European safety regulators are ordering carriers to fit secondary casings to Airbus A330 and A340 auxiliary power units to prevent possible damage from uncontained failure.

Investigators are still trying to understand the mechanism for the failures - experienced by at least two A330 operators on the ground - but the European Aviation Safety Agency states that collapse of the APU's drive-end bearing appears to be a preceding event.

EASA has already issued emergency directives requiring inspection of APU sections for evidence of debris, a possible precursor to drive-end bearing problems.

It says the previous failures have led to structural damage to the APU compartment and even the stabiliser compartment. The same APU design is used on both A330s and A340s.

In an updated emergency directive EASA is requiring operators to fit a secondary housing on the APU generator. Airbus has developed such a housing designed to contain generator parts should the APU fail.

"Loose APU generator parts can lead to damage to the APU firewall, reducing its fire-extinguishing capability and potentially leading to a temporary uncontrolled fire," says the directive.

EASA says the secondary housing should be fitted no later than 31 March next year.

Source: Flight International