Swiss pleasure flights operator JU-Air has temporarily suspended operations after the fatal accident involving a Junkers Ju 52/3m in the Alps.

The aircraft (HB-HOT) crashed on the slopes of the Piz Segnas peak, situated 75km south-east of Zurich, at around 17:00 on 4 August.

It collided with the western flank of the mountain, at an altitude of 2,540m above sea level, states the Grisons cantonal police service. None of the 20 occupants – comprising 17 passengers and three crew members – survived the accident.

Switzerland’s civil aircraft register lists the three-engined transport as having been manufactured in 1939. The Ju 52 was one of only a handful of airworthy examples still operating.

It departed Locarno-Magadino airport at 16:10, bound for the airfield at Dubendorf, a return service as part of a two-day excursion to the canton of Ticino.

Swiss air accident investigation authority SUST is co-operating with federal and regional prosecutors, as well as law enforcement, in an effort to understand the cause of the crash.

JU-Air says its flight operations are being suspended “until further notice”.

The aircraft, powered by three BMW 132 engines, was registered with the Swiss federal office for civil aviation in July 1985.

Its operator is identified as Verein der Freunde der schweizerischen Luftwaffe, the legal entity which oversees JU-Air.

The federal office says it has imposed temporary airspace restrictions around the site of the crash. This restriction, which bans civil air traffic except for emergency medical helicopters from operating within a 5nm radius of Segnaspass, up to 13,000ft, features in a NOTAM lasting until 8 August.

SUST investigation chief Daniel Knecht says the aircraft appears to have struck terrain “nearly vertically” at high speed, and did not collide with another aircraft or alpine cable prior to the accident. The inquiry is likely to be complicated by the absence of flight recorders.

Lufthansa’s historic flight arm operates an older Ju 52, a 1936-vintage aircraft, fitted with Pratt & Whitney R-1340 Wasp engines.

The aircraft has recently undergone a major overhaul, around the time of its 80th anniversary, after structural damage was discovered in the wing, which forced the company to halt pleasure flights over safety concerns.

Maintenance teams had to undertake a complex repair of a key wing spar, a repair which involved manufacturing new parts and sourcing suitable substitute materials. The difficulties involved led to several delays to the aircraft’s 2016 flying season.

But the Swiss civil aviation office states that it last checked HB-HOT on 6 April, four months before the crash, and cleared the Ju 52 for a further two years of operations. "No discrepancies were found during the inspection," the office adds.

JU-Air says two captains, former air force and airline pilots, were in charge of the cockpit. One had logged 943h with the company, flying Ju 52s for the carrier since 2004, while the other had 297h on the type.