiling of FL130; during “MIL OFF” this rises to FL150.  At standard QNH, these levels equate to 3950m and 4550m ASL.

However, the uncontrolled class G airspace extends as a layer all over Switzerland from ground level to 600m AGL (with the exception of CTRs, which descend to the ground, and TMAs with lower limits, e.g. for Sion TMA it’s 300m AGL).

Airspace categories

Consequently, we may exceed these altitude limits as long as we remain less than 600m directly above terrain, so (for example) even during “MIL ON” we are allowed to fly over the 4274m Finsteraarhorn provided that we remain within this class G layer.  However, you would be in breach of air law if you went on a glide from there towards lower terrain, and were still over 3950m ASL when the ground directly underneath you had dropped away further than 600m.

On the other hand, in the Zürich air lane, the altitude limit is always FL130, with no increased allowance during “MIL OFF” times.