Disclaimer: I take no responsibility for the accuracy of any of the information on this page, and you should not rely on it in order to avoid violating airspace requirements!
At Sion, there is a CTR and a complex TMA, which belong to Class D airspace. The CTR is permanently out of bounds to us, but the status of the TMA, which is divided into three sections, can be established every day by checking the Daily Airspace Bulletin for Switzerland. Unless you know otherwise, it is safest to assume that throughout MIL ON times, the entire TMA is active. If you have no internet access or forget to check, you can usually find out the status of the TMA by asking other pilots in the cabin or at take-off. All this airspace has a ceiling of 3950m. The lower limit of TMA 1 is 900m, of TMA 2 is 1850m, and of TMA 3 is 3050m; however, it may be possible to negotiate the TMA (but not the CTR) legally by remaining less than 300m above the ground. Permission to fly through the TMA may be requested from the control tower on 118.275 MHz (in English or French).
To add to the complexity of this airspace, in 2016 three new sectors (LS-RGS 1 2 and 3) were created within the area covered by the TMA. We are allowed to enter these sectors (when the TMA is inactive) but must observe increased minimum distances from cloud there (1.5 km horizontally, 300 m vertically):
So if you cannot enter the TMA, what does this mean in practice?
◾The most straightforward option is to simply fly around all the airspace (see the track log below).
◾Crossing the Rhône valley around Crans-Montana is effectively blocked by the requirement to stay below 900m to avoid TMA 1.
◾A route along the north side of the Rhône valley under TMA 2 (staying under 1850m or less than 300m above the ground) may be possible, but you would not want to try to start crossing here with such little height.
◾Staying below 3050m or within 300m of the ground in TMA 3 allows you to negotiate this area with care.
◾Sneaking under the TMA by staying less than 300m above the ground throughout may be legal, but would be tricky.