An aircrew union is legally challenging the Director of Civil Aviation for allowing a Cathay Pacific flight from Melbourne to Hong Kong to operate with only two pilots instead of three.

In a writ filed with the High Court, the Hong Kong Aircrew Officers' Association expressed its concern over aviation safety after Hong Kong's Civil Aviation Department waived the three-pilot requirement in February.

The association is seeking a judicial review asking the court to declare the decision unlawful and improper.

According to the writ, flight CX168 was scheduled to fly overnight from Melbourne to Hong Kong on February 27 but one pilot fell sick.

According to the union, the operation manual requires three pilots because the flight covers a long distance.

The union also cited a Civil Aviation Department standard provision that said: "If the planned flight-duty period for a two-crew aircraft includes a scheduled sector length in excess of nine hours, one additional pilot must be on board to meet the relief crew qualifications."

According to the writ, Cathay Pacific is said to have asked the department to allow the flight to proceed without the full crew complement.

The union wrote to the department on May 8 seeking clarification.

In a May 27 reply, the department said the two pilots were well rested and had been rostered for a number of days before the flight in question.

But the association rejected the explanation.

In the writ, the association argued that "fatigue is recognized to be insidious in nature such that an individual often cannot accurately assess or forecast their own fatigue, especially for crew members before a long-haul flight."

The writ also said members of the crew have to be constantly alert to deal with any technical or meteorological difficulties and any possible abnormal or emergency situations.

"Thus, a high degree of alertness and reliability of the crew member is pertinent to the safety of the flight that they operate."

The union is also seeking an injunction from the court to refrain the director from acting in breach of the requirement.

Cathay Pacific refused to comment, saying the airline is not one of the parties in the litigation. In its reply last night, the Civil Aviation Department said it "has never and will never compromise safety. This applies equally to the flight in question on February 27 from Melbourne to Hong Kong."