Challenger 350 TCAS RA on the approach to Sion, 2025

What happened

1. Before 0935 UTC, the Challenger is on the instrument approach to runway 25 at Sion, inside the Sion TMA. The TMA is a temporary one and is not active that day, so the airspace is class E.

2. On final approach, the Sion Tower controller gives the crew two traffic information calls about a helicopter. It is flying north at about 11,000 ft AMSL through Sion TMA 3.

3. 0935

   About 17 NM from the threshold, at about 12,000 ft AMSL, TCAS issues a resolution advisory. The crew follow it.

4. After the RA, the crew start the missed approach procedure.

5. Later, the crew fly a second approach and land safely at Sion.

All times are UTC. 0935 UTC was 1035 local.

Why it happened

1. The bulletin does not state a cause. It records one condition: the temporary Sion TMA was not activated that day, so the airprox happened in class E airspace. In class E, VFR flights need no ATC clearance and ATC does not separate them from IFR traffic.¹

2. The STSB points to a comparable case: an airprox between a business jet and a light aircraft in the Sion TMA on 18 December 2020, its final report no. 2393.¹

3. The same bulletin records two more airprox events near Sion on 18 July 2025. In one, an Embraer EMB-505 on the approach to runway 25 got an RA about 16 NM east at about 10,300 ft and went around; the pilot of the light aircraft was on the tower frequency but had not called. In the other, a Cessna Citation CJ2 departing west got an RA near a glider about 8 NM from the runway.¹

4. Bulletin 2026/1 records a further Challenger 350 RA about 10 NM east of Sion on 8 February 2025, again with the TMA not active.³

The bulletin makes no recommendations.¹

What a crew can take from it

1. Find out if the Sion TMA is active. When it is not, the approach runs through class E airspace. VFR traffic can be in it without a clearance, and the controller will not separate you from it. Check its status before the approach and ask on first contact, then brief the approach with that in mind.

2. Use the traffic information to find the target. The tower called the helicopter twice before the RA. Put the traffic on the display, set a range that shows it, and ask for its altitude and track if you do not have them. The tower reported the helicopter at about 11,000 ft. The jet was at about 12,000 ft when the RA came.

3. Follow the RA. The crew complied with it. On a mountain approach that can feel wrong, close to terrain and with the approach set up. The RA is built to give separation from the other aircraft, and it takes priority over the approach.

4. Plan for a go-around after an RA on approach. After the RA the crew flew the missed approach and came back for a second one. In the February 2025 case the crew continued the approach. Brief the missed approach for each approach at Sion before you start it. Then the go-around after an RA is already planned when it comes.

5. Expect helicopters and gliders at approach altitudes. The STSB recorded RAs with a helicopter, a light aircraft and a glider around Sion in 2025. They fly the valleys at the same levels the approaches use.

Study it on the course

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Sources

1. Swiss Transportation Safety Investigation Board (STSB). Prevention Bulletin 2025/3, page 2, entry of 5 March 2025, 0935 UTC, "17 NM east of Sion Airport (LSGS)". Dated Bern, 21 October 2025.
2. STSB, press release Prevention bulletin 2025/3 aviation, 2 December 2025.
3. STSB. Prevention Bulletin 2026/1, entry of 8 February 2025 (Challenger 350 RA 10 NM east of Sion).