Loss of control on a slope

A touring glider flight ends fatally in the Alps. Does the risk of a stall need to be better addressed and what about stall warnings in gliders?

Author: Evelyn Pesentheiner

On 23 August 2024 shortly after 15:53, the flight of an Arcus M ended around one kilometer southeast of the Chistehorn near Niedergesteln (VS). A witness had observed how the glider "plunged almost vertically downwards in a roll and hit the ground nose-first". Prior to this, the aircraft had flown figure-eight loops for almost twelve minutes, gaining altitude in the process. On the day of the accident, there was blue sky with a few small cumuli. Both pilots, a 46-year-old and a 72-year-old Swiss man, suffered fatal injuries in the crash.

The Arcus M, registered in Austria as OE-9485, took off at 13:28 on its own at Birrfeld airfield (LSZF). The men, both in possession of a gliding license, intended to undertake a touring glider flight over the main Alpine ridge to Grenoble in France. On the following
The flight back to Langenthal (LSPL) was scheduled for the following day. The pilots had rented the high-performance glider with self-launch capability from a third party.

In stalled flight condition

No technical defects were found on the glider. The Swiss Transportation Safety Investigation Board (STSB) came to the conclusion that "the accident, in which the glider entered an almost vertical spiral dive from a low altitude above the terrain, was due to a loss of control as a result of exceeding the critical angle of attack." In other words, an abrupt stall due to stalling, as a result of which the aircraft could no longer be intercepted due to its proximity to the terrain. The center of gravity, which may have been at the rear but within the permissible limits, could also have contributed to the accident.

According to the gliding weather forecast, there were moderate thermals for the Central Plateau and the western Alps on the day of the accident, while hardly any thermals were expected for the southern side of the Alps. On the way via Lake Hallwil, Lake Sempach, west past Lucerne and via Interlaken and Kandersteg to the Gemmi Pass, the pilots operated the engine several times to gain altitude. At an altitude of 2991 m/M, they switched off the engine for the last time and then glided along the southern slopes of the Rhone Valley. The course of the flight with repeated use of the engine underlines the less than ideal thermal conditions. However, the accident report shows that it would have been possible to land in gliding flight at the Raron airfield at any time. The report also refers to the fact that several cross-country flights along the southern slopes of the Upper Valais were documented on the WeGlide internet platform on the day of the accident. The pilots turned around in the area north of Raron. They mostly reached altitudes of over 3000 m/M. It was not a hopeless situation in the mountains, but the danger of a stall that was not recognized in time that led to the tragic accident.

Possibilities of a warning system

In the final report on the accident, the STSB lists the following for the period from July 2008 to the crash of the Arcus M
19 accidents involving gliders or motorized gliders due to loss of control in flight. A strikingly high number of similar cases. How does this happen?

In powered flight, the stall warning, i.e. a stall warning system, is part of the basic equipment of an aircraft. A metal tongue or a vacuum hole is located on the wing tip.
leading edge. If the angle of attack rises above the critical point, the construction triggers an acoustic signal and warns the pilot of the impending stall. A stall warning is not mandatory in gliding. Only a few types of gliders have a corresponding device.

The STSB has taken the recent accident as an opportunity to emphatically recall a safety recommendation it made 17 years ago. According to this recommendation, manufacturers should be obliged by the EASA to equip gliders with an appropriate warning system. Did the authorities and the industry miss the appeal?

It's not that simple. Some manufacturers offer a stall warning system for their models. However, a system that works for all types of gliders is not yet on the market. The topic, as FOCA Head of Communications Christine Caron explains, has been discussed within the Organization Scientifique et Technique du Vol à Voile (OSTIV) and is being pursued by this body. As gliders are often operated in a range close to the minimum speed, a technical solution analogous to powered flight is not expedient. This is also explained by Roland Bieri, Head of Training at the Swiss Gliding Association. Classic stall warnings would disrupt the glider's airflow and would be of little use, as they also give a warning when it is not necessary. A system that gives too many warnings encourages pilots to ignore them, as the FOCA also sees it.

Sensitization takes place on an ongoing basis

The second safety recommendation made by the STSB following the Arcus M accident concerns prevention. Together with the gliding association, the FOCA should "promptly raise awareness among the gliding community in an appropriate manner with regard to loss of control, particularly in slope flying." What do the people involved say? The safety officers regularly address safety-related topics in the individual gliding groups. Serious accidents are also discussed in the safety workshop, says Roland Bieri. The gliding association holds these every year at the end of January. The FOCA draws attention to its "Stay Safe" safety campaign for general aviation. Articles on safety have been published on this website for over ten years, including for glider pilots. A new video specifically for gliding is currently being produced and will be published shortly before the next gliding season in spring 2026. The FOCA also shows the STSB's explanatory video on the Ju 52 accident, which demonstrates the relationship between angle of attack/lift versus speed, at further training events. 

Final report no. 2431 on the accident involving the Arcus M gliderOE-9485, dated August 23, 2024, around 1 km southeast of the Chistehorn, municipality of Niedergesteln (VS).