Swiss regulators have grounded two remaining Junkers Ju 52s operated by Ju-Air over airworthiness concerns, three months after a fatal accident involving another of its aircraft.

The Swiss civil aviation office has ordered the certificates of airworthiness for the tri-motored aircraft – registered HB-HOP and HB-HOS – to be surrendered by 21 November.

None of the 20 occupants on board a Ju-Air Ju 52 (HB-HOT) survived when, during a 4 August flight, the aircraft’s nose dropped as it entered a left turn, and it spiralled vertically into terrain.

Swiss accident investigation authority SUST has yet to determine the cause of the crash, but inspection of the wreckage has revealed areas of concern regarding the aircraft’s structural condition.

It stresses that the deficiencies are “not related” to the accident, and there is no evidence of a pre-existing technical problem which might have caused the crash.

But it states that Ju-Air’s other two Ju 52s are of similar age to the aircraft lost, and underwent similar operations.

“It is to be expected that comparable deficiencies, which could compromise aviation safety, are also present in [these other two aircraft],” adds SUST.

Inspection of the HB-HOT debris found “significant corrosion” in spars, hinges, wing components and cabin floor parts.

Two of the aircraft’s three engines were fitted with newly-manufactured cams, but the cam discs were discovered to have quality issues. The Ju 52 was powered by BMW 132A3 radial engines.

The inquiry has also turned up “various shortcomings” in the documentation of maintenance work, says SUST, particularly relating to modification and management of spare parts.

“Such deficiencies pose a potential risk,” it states.

The Swiss civil aviation office had last checked the aircraft on 6 April, four months before the accident, without noting any problems.

Ju-Air is not contesting the grounding, but insists that the damage to HB-HOT was located in a section which could not be fully examined with the inspection methods employed.

“It only came to light when the aircraft was completely destroyed in the crash,” says the operator.

Ju-Air says it has embarked on a project to ensure its other two aircraft are fully airworthy, and the company has ambitions to introduce a third Ju 52 next year by recommissioning a parked aircraft, HB-HOY, which is a Spanish licence-built aircraft, 10 years younger than the others in the fleet.

Preparations are under way to retrieve the aircraft from its base in Monchengladbach, and fly it to Dubendorf for a structural overhaul.