Dragonair would not sack anyone stating 'following a work to rule' as the reason because they may have to explain themselves in a court of law. Under Hong Kong law any employee can be sacked as long as the employer gives the agreed notice and any payments and allowances due to the employee. Therefore, although following a work to rule could be the actual reason for any sackings, anyone who gets sacked would just receive a letter of termination with no stated reason. It has happened before on several occasions and probably will happen again.
This is not a company to rely on for a career.
