Description

On 18 December 2020, a Bombardier Challenger 850 (9H-ILB) being operated by VistaJet on a non-scheduled international passenger flight from Kortrijk-Wevelgem to Sion as VistaJet 645 and on the destination ILS and a privately-operated Diamond DA-40 (HB-SDV) on a domestic flight from Lausanne to Lugano were on crossing tracks and working different ATC frequencies. The Challenger crew received and followed a TCAS RA to climb but the DA-40 began visual avoidance by also climbing before sighting the Challenger doing the same and changing to a descent. Minimum separation was 100 feet vertically and just over 1 nm laterally before vertical separation began to increase.

Investigation

An Investigation was carried out by the Swiss Transportation Safety Investigation Board (STSB). Relevant data from ATC radar recordings were available.

It was noted that the Bombardier Captain, (a Dutch citizen born in 1975) had a total of 5,800 hours flying experience which included 2,400 hours on type. The First Officer’s flying experience was not documented. The DA40 Pilot (an Italian citizen born in 1964) had a total of 8,100 hours flying experience which included 50 hours on type.  

What Happened

The Challenger was in contact with Sion TWR and on an IFR flight plan as it descended for an ILS approach to runway 25 at Sion within the Sion TMA whilst it was not activated and therefore downgraded to Class ‘E’ airspace. Sion TWR advised the flight of unidentified and potential conflicting VFR traffic likely to cross right to left at a 1 o’clock relative position at a range of 7nm at an altitude of 13,000 feet. The Sion TWR controller then contacted the Geneva Information officer who was working the other traffic - a Diamond DA-40 - and this controller transmitted the position of the jet traffic to the light aircraft involved.

Soon after this, a TCAS ‘CLIMB’ RA occurred on the Challenger and was followed. At the same time, the DA-40 pilot saw the Challenger and transmitted “in sight, we climb a little bit”. As both aircraft climbed, their separation reduced and 17 seconds after announcing his intention to climb, he transmitted that “we have to descend because he is climbing”. Separation at that point was 100 feet vertically and 1.1nm but thereafter the vertical separation increased as the horizontal separation reduced becoming 375 feet at 0.7nm and then 900 feet at 0.1nm.

It was noted that some civil and military airports in Switzerland have temporary TMAs which are “generally designated on the topographic aeronautical chart using the abbreviation HX”. This kind of TMA is supposed to be able to be activated at short notice via voice radio if required and “must be activated during IFR approaches and departures”. Contrary to this arrangement, the Sion TMA is marked on the topographic aeronautical chart by a lighter border rather than the abbreviation HX which indicates that it must be activated by NOTAM issued in advance and the activation included in the Daily Airspace Bulletin. There was only one other temporary TMA subject to advance designation in this way.

The conflicting tracks annotated with timed positions and minimum separation. [Reproduced from the Official Report]

The vertical flight profiles showing both aircraft climbing. [Reproduced from the Official Report]

Why It Happened 

The ILS procedure for runway 25 at Sion is within the Sion CTZ (Class ‘D’airspace) once approximately 5nm from the runway threshold and the remainder is within the Sion TMA when active. At the time of the event and during the course of the Investigation, it was “generally not being activated” so that VFR traffic, with or without a transponder switched on, “must be expected at all times”.

However, the Investigation found that in the current case, “the ‘see and avoid’ principle applicable in Class ‘E’ airspace had been pushed to the limit despite the transmission of traffic information”. Only the TCAS RA had “ensured that a safe vertical distance was achieved by the time the horizontal distance between the two aircraft had reduced to practically zero”.

A marked increase in “very close encounters” in the Class ‘E’ airspace around Sion between business jets like the one involved in the event under investigation also making ILS approaches to runway 25 there and VFR light aircraft was noted to have occurred since it happened with, by way of example, five such conflicts occurring in a three month period in 2022.

The advance notification of the Sion TMA activation was found to have originated when the aerodrome was routinely used only for now discontinued military flight operations whereas it has for many years now been a civil use airport, frequently visited by business jets on IFR flight plans using the runway 25 ILS. This situation had already been assessed by the STSB as a “safety deficit” and led it to issue several safety recommendations after similar near miss events.

It was concluded that “it is very clear that the inflexible procedure for activating the Sion TMA had contributed to the occurrence”. However, the safety deficit was not new and “to date no adjustments have yet been made to the airspace structure or to the conditions for use of the airspace around the airport”. In particular, it was noted that as long ago as 2018, ANSP Skyguide had set up a working group called “Airspace Echo Task Force” which in 2022 had indicated that “the entire Rhone Valley was a hotspot” Although it had been suggested that “to improve flight safety, consideration should be given to discussing with the State Safety Regultor FOCA, the possibility of establishing a TMZ with a listening squawk for this area”, this had not happened.

The STSB therefore determined that two Safety Notices should be issued to make flight crews more aware of the current situation, one addressed to “flight crews performing an instrument approach to Sion” and the other addressed to pilots making VFR flights within the “not normally activated” Sion TMA.

The Cause of the loss of separation was recorded as “the fact that both aircraft crews failed to visually recognise the other early enough to ensure sufficient distance based on the “see and avoid” principle".

A Contributory Factor was identified as “outdated procedures preventing activation of the Sion TMA at short notice”.

One Safety Recommendation was made based on the findings of the Investigation as follows:

• that the Federal Office of Civil Aviation (FOCA) should promptly take suitable measures to reduce the risk of airprox incidents in the Sion TMA area arising from IFR traffic passing through class E airspace, for example by permanently activating the current TMA via NOTAM ("TMA TEMPO") or by introducing a TMA that can be activated at short notice via voice radio if necessary ("TMA HX").

The Final Report was approved on 31 October 2023 and subsequently published on 13 November 2023.  

Related Articles

• Loss of Separation
• Classification of Airspace
• See and Avoid
• ACAS Resolution and Traffic Advisories
• Separation Standards
• Transponder
• Visual Flight Rules (VFR)

Further Reading

• Flight Safety Information: IFR / VFR – Air Traffic in Airspace E, Sympathetic Cooperation, BFU Germany, Dec 2011