Swiss investigators have instructed Junkers Ju 52 operator JU-Air to ensure passengers remain seated during pleasure flights and have insisted that the carrier introduces a greater altitude safety margin when it resumes services.

The operator suffered a fatal Ju 52 accident on 4 August and investigation authority SUST has yet to determine the cause.

Switzerland’s federal office for civil aviation says there are “no indications” of a general technical problem with the type, and it cannot justify grounding the JU-Air fleet.

But while it has permitted JU-Air to resume flights with two Ju 52s on 17 August, it has set conditions for the operation. The aircraft will need to be fitted with GPS data recording devices capable of tracking the flightpath.

The federal office is demanding that all passengers on the aircraft must remain seated and belted during the flight, and not permitted to “circulate freely”. This restriction will also apply to cockpit visits.

JU-Air will also need to operate at a minimum altitude threshold above the legal requirement.

Switzerland’s regulator has not detailed the rationale behind the limitations, beyond stating that they are “precautionary”. There is no immediate suggestion that balance or centre-of-gravity issues might have contributed to the crash.

These measures, already agreed by JU-Air, must remain in place at least until the accident investigation turns up definite findings.

None of the 20 occupants of the aircraft involved in the accident (HB-HOT) survived the crash.