Toilets on the Hong Kong-based airline's A330 and A340 Airbuses have become blocked three times in the space of 11 days, with one flight diverted and two others forced to reduce passenger numbers.

In the worst case, a flight from Riyadh to Hong Kong on Nov 17 with 278 passengers on board had to divert to Mumbai when flight attendants discovered none of the 10 toilets on board were working.

The diversion resulted in an 18-hour delay for repairs and a crew change for what should have been a routine eight-hour flight. Passengers received an apology and vouchers as compensation.

Two other Hong Kong-bound flights -- one from Rome Nov 9 and another from Dubai Nov 19 -- had to cut passenger numbers when crew found before take-off all toilets on one side of the plane blocked.

Cathay Pacific and Airbus both said they were unsure about the exact cause of the blockages but an airline spokeswoman said the problem was being tackled with a cross-fleet maintenance operation.

Passengers might be partly to blame, she suggested. "You would be amazed at what we find in the pipes when we clean the system -- not just face towels but medicine bottles, socks and even children's stuffed toys," the spokeswoman said.

Aircraft toilets use a high-speed pipe mechanism that carries waste into a holding tank. Two vacuum systems operate separately down each side of the plane, meaning a blockage usually affects all toilets on one side of the aircraft.

Cathay Pacific regulations stipulate planes should have a minimum of one toilet per 80 passengers.

Engineers are now fitting new pipes and carrying out deep-cleaning treatment on the toilets of its entire fleet, starting with its worst-hit 15 Airbus A340s and 32 Airbus A330s.

In a memo to staff, the airline's director of flight operations Nick Rhodes said the problem was "possibly due to a change in the cleaning procedure introduced approximately six months ago".

Sean Lee, Singapore-based Asia communications director for Airbus, said the plane maker was working with Cathay Pacific engineers to try to find out the cause of the recent spate of toilet blockages.

There were no similar outbreaks of toilet blockages on other airlines, he said, adding: "We appreciate that it is very inconvenient for passengers and we are looking into it."

--DPA