The retired civil aviation chief, Peter Lok Kung-nam, has a duty to explain publicly his involvement with China National Aviation Corporation (CNAC). Ever since the Beijing-based airline applied for a certificate to operate in Hong Kong, Cathay Pacific has complained that it will not face a level playing field in competing with its future rival. In the past, such concern focused on the post-1997 position, since CNAC is a wholly-owned subsidiary of the mainland's air regulator. Now, the disclosure that a retired civil servant is advising CNAC on its application raises more immediate fears that it already enjoys an advantage Cathay does not share. There is no suggestion that Mr Lok has acted improperly. But he should be the first to acknowledge what a valuable asset his 39 years of experience with the Civil Aviation Department will be for CNAC as it prepares to do battle for routes now flown by Cathay and Dragonair. The question of how far former senior civil servants should be restricted in the jobs they undertake after leaving the service is a difficult one. An advisory body is supposed to monitor the issue and vet applications. But it rarely holds formal meetings. The Government has issued some guidelines. But these cannot cover every possible scenario, as shown by the row last year over former police chief Li Kwan-ha's job with Cheung Kong. That led to a belated rewriting of the rules: legislators believe they are still inadequate. Given such doubts, it would be unwise to solely rely on these guidelines. Instead, all retirees have a moral duty to consider if any job they do might give rise to even an appearance of a conflict of interest. Mr Lok's work with CNAC has such potential. That is why he has a duty to explain why no conflict of interest does, in fact, arise. Continuing confidence in Hong Kong's public service, and a level playing field between rival airlines, demand no less.